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Nonionic amphiphile nanoarchitectonics: self-assembly into micelles and lyotropic liquid crystals
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Nanotechnology Nanotechnology 26 (2015) 204002 (11pp)
doi:10.1088/0957-4484/26/20/204002
Nonionic amphiphile nanoarchitectonics: self-assembly into micelles and lyotropic liquid crystals Lok Kumar Shrestha1, Karolina Maria Strzelczyk1,2, Rekha Goswami Shrestha3, Kotoko Ichikawa4, Kenji Aramaki4, Jonathan P Hill1 and Katsuhiko Ariga1 1
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan 2 Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141 St, 02507 Warszawa, Poland 3 Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 4 Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Yokohama 240-8501, Japan E-mail: [email protected] and [email protected] Received 19 November 2014, revised 6 March 2015 Accepted for publication 23 March 2015 Published 27 April 2015 Abstract
Amphiphiles, molecules that possess both hydrophilic and hydrophobic moieties, are architecturally simple molecules that can spontaneously self-assemble into complex hierarchical structures from lower to higher dimensions either in the bulk phase or at an interface. Recent developments in multifunctional nanostructure design using the advanced concept of nanoarchitectonics utilize this simple process of assembly. Amphiphilic self-assemblies involving lipids or proteins mimic the structure of biological systems, thus highlighting the necessity of a fundamental physical understanding of amphiphilic self-assembly towards a realization of the complex mechanisms operating in nature. Herein, we describe self-assembled microstructures of biocompatible and biodegradable tetraglycerol lauryl ether (C12G4) nonionic surfactant in an aqueous solvent system. Temperature-composition analyses of equilibrium phases identified by using small-angle x-ray scattering (SAXS) provide strong evidence of various spontaneously selfassembled mesostructures, such as normal micelles (Wm), hexagonal liquid crystal (H1), and reverse micelles (Om). In contrast to conventional poly(oxyethylene) nonionic surfactants, C12G4 did not exhibit the clouding phenomenon at higher temperatures (phase separation was not observed up to 100 °C), demonstrating the greater thermal stability of the self-assembled mesophases. Generalized indirect Fourier transformation (GIFT) evaluation of the SAXS data confirmed the formation of core–shell–type spherical micelles with a maximum dimension ca. 8.7 nm. The shape and size of the C12G4 micelles remained apparently unchanged over a wide range of concentrations (up to 20%), but intermicellar interactions increased and could be described by the Percus–Yevick (PY) theory (after Carnahan and Starling), which provides a very accurate analytical expression for the osmotic pressure of a monodisperse hard sphere. S Online supplementary data available from stacks.iop.org/NANO/26/204002/mmedia Keywords: amphiphiles, nanoarchitectonics, micelle, liquid crystal, SAXS, rheology (Some figures may appear in colour only in the online journal)
0957-4484/15/204002+11$33.00
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1. Introduction
and pharmaceutical formulations, and more recently in functional materials design [31, 35]. It has been found that the intrinsic geometry of an individual amphiphile (or surfactant) has a strong influence on the shape of the micelles. The different shapes of micellar aggregates can be characterized by the critical packing parameter (cpp) defined as [36],
Recent developments in materials science (both soft and hard) and nanotechnology have enabled the preparation of various functional nanomaterials with advanced properties and functions, which are suitable for integration into nanosystems for the design of solar cells, energy storage devices, and optoelectronics or spintronics devices [1–11]. In most cases, sophisticated fabrication techniques based on top-down approaches are used for the preparation of nanosystems, using an object of macroscopic dimesnions as the starting point [12–14]. However, this approach suffers from a fundamental limitation due to the minimum possible structural dimensions available by lithographic techniques. On the other hand, using an alternative bottom-up, spontaneous self-assembly process, nanomaterials with different length scales and controlled morphologies, such as nanosheets and nanotubes, have been fabricated [15–21]. However, their individual functions are limited when compared with the huge potential available from integrated systems. To address these fundamental and technological challenges, the novel and unique concept of ‘materials nanoarchitectonics’ has been developed [22–24], which allows synthesis of nanomaterials using self-assembly of the required building blocks into hierarchic structures with atomic/molecular level precision and organized/reorganized structures in a rational way for construction of advanced functional nanomaterials with ‘smart’ functions and properties [25–30]. Self-assembly is the organization of atoms, molecules, or functional building blocks into well-defined nanostructures in the absence of any external forces [15]. This process is common in nature and has also been used in advanced technologies. Self-assembly is one of the few practical strategies for the creation of ensembles of nanostructures over different length scales and has become one of the essential aspects of materials nanoarchitectonics. Amphiphilic molecules such as surfactants or block copolymers undergo spontaneous selfassembly into a variety of mesophases, both in aqueous and nonaqueous systems [31–34]. The self-assembled, complex, hierarchical geometries can be flexibly controlled or tuned by surfactant or solvent molecular architectures. Surfactants often exhibit remarkable phase behaviour in water, forming various phases (micelles, liquid crystals, vesicles, and reverse micelles), depending on surfactant composition and temperature [35]. One of the most intriguing features of surfactants in solution is that the physicochemical properties of those solutions undergo abrupt changes over a very narrow concentration range. The concentration at which the physicochemical properties change occurs is referred to as the critical micelle concentration (CMC). Just above the CMC, hydrophilic surfactants with a bigger headgroup generally assemble into spherical micelles. However, the spherical morphology can be modulated to rods or disks and also to ordered liquid crystalline phases by varying certain control parameters, including concentration, temperature, the addition of cosurfactants, controlling salinity, and pH. Self-assembled structures of amphiphiles have been extensively used in different applications including as food additives, in cosmetic
cpp =
v a 0 lc
(1)
where a0 is the effective cross-sectional area of the hydrophilic head group and v and lc are the volume and critical chain length of the hydrophobic chain, respectively. The cpp values for spherical, cylindrical, and lamellar particles are ∼1/3, 1/3 < cpp < 1/2, and 1/2 < cpp < 1, respectively. In this paper, we present self-assembled microstructures of biofriendly tetraglycerol lauryl ether (C12G4) nonionic surfactant in an aqueous system. The binary phase diagram of the C12G4/water system was constructed by visual observation through a crossed polarizer, and equilibrium phases were identified by using small-angle x-ray scattering (SAXS) technique. Shape, size, and internal structure of C12G4 micelles were determined by the generalized indirect Fourier transformation (GIFT) [37–40] method, and the results are also complemented by rheological measurements. In the dilute regime, C12G4 spontaneously self-assembles into core– shell–type spherical micelles and, at higher surfactant concentrations, into a lyotropic hexagonal liquid crystal phase that resists phase separation and possesses good thermal stability.
2. Experimental 2.1. Materials
Tetraglycerol lauryl ether (abbreviated as C12G4) nonionic surfactant was purchased from Taiyo Kagaku Co. Japan and used as received. Milli-Q filtered water was used to prepare samples for phase, SAXS, and rheology studies. 2.2. Methods 2.2.1. Sample preparation. C12G4 and water were weighed
out into clean and dry glass tubes with screw cap (10 mL) in the preparation of C12G4/H2O binary mixtures with compositions in the range from 2–100 wt%. Samples were immediately sealed and mixed using a dry thermo bath, vortex mixer, and repeated centrifugation until homogeneous. Phase behaviour was then studied by visual observation over a wide temperature range (0–100 °C) in a temperaturecontrolled water bath. Optical properties of samples (birefringent and nonbirefringent) were identified by viewing samples through a crossed polarizer. Samples were kept in equilibrium for 1 h at each temperature before observing the phases. The structures of liquid crystalline phases and their textures were identified by using SAXS and polarizing optical microscopy, respectively. For the SAXS 2
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studies of the micellar structure, samples were prepared separately and, after their thorough mixing, stored in a water bath at 25 °C for 24 h prior to the SAXS measurements. The same samples were used for rheological measurements.
where p(r) is the pair-distance distribution function giving structural information regarding the scattering particles in real space. The p(r) function directly represents a histogram of distances within the particle for homogeneous scattering particles [44]. Generally, the intraparticle scattering contributions are related to the form factor P(q) and theoretically correspond to the Fourier transformation of the p(r). Whereas the interparticle interference scattering is connected to the structure factor S(q), corresponding to the Fourier transformation of the total correlation function h(r) = g(r)–1, as
2.2.2. Optical microscopy. Optical micrographs of liquid
crystals were recorded using an Olympus BH-2 microscope equipped with an integrated charge-coupled device (CCD) camera. Images were taken in transmission, and reflectance modes from samples deposited on glass slides and sandwiched with a thin glass plate.
S (q ) = 1 + 4 πn 2.2.3. Small-angle x-ray scattering (SAXS). The SAXS
4π sin(θ /2) λ
∫0
∞
p (r )
sin qr dr qr
[g (r ) − 1 ] r 2
I (q) = nP (q) S (q)
sin qr dr qr
(4)
(5)
It is known that interparticle interactions are very weak for dilute systems (particularly at volume fraction 1%), S(q) ≠ 1 due to considerable particle interactions. Therefore, we have analyzed the experimental scattering data using the GIFT method [37–40], which potentially can be used to determine both the form factor and the structure factor without any model assumption for the form factor. Nevertheless, appropriate assumptions for the interparticle interaction potentials and the closure relation for the structure factor are required. In the calculation, we used an averaged structure factor model of hard-sphere (HS) interactions, S(q)av [49, 50], which considers the Gaussian distribution of the interaction radius σ for individual monodisperse systems for polydispersity μ, and the Percus–Yevick (PY) closure relation to solve the Ornstein–Zernike (OZ) equation [51, 52].
(2)
Measured SAXS intensities were calibrated for transmission by normalizing a zero-q attenuated primary beam intensity to unity, and the I(q)s were corrected for background scattering (capillary and water), and an absolute scale calibration was constructed using water as a secondary standard solvent [41]. With alignment during SAXS measurement, this system allows maximum resolution of qmin ∼ 0.08 nm–1, which corresponds to ∼40 nm as the detectable dimension for scattering objects. The scattering intensity I(q) is the complex square of the scattering amplitude F(q), which represents the Fourier transform of the scattering length density difference Δρ(r), and thus describes the structure of the particle in real space [42, 43]. In scattering experiments of micellar solutions, we measured the spatial average of these functions, which is defined by the following expression: I (q ) = 4 π
∞
where n is the particle number density of scatterers and g(r) is their pair-correlation function [45]. For an ideal monodisperse spheroid system, the total scattering intensity I(q) can be expressed as
technique was used to identify the liquid crystalline phase and micellar structure. To study the effect of surfactant concentration on the shape, size, and structure of micelles, SAXS measurements were carried over a wide concentration range 5–20% at 25 °C and, to observe the effect of temperature on the micellar structure, measurements were carried out on the 10% sample in the temperature range of 25–85 °C. SAXS measurements were carried out using a SAXSess camera (Anton Paar, Austria) attached to a PW3830 sealed-tube anode x-ray generator (PANalytical, Netherlands) operating at 40 kV and 50 mA. A monochromatic x-ray beam of Cu–Kα radiation (λ = 0.1542 nm) with a well-defined, focused line-shape was obtained using a Göbel mirror fitted with a block collimator. The thermostated sample holder unit (TCS 120, Anton Paar) supplied with the SAXSess system offers an accuracy of temperature ±0.1 °C. Two-dimensional (2D) scattering patterns were first recorded on an image plate (IP) detector (Cyclone, Perkin Elmer, USA) and with subsequent integration into one-dimensional (1D) scattering intensities I(q) as a function of the magnitude of the scattering vector q using the SAXSQuant software (Anton Paar). The scattering vector q is related to the total scattering angle θ as q=
∫0
(3)
3
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In the case of cylindrical micelles, the internal crosssectional structure (cross-section diameter of the cylinder) can be determined by the deconvolution of SAXS data, if Δρc(r) is only a function of the radial position within the cross-section. For high-aspect-ratio cylindrical micelles (axial length at least three times longer than the crosssectional diameter), radial electron density profile Δρc(r), which is related to the pair-distance distribution function (PDDF) of cross-section pc(r) as described in equation (7), can be determined. pc (r ) = rΔρ˜c 2 (r )
(7)
The cross-sectional pc(r) can be further calculated from the experimental I(q) via I (q) q = πLIc (q) = 2π 2L
∫0
Figure 1. Phase behaviour of the C12G4/water system over a wide range of temperatures and compositions. Wm, H1, S, and Om, respectively, denote the normal micellar phase, hexagonal liquidcrystal phase, solid phase, and inverse micellar phase. Cartoons of normal spherical micelles, hexagonal liquid-crystal phase, and an optical micrograph of the 70% C12G4 system are also shown in the phase diagram.
∞
pc (r ) J0 (qr )dr .
(8)
where J0(qr) is the zeroth-order Bessel function. The IFT of equation (8) yields pc(r), which is then used to calculate Δρc(r) by the deconvolution technique [46, 47]. 2.2.4. Rheometry. Flow properties of C12G4 micellar
moiety [53–56], C12G4 micelles did not exhibit phase separation upon heating (up to ∼100 °C), i.e., C12G4 surfactant does not exhibit lower critical solution temperature (LCST) behaviour. This could be due to the fact that glycerolbased nonionic surfactants display a stronger hydration effect (hydrogen bonding of water to the glycerol moiety of the surfactant) compared to the EO-based surfactant, thereby increasing its thermal stability. At lower temperatures below 10 °C, C12G4 appears as a solidlike phase. The C12G4 surfactant, which is a fluid at 25 °C, self-assembles into inverse micelles (Om, with an inverted, watery, cored structure, compared to normal, oily, cored micelles in the dilute region) upon addition of a small amount of water. The Om phase swells with water, followed by an Om–lyotropic liquid-crystal phase transition above 5% water (that is, the liquid-crystal phase appears for C14G2 content 40%) with the Wm–H1 phase transition eventually occurring at 54%. Optically isotropic nonbirefriengent viscous liquid formed just below the H1 phase was anticipated to be due to a micellar cubic phase (I1), but its broad SAXS pattern (data not shown) confirmed that the viscous liquid is a micellar phase. With increasing temperature, viscosity gradually decreased, leading eventually to a less viscous, easily flowing solution. Note that contrary to the conventional ethylene oxide (EO)-based nonionic surfactants, which usually undergo phase separation (clouding) upon heating due to dehydration of hydrophilic EO 4
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Figure 2. (a) SAXS patterns of the C12G4/water system recorded at 25 °C at different surfactant weight fractions, WS as typical examples, (b) variation of calculated d-spacing at different concentrations at 25 °C, and (c) variation of d-spacing in the 70% C12G4/water system at different temperatures.
spacings for 70% C12G4 at different temperatures are shown in figure 2(c). The SAXS patterns presented in figure 2(a) clearly show that the H1 phase in surfactant-rich regions incorporates a significant amount of water. The phase boundary was determined by plotting d-spacing as a function of 1/Ws. As can be seen in figure 2(b), d100 increases almost linearly, with the weight fraction of water indicating that the H1 phase swells without variation of the molecular arrangement of cylindrical micelles in their hexagonal packing. The d100 increases from ca. 4.419 to 5.236 nm when Ws decreases from 0.90 to 0.59 and then does not change, demonstrating that the H1 phase becomes saturated with water at Ws = 0.59 and upon further addition of water, the excess water separates out from the H1 phase. Therefore, a constant d is expected in the two-phase region. SAXS data indicate that the H1 phase incorporates around 35% water at 25 °C. The phase diagram indicates clearly that the H1 phase of C12G4 exists over a wide temperature range. We have studied the thermal stability of the H1 phase by measuring SAXS spectra over a wide temperature range. Figure 2(c) shows the d-spacings of 70% C12G4 at different temperatures (25, 35, 45, 55, 65, and 75 °C). Three SAXS peaks in the ratio of 1:1/√3:√2 are observed at all temperatures studied (figure S2). With increasing temperature, the SAXS pattern of the H1 phase shifts towards higher-q region, indicating a decreasing d-spacing, shown in figure 2(c). A monotonous decrease in d-spacing with temperature indicates that the H1 phase does not undergo phase separation over the temperature range studied. Moreover, phase
separation was not observed during careful visual observations with viewing through crossed polarizers.
3.2. Structure of C12G4 micelles in aqueous system
In order to investigate the morphology of C12G4 micelles in the dilute region, a series of SAXS measurements was carried out and the SAXS data were evaluated by the GIFT method. First, we will discuss the form of micelles in the 5% C12G4 system at 25 °C. Figure 3 shows SAXS data as a typical example. We can see that I(q) decays with q, giving a clear indication of the formation of micellar aggregates. In the absence of micellar aggregate structure, I(q) is independent of q due to the lack of electron density fluctuations. The I(q) curve follows approximately q0 behaviour in the low-q region with a local minimum at q ∼ 0.82 nm-1 (figure 3(a)), indicating spherical micelles with a core–shell type structure [59– 61]. Micelles seem to interact only weakly with neighbouring micelles, as indicated by the lack of any SAXS peaks in the lower-q region. As mentioned earlier, GIFT enables calculation of the structure factor and form factor simultaneously without the need to consider any form factor model. However, in order to reduce the influence of interparticle interference scattering on the estimation of p(r), an interaction potential model for S(q) is required for which we chose an averaged structure factor model of HS and PY closure relation to solve the OZ equation [51, 52, 62]. The S(q) curve presented in figure 3(a) indicates relatively weak intermicellar interactions (small peak in S(q) curve at q ∼ 0.534 nm–1), although the 5
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Figure 3. (a) Normalized x-ray scattering intensity I(q) of 5% C12G4/water system on absolute scale (symbol) and calculated static structure factor S(q), considering an average structure model of hard-sphere interaction potential, and (b) the corresponding pair-distance distribution functions p(r). Solid (green) and broken lines (red) in panel (a) represent the GIFT fit and calculated form factor for n particles existing in unit volume nP(q), respectively. The downward arrow in the low- and higher-r side of the p(r) curve in the panel (b) indicates the radius of micellar core Rc and maximum dimension of micelle Dmax, respectively. The cartoon in panel (b) displays a cross-sectional view of the core– shell structure of spherical micelles.
interactions strengthen with the volume fraction of surfactant. We will discuss this feature in the following section. The real-space p(r) function presented in figure 3(b) gives microstructural information with respect to the shape, size, and inhomogeneity of C12G4 micelles. We have already mentioned that the shape of the p(r) curve resembles the shape for micelles and the value of r at which p(r) reaches zero at the higher-r side can be used as an estimate of the maximum dimension Dmax [61]. The shape of the p(r) curve is nearly symmetrical, with a local maximum (at r ∼ 1.17 nm) and minimum (at r ∼ 2.88 nm) in the lower-r side, which confirms the spherical shape of micelles with a core–shell (inhomogeneous) structure with maximum diameter ca. 8.7 nm. Note that the local maximum and minimum in the lower-r side of the p(r) curve result from the respective negative and positive electron densities from the hydrophobic core and hydrophilic shell.
shape and size of the micelles remain constant, as indicated by the similar shape of p(r) curves and the fixed position of Dmax (at 8.7 nm). Figure 4(c) shows the normalized p(r) functions by weight fractions of C12G4, p(r)/Ws, which further confirm that the micelles do not grow (in case of growth, the p (r)/Ws would increase with Ws), with increasing concentration at least until 20% of C12G4. Nevertheless, we cannot avoid the possibility of formation of elongated rodlike micelles at higher concentrations below the phase boundary line. Since the SAXS data analysis for the micellar solution at higher volume fractions of surfactant suffers from a complicated structure factor effect and it is difficult to retrieve reliable structural information, we did not evaluate the micellar structure at higher concentrations before the phase boundary line. Note that the inflection point seen at r ∼ 1.6 nm (indicated by an arrow), which is a semi-quantitative measure of the hydrophobic core radius Rc, also remains unchanged with increasing C12G4 concentration, indicating that concentration does not modulate the internal structures of micelles. Thus, p(r) function provides quantitative information on the internal structure of the micelles without any theoretical modelling. The S(q) curves presented in figure 4(d) show that intermicellar interactions intensify with increasing C12G4 concentration and these are responsible for the growing SAXS peaks in the low-q of the I(q) curves in figure 4(a). The S(q) curves reveal that the structure factor peak, which measures the mean distance between neighbouring micelles, shifts towards higher-q with C12G4 concentration (as indicated by arrows), indicating that an increase in C12G4 concentration decreases the mean distance between neighboring micelles. In figure 4(d) extrapolation of S(q) to zero-q, S(q → 0), indicates the osmotic compressibility of the system and, for HS with monodisperse size distribution, S(q → 0) is determined by the packing fraction of HS [62]. We note that this could be a good approximation for nonionic micelles. A monotonous decrease of S(q → 0) with C12G4 concentration demonstrates the reduced osmotic compressibility of the system. We calculated
3.3. Effect of C12G4 concentration on micellar structure
The effect of C12G4 concentration on micellar structure was investigated by performing SAXS measurements on 5–20% C12G4 systems at 25 °C. Figure 4 shows the dependencies of I (q), p(r), S(q), and the radial electron density profiles Δρ(r) on C14G2 concentration. The scattering intensity increases over the entire q-range with increasing C12G4 concentration from 5–10%, which is expected based on an increase in the number density of micelles in the unit-scattering volume. A small interaction peak that appears in the I(q) curve for 10% C12G4 increases in intensity and shifts towards the higher-q regime, with further increases in C12G4 concentration followed by a decrease in the I(q = 0) value. However, the position of the local minimum persists unchanged at q ∼ 0.82 nm-1. From these observations, it can be concluded that increases in C12G4 concentration cause an increase in the strength of intermicellar interactions, keeping the microstructure of micelles unchanged [59, 61]. The micellar structure can be estimated from the real-space p(r) functions (figure 4(b)), which show that despite the increase in C12G4 concentration, 6
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Figure 4. Effect of surfactant concentration on the micellar structure: (a) Normalized x-ray scattering intensities I(q), (b) pair-distance
distribution function p(r), (c) normalized p(r) by the weight fraction of the surfactant Ws, (d) effective static structure factor S(q) curves, (e) comparison of the S(q → 0) values as obtained by SAXS experiments (filled circles) and those calculated from an extended PY theory (solid curve) for a dispersion of the hard-sphere, and (f) radial electron density profile obtained by the deconvolution of p(r) using DECON program for the 5–20% C12G4/water system at 25 °C. The solid and broken lines in panel (a) represent the GIFT fit and the calculated form factor for n particles existing in unit volume nP(q), respectively. Rc, Dmax, and Rmax in panels (b) and (d) represent core radius, maximum dimension, and maximum radius of micelle, respectively.
values of S(q → 0) using the semiempirical extension of the PY theory derived by Carnahan and Starling [63], which provides an accurate analytical expression for the osmotic pressure of a suspension of monodisperse HS as S (q → 0 ) =
( 1 − φHS )4 1+4φHS + 4φHS 2 + 4φHS 3 + 4φHS 4
microemulsion droplets formed by nonionic surfactants. Using the density of solvent, micellar solutions, and C12G4, we have converted the weight fraction of the surfactant Ws into the surfactant volume fraction ϕs and assuming that the surfactant volume fraction ϕs is virtually identical to the micellar volume fraction ϕmic, we have calculated the concentration dependence of S(q → 0). Figure 4(e) compares S(q → 0) from the results of GIFT and as obtained from extended PY theory. S(q → 0) for the C12G4 micelles follows a similar trend as predicted for an HS system, implying that the osmotic compressibility of the C12G4 micellar system can
(9)
where ϕHS is the volume fraction of HS. This equation has been successfully applied, for example, to aqueous micelles, nonaqueous inverse micelles, and oil-in-water or water-in-oil 7
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be explained in terms of the excluded volume of C12G4 micelles. The structure factor parameters, effective volume fraction (Φeff), polydispersity (σ), and interaction radius (r) obtained from the GIFT evaluation of the SAXS data for the C12G4/ water system at different concentrations 0.05 ⩽ Ws ⩾ 0.2 are supplied in table 1 of the supporting information (available at stacks.iop.org/NANO/26/204002/mmedia). The polydispersity of micelles lies within 5% at all concentrations, demonstrating that C12G4-based spherical micelles are rather monodisperse. Note that calculated Φeff from GIFT is higher than Ws due to extensive hydration of the glyceryl units of the hydrophilic moiety. The radial electron density profile Δρ(r) obtained from deconvolution of the p(r) function is shown in figure 4(f). The negative electron density in the low-r side of the Δρ(r) profile indicates that the micellar core consists of relatively electrondeficient hydrocarbon chains [59]. The Δρ(r) increases as r becomes positive, and eventually reaches zero at the higher-r side. It should be noted that value of r at which Δρ(r) reaches zero in the lower-r side provides a quantitative estimation of micellar core radius Rc. From the radial electron density profile, Rc is ca. 1.59 nm, which is close to the semiquantitative estimation made from the total p(r) function, and the Rc remains unchanged at all concentrations. We note that the experimental Rc calculated from the SAXS data (from the Δρ(r) curve) is slightly smaller than the extended hydrocarbon chain of the C12G4 surfactant, which is ca. 1.66 nm when obtained using Tanford’s equation. These observations reveal that the hydrocarbon chain of the surfactant is not in fully extended form at the interior of micelles. Based on the radial electron density profile, the maximum radius of micelle Rmax is ca. 4.23 nm, which gives a micellar shell thickness of 2.65 nm. This shell thickness is much larger than the extended length of the hydrophilic moiety of the surfactant. The length of one molecule of glycerol is ∼0.4–0.5 nm, which implies that the total length of the hydrophilic moiety of the C12G4 surfactant should be ∼1.60–2.0 nm. Therefore, the experimentally obtained shell thickness (2.65 nm) should be the joint symptoms of hydration of the micelles, the chain distribution of the polyglycerol unit of the surfactant, and position distribution of surfactant molecules normal to the interface. Surfactant molecules are not always fixed at the interface. Sometimes, some molecules pop up from the interface, which also contribute to larger diffused shell thickness. It is well known that macroscopic flow properties of surfactant solutions depend on the microstructure (shape and size of micelles), volume fraction of surfactant, and number density of micelles. In general, viscosity of spherical micelles (just above CMC) is comparable to that of pure solvent (water). An increase in viscosity of micellar solution with the volume fraction of surfactant could either be due to microstructure transformation into rods or to strong intermicellar interactions. Viscosity of rodlike micelles and their network structure (wormlike micelles) is reported to be much higher than the pure solvent and/or spherical micelles [40, 64]. Figure 5(a) shows the measured viscosity η of aqueous binary
mixtures of C12G4 as a function of shear rate at 25 °C at different surfactant weight fractions (0.03–0.4), while figure 5(b) shows the relative zero-shear viscosity (ηR = η0 (micelle)/η0(water), where η0(micelle) and η0(water) represent zeroshear viscosity of micellar solution and pure water, respectively). Steady-shear rheology data confirmed that C12G4 micellar solutions display Newtonian fluidlike behaviour, which can be judged from the shear-independent viscosity. In systems containing long cylindrical micelles with a network structure, a shear-thinning behaviour is observed where viscosity decays after a critical shear rate. The experimental relative viscosity ηR remains unaffected by Ws at lower concentrations below Ws = 0.2. However, the ηR increases steadily, followed by a sharp increase above Ws = 0.3 and the ηR curve deviates significantly from that predicted for the HS as proposed by the Krieger–Dougherty (K–D) equation [65] ⎛ φHS ⎞−[η] φ ⁎ ηR = ⎜ 1 − ⎟ φ⁎ ⎠ ⎝
(10)
where [η] is a fitting parameter based on the shape of particles, often called intrinsic viscosity, and taken equal to 2.5 for the spherical shape of particles, ϕHS is the volume fraction of HS, and ϕ* is the volume fraction of maximum packing (for spherical particles, ϕ* = 0.64). The K-D equation was developed for noninteracting HS dispersions and can be applied to polymer-stabilized colloidal dispersions, provided the thickness of the stabilizing layer is considered to determine the volume fraction of colloidal dispersions [66]. Here we assume that the ηR of C12G4 aqueous micelles can be described by the K-D model until Ws = 0.2, which is in agreement with the SAXS data, namely that the shape and size of C12G4 micelles remain essentially the same over a wide concentration range up to Ws = 0.2. A slight deviation of the ηR until Ws = 0.3 may account for the increased interactions among C12G4 micelles [67]. However, a significant deviation of experimental ηR from the K-D model at Ws = 0.4 cannot be solely explained in terms of strong micellar interactions. Due to the significant deviation from the K-D model, in combination with the fact that the micellar phase transforms into a hexagonal liquid-crystal phase at higher concentrations above Ws = 0.54 (figure 1), we anticipate that elongated rodlike micelles may present just below the phase boundary. 3.4. Effect of temperature on micellar structure
To investigate the thermoresponsive behaviour of C12G4 micelles, SAXS measurements were carried out at a fixed composition (10% C12G4) over a wide temperature range (25–85 °C). Figure 6 shows these SAXS results. Generally, an increase in temperature favours a sphere-to-rod type transition in the micellar structure of conventional EO-based nonionic surfactants in an aqueous system due to dehydration of the surfactant’s hydrophilic headgroup [68]. Dehydration caused by heating reduces the effective cross-sectional area of the surfactant’s headgroup and consequently increases the critical packing parameter (equation (1)) favouring micellar growth in 8
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Figure 5. Rheological properties of C12G4 micellar solutions: (a) viscosity versus shear rate curves for different weight fractions of C12G4 at 25 °C, and (b) the corresponding relative zero-shear viscosity ηR (experimental) and calculated using the Krieger–Dougherty equation for noninteracting hard-spheres.
Figure 6. Effect of temperature on the micellar structure: (a) normalized x-ray scattering intensities I(q) on absolute scales, and (b) pair-
distance distribution function p(r) for the 10% C12G4/water system at different temperatures (25, 50, 75, and 85 °C). The solid and broken lines in panel (a) represent the GIFT fit and the calculated form factor for n particles existing in unit volume nP(q), respectively. Rc, and Dmax in panel (b) represent micellar core radius and maximum dimension of micelle, respectively.
4. Conclusions
aqueous systems. However, the opposite trend is generally observed in reverse micelles in nonaqueous media [40] due to interpenetration of solvent towards the micellar interface. The I(q) curves presented in figure 6(a) show that the effect of temperature on the C12G4 aqueous micelles is not simple. The scattering intensity decreases with increasing temperature, with the position of the local minimum in the low-q region maintained at q ∼ 0.82 nm-1. Nevertheless, the maximum dimension of micelles estimated from the realspace p(r) curves (figure 6(b)) remains at 8.7 nm despite the increase in temperature from 25 to 85 °C. Thus, the decreasing scattering intensity with increasing temperature can be attributed to the different contrast of the system. In general, increases in temperature would lower the contrast (electron density difference of solvent and aggregates) of amphiphilic core–shell systems, which might lead to a critical underestimation of micellar size. Closer observation of the p (r) curve, particularly in the higher-r side, reveals that its shape at higher temperature (85 °C) is not perfectly symmetrical, indicating the possibility of rodlike micelles at temperatures higher than 85 °C.
An aqueous binary phase diagram of glycerol-based nonionic surfactant tetraglycerol lauryl ether (C12G4) was constructed over a wide range of temperatures and compositions, and structural characterization of the self-assembled mesostructures was performed by using the SAXS technique. SAXS data were evaluated using a GIFT method to obtain microstructural information (shape, size, and internal structure) of the C12G4 micelle depending on composition and temperature. Due to its strong amphiphilicity, C12G4 self-assembled into normal micelles (Wm), a hexagonal liquid crystal (H1), and inverse micelles (Om) depending on the composition. The lyotropic H1 phase extended over a wide range of concentration (54–95%) and temperature (beyond 100 °C). In contrast to the conventional nonionic counterparts, C12G4 did not exhibit LCST behaviour, demonstrating the higher thermal stability of C12G4 micelles due to strong hydration effects. Bulky hydrophilic groups result in the formation of spherical core–shell type micelles in the dilute region, whose shape, size, and structures did not vary with increases in concentration up to surfactant weight fraction Ws = 0.2. In the 9
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lower concentration region (Ws < 0.2), the relative viscosity ηR remains unaffected by Ws. However, at higher concentrations (Ws ⩾ 0.3), the ηR increases and deviates significantly from that predicted for the HS as proposed by the K-D equation, demonstrating the presence of interacted micelles. Intermicellar interactions expressed in terms of structure factor obtained by the GIFT method confirmed that micelles interact particularly at higher C12G4 concentration, and this is responsible for the reduced osmotic compressibility of the system. This feature was explained in terms of the excluded volume of C12G4 micelles as proposed by the Carnahan and Starling model for manodisperse HS.
[12] Takahashi S, Suzuki K, Okano M, Imada M, Nakamori T, Ota Y, Ishizaki K and Noda S 2009 Direct creation of threedimensional photonic crystals by a top-down approach Nat. Mater. 8 721–5 [13] Wu S, Tsuruoka T, Terabe K, Hasegawa T, Hill J P, Ariga K and Aono M 2011 A polymer-electrolyte-based atomic switch Adv. Mater. 21 93–9 [14] Hasegawa T, Terabe K, Tsuruoka T and Aono M 2012 Atomic switch: atom/ion movement controlled devices for beyong von-neumann computers Adv. Mater. 24 252–67 [15] Whitesides G M and Grzybowski B 2002 Self-assembly at all scales Science 295 2418–21 [16] Pileni M P 2003 The role of soft colloidal templates in controlling the size and shape of inorganic nanocrystals Nat. Mater. 2 145–50 [17] Kowalczyk B, Bishop K J M, Lagzi I, Wang D, Wei Y, Han S and Grzybowski B A 2012 Charged nanoparticles as supramolecular surfactants for controlling the growth and stability of microcrystals Nat. Mater. 11 227–32 [18] Shrestha L K, Yamauchi Y, Hill J P, Miyazawa K and Ariga K 2013 Fullerene crystals with bimodal pore architectures consisting of macropores and mesopores J. Am. Chem. Soc. 135 586–9 [19] Rubenstein M, Cornejo A and Nagpal R 2014 Programmable self-assembly in a thousand-robot swarm Science 345 795–9 [20] Richards G J, Hill J P, Labuta J, Wakayama Y, Akada M and Ariga K 2011 Self-assembled pyrazinacene nanotubes Phys. Chem. Chem. Phys. 13 4868–76 [21] Wang L and Sasaki T 2014 Titanium oxide nanosheets: graphene analogues with versatile functionalities Chem. Rev. 114 9455–86 [22] This terminology was first proposed by Dr Masakazu Aono at 1st Int. Symp. on Nanoarchitectonics Using Suprainteractions (NASI-1) at Tsukuba in 2000 [23] Weiss P S 2007 A conversation with Dr Masakazu Aono: leader in atomic-scale control and nanomanipulation ACS Nano 1 379–83 [24] Aono M, Bando Y and Ariga K 2012 Nanoarchitectonics: pioneering a new paradigm for nanotechnology in materials development Adv. Mater. 24 150–1 [25] Ariga K, Ji Q, Hill J P, Bando Y and Aono M 2012 Forming nanomaterials as layered functional structures toward materials nanoarchitectonics NPG Asia Mater. 4 e17 [26] Ariga K, Ji Q, Mori T, Naito M, Yamauchi Y, Abe H and Hill J P 2013 Enzyme nanoarchitectonics: organization and device application Chem. Soc. Rev. 42 6322–45 [27] Hill J P, Xie Y, Akada M, Wakayama Y, Shrestha L K, Ji Q and Ariga K 2013 Controlling porphyrin nanoarchitectonics as solid interfaces Langmuir 29 7291–9 [28] Shrestha L K, Ji Q, Mori T, Miyazawa K, Yamauchi Y, Hill J P and Ariga K 2013 Fullerene nanoarchitectonics: from zero to higher dimensions Chem. Asian J. 8 1662–79 [29] Ariga K, Yamauchi Y, Rydzek G, Ji Q, Yonamine Y, Wu K C-W and Hill J P 2014 Layer-by-layer nanoarchitectonics: invention, innovation, and evolution Chem. Lett. 43 36–68 [30] Rajendran R, Shrestha L K, Minami K, Subramanian M, Jayavel R and Ariga K 2014 Dimensionally integrated nanoarchitectonics for a novel composite from 0D, 1D, and 2D nanomaterials: RGO/CNT/CeO2 ternary nanocomposites with electrochemical performance J. Mater. Chem. A 2 18480–7 [31] Ramanathan M, Shrestha L K, Mori T, Ji Q, Hill J P and Ariga K 2013 Amphiphile nanoarchitectonics: from basic physical chemistry to advanced applications Phys. Chem. Chem. Phys. 15 10580–611 [32] Shrestha R G, Shrestha L K, Sharma S C and Aramaki K 2008 Phase behavior and microstructures of nonionic fluorocarbon surfactant in aqueous systems J. Phys. Chem. B 112 10520–7
Acknowledgments LKS thanks the Japan Society for the Promotion of Science (JSPS) for Grant-in-Aid for Young Scientists B (25790021).
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Nonionic amphiphile nanoarchitectonics: self-assembly into micelles and lyotropic liquid crystals
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Nanotechnology Nanotechnology 26 (2015) 204002 (11pp)
doi:10.1088/0957-4484/26/20/204002
Nonionic amphiphile nanoarchitectonics: self-assembly into micelles and lyotropic liquid crystals Lok Kumar Shrestha1, Karolina Maria Strzelczyk1,2, Rekha Goswami Shrestha3, Kotoko Ichikawa4, Kenji Aramaki4, Jonathan P Hill1 and Katsuhiko Ariga1 1
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan 2 Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141 St, 02507 Warszawa, Poland 3 Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 4 Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Yokohama 240-8501, Japan E-mail: [email protected] and [email protected] Received 19 November 2014, revised 6 March 2015 Accepted for publication 23 March 2015 Published 27 April 2015 Abstract
Amphiphiles, molecules that possess both hydrophilic and hydrophobic moieties, are architecturally simple molecules that can spontaneously self-assemble into complex hierarchical structures from lower to higher dimensions either in the bulk phase or at an interface. Recent developments in multifunctional nanostructure design using the advanced concept of nanoarchitectonics utilize this simple process of assembly. Amphiphilic self-assemblies involving lipids or proteins mimic the structure of biological systems, thus highlighting the necessity of a fundamental physical understanding of amphiphilic self-assembly towards a realization of the complex mechanisms operating in nature. Herein, we describe self-assembled microstructures of biocompatible and biodegradable tetraglycerol lauryl ether (C12G4) nonionic surfactant in an aqueous solvent system. Temperature-composition analyses of equilibrium phases identified by using small-angle x-ray scattering (SAXS) provide strong evidence of various spontaneously selfassembled mesostructures, such as normal micelles (Wm), hexagonal liquid crystal (H1), and reverse micelles (Om). In contrast to conventional poly(oxyethylene) nonionic surfactants, C12G4 did not exhibit the clouding phenomenon at higher temperatures (phase separation was not observed up to 100 °C), demonstrating the greater thermal stability of the self-assembled mesophases. Generalized indirect Fourier transformation (GIFT) evaluation of the SAXS data confirmed the formation of core–shell–type spherical micelles with a maximum dimension ca. 8.7 nm. The shape and size of the C12G4 micelles remained apparently unchanged over a wide range of concentrations (up to 20%), but intermicellar interactions increased and could be described by the Percus–Yevick (PY) theory (after Carnahan and Starling), which provides a very accurate analytical expression for the osmotic pressure of a monodisperse hard sphere. S Online supplementary data available from stacks.iop.org/NANO/26/204002/mmedia Keywords: amphiphiles, nanoarchitectonics, micelle, liquid crystal, SAXS, rheology (Some figures may appear in colour only in the online journal)
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1. Introduction
and pharmaceutical formulations, and more recently in functional materials design [31, 35]. It has been found that the intrinsic geometry of an individual amphiphile (or surfactant) has a strong influence on the shape of the micelles. The different shapes of micellar aggregates can be characterized by the critical packing parameter (cpp) defined as [36],
Recent developments in materials science (both soft and hard) and nanotechnology have enabled the preparation of various functional nanomaterials with advanced properties and functions, which are suitable for integration into nanosystems for the design of solar cells, energy storage devices, and optoelectronics or spintronics devices [1–11]. In most cases, sophisticated fabrication techniques based on top-down approaches are used for the preparation of nanosystems, using an object of macroscopic dimesnions as the starting point [12–14]. However, this approach suffers from a fundamental limitation due to the minimum possible structural dimensions available by lithographic techniques. On the other hand, using an alternative bottom-up, spontaneous self-assembly process, nanomaterials with different length scales and controlled morphologies, such as nanosheets and nanotubes, have been fabricated [15–21]. However, their individual functions are limited when compared with the huge potential available from integrated systems. To address these fundamental and technological challenges, the novel and unique concept of ‘materials nanoarchitectonics’ has been developed [22–24], which allows synthesis of nanomaterials using self-assembly of the required building blocks into hierarchic structures with atomic/molecular level precision and organized/reorganized structures in a rational way for construction of advanced functional nanomaterials with ‘smart’ functions and properties [25–30]. Self-assembly is the organization of atoms, molecules, or functional building blocks into well-defined nanostructures in the absence of any external forces [15]. This process is common in nature and has also been used in advanced technologies. Self-assembly is one of the few practical strategies for the creation of ensembles of nanostructures over different length scales and has become one of the essential aspects of materials nanoarchitectonics. Amphiphilic molecules such as surfactants or block copolymers undergo spontaneous selfassembly into a variety of mesophases, both in aqueous and nonaqueous systems [31–34]. The self-assembled, complex, hierarchical geometries can be flexibly controlled or tuned by surfactant or solvent molecular architectures. Surfactants often exhibit remarkable phase behaviour in water, forming various phases (micelles, liquid crystals, vesicles, and reverse micelles), depending on surfactant composition and temperature [35]. One of the most intriguing features of surfactants in solution is that the physicochemical properties of those solutions undergo abrupt changes over a very narrow concentration range. The concentration at which the physicochemical properties change occurs is referred to as the critical micelle concentration (CMC). Just above the CMC, hydrophilic surfactants with a bigger headgroup generally assemble into spherical micelles. However, the spherical morphology can be modulated to rods or disks and also to ordered liquid crystalline phases by varying certain control parameters, including concentration, temperature, the addition of cosurfactants, controlling salinity, and pH. Self-assembled structures of amphiphiles have been extensively used in different applications including as food additives, in cosmetic
cpp =
v a 0 lc
(1)
where a0 is the effective cross-sectional area of the hydrophilic head group and v and lc are the volume and critical chain length of the hydrophobic chain, respectively. The cpp values for spherical, cylindrical, and lamellar particles are ∼1/3, 1/3 < cpp < 1/2, and 1/2 < cpp < 1, respectively. In this paper, we present self-assembled microstructures of biofriendly tetraglycerol lauryl ether (C12G4) nonionic surfactant in an aqueous system. The binary phase diagram of the C12G4/water system was constructed by visual observation through a crossed polarizer, and equilibrium phases were identified by using small-angle x-ray scattering (SAXS) technique. Shape, size, and internal structure of C12G4 micelles were determined by the generalized indirect Fourier transformation (GIFT) [37–40] method, and the results are also complemented by rheological measurements. In the dilute regime, C12G4 spontaneously self-assembles into core– shell–type spherical micelles and, at higher surfactant concentrations, into a lyotropic hexagonal liquid crystal phase that resists phase separation and possesses good thermal stability.
2. Experimental 2.1. Materials
Tetraglycerol lauryl ether (abbreviated as C12G4) nonionic surfactant was purchased from Taiyo Kagaku Co. Japan and used as received. Milli-Q filtered water was used to prepare samples for phase, SAXS, and rheology studies. 2.2. Methods 2.2.1. Sample preparation. C12G4 and water were weighed
out into clean and dry glass tubes with screw cap (10 mL) in the preparation of C12G4/H2O binary mixtures with compositions in the range from 2–100 wt%. Samples were immediately sealed and mixed using a dry thermo bath, vortex mixer, and repeated centrifugation until homogeneous. Phase behaviour was then studied by visual observation over a wide temperature range (0–100 °C) in a temperaturecontrolled water bath. Optical properties of samples (birefringent and nonbirefringent) were identified by viewing samples through a crossed polarizer. Samples were kept in equilibrium for 1 h at each temperature before observing the phases. The structures of liquid crystalline phases and their textures were identified by using SAXS and polarizing optical microscopy, respectively. For the SAXS 2
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studies of the micellar structure, samples were prepared separately and, after their thorough mixing, stored in a water bath at 25 °C for 24 h prior to the SAXS measurements. The same samples were used for rheological measurements.
where p(r) is the pair-distance distribution function giving structural information regarding the scattering particles in real space. The p(r) function directly represents a histogram of distances within the particle for homogeneous scattering particles [44]. Generally, the intraparticle scattering contributions are related to the form factor P(q) and theoretically correspond to the Fourier transformation of the p(r). Whereas the interparticle interference scattering is connected to the structure factor S(q), corresponding to the Fourier transformation of the total correlation function h(r) = g(r)–1, as
2.2.2. Optical microscopy. Optical micrographs of liquid
crystals were recorded using an Olympus BH-2 microscope equipped with an integrated charge-coupled device (CCD) camera. Images were taken in transmission, and reflectance modes from samples deposited on glass slides and sandwiched with a thin glass plate.
S (q ) = 1 + 4 πn 2.2.3. Small-angle x-ray scattering (SAXS). The SAXS
4π sin(θ /2) λ
∫0
∞
p (r )
sin qr dr qr
[g (r ) − 1 ] r 2
I (q) = nP (q) S (q)
sin qr dr qr
(4)
(5)
It is known that interparticle interactions are very weak for dilute systems (particularly at volume fraction 1%), S(q) ≠ 1 due to considerable particle interactions. Therefore, we have analyzed the experimental scattering data using the GIFT method [37–40], which potentially can be used to determine both the form factor and the structure factor without any model assumption for the form factor. Nevertheless, appropriate assumptions for the interparticle interaction potentials and the closure relation for the structure factor are required. In the calculation, we used an averaged structure factor model of hard-sphere (HS) interactions, S(q)av [49, 50], which considers the Gaussian distribution of the interaction radius σ for individual monodisperse systems for polydispersity μ, and the Percus–Yevick (PY) closure relation to solve the Ornstein–Zernike (OZ) equation [51, 52].
(2)
Measured SAXS intensities were calibrated for transmission by normalizing a zero-q attenuated primary beam intensity to unity, and the I(q)s were corrected for background scattering (capillary and water), and an absolute scale calibration was constructed using water as a secondary standard solvent [41]. With alignment during SAXS measurement, this system allows maximum resolution of qmin ∼ 0.08 nm–1, which corresponds to ∼40 nm as the detectable dimension for scattering objects. The scattering intensity I(q) is the complex square of the scattering amplitude F(q), which represents the Fourier transform of the scattering length density difference Δρ(r), and thus describes the structure of the particle in real space [42, 43]. In scattering experiments of micellar solutions, we measured the spatial average of these functions, which is defined by the following expression: I (q ) = 4 π
∞
where n is the particle number density of scatterers and g(r) is their pair-correlation function [45]. For an ideal monodisperse spheroid system, the total scattering intensity I(q) can be expressed as
technique was used to identify the liquid crystalline phase and micellar structure. To study the effect of surfactant concentration on the shape, size, and structure of micelles, SAXS measurements were carried over a wide concentration range 5–20% at 25 °C and, to observe the effect of temperature on the micellar structure, measurements were carried out on the 10% sample in the temperature range of 25–85 °C. SAXS measurements were carried out using a SAXSess camera (Anton Paar, Austria) attached to a PW3830 sealed-tube anode x-ray generator (PANalytical, Netherlands) operating at 40 kV and 50 mA. A monochromatic x-ray beam of Cu–Kα radiation (λ = 0.1542 nm) with a well-defined, focused line-shape was obtained using a Göbel mirror fitted with a block collimator. The thermostated sample holder unit (TCS 120, Anton Paar) supplied with the SAXSess system offers an accuracy of temperature ±0.1 °C. Two-dimensional (2D) scattering patterns were first recorded on an image plate (IP) detector (Cyclone, Perkin Elmer, USA) and with subsequent integration into one-dimensional (1D) scattering intensities I(q) as a function of the magnitude of the scattering vector q using the SAXSQuant software (Anton Paar). The scattering vector q is related to the total scattering angle θ as q=
∫0
(3)
3
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In the case of cylindrical micelles, the internal crosssectional structure (cross-section diameter of the cylinder) can be determined by the deconvolution of SAXS data, if Δρc(r) is only a function of the radial position within the cross-section. For high-aspect-ratio cylindrical micelles (axial length at least three times longer than the crosssectional diameter), radial electron density profile Δρc(r), which is related to the pair-distance distribution function (PDDF) of cross-section pc(r) as described in equation (7), can be determined. pc (r ) = rΔρ˜c 2 (r )
(7)
The cross-sectional pc(r) can be further calculated from the experimental I(q) via I (q) q = πLIc (q) = 2π 2L
∫0
Figure 1. Phase behaviour of the C12G4/water system over a wide range of temperatures and compositions. Wm, H1, S, and Om, respectively, denote the normal micellar phase, hexagonal liquidcrystal phase, solid phase, and inverse micellar phase. Cartoons of normal spherical micelles, hexagonal liquid-crystal phase, and an optical micrograph of the 70% C12G4 system are also shown in the phase diagram.
∞
pc (r ) J0 (qr )dr .
(8)
where J0(qr) is the zeroth-order Bessel function. The IFT of equation (8) yields pc(r), which is then used to calculate Δρc(r) by the deconvolution technique [46, 47]. 2.2.4. Rheometry. Flow properties of C12G4 micellar
moiety [53–56], C12G4 micelles did not exhibit phase separation upon heating (up to ∼100 °C), i.e., C12G4 surfactant does not exhibit lower critical solution temperature (LCST) behaviour. This could be due to the fact that glycerolbased nonionic surfactants display a stronger hydration effect (hydrogen bonding of water to the glycerol moiety of the surfactant) compared to the EO-based surfactant, thereby increasing its thermal stability. At lower temperatures below 10 °C, C12G4 appears as a solidlike phase. The C12G4 surfactant, which is a fluid at 25 °C, self-assembles into inverse micelles (Om, with an inverted, watery, cored structure, compared to normal, oily, cored micelles in the dilute region) upon addition of a small amount of water. The Om phase swells with water, followed by an Om–lyotropic liquid-crystal phase transition above 5% water (that is, the liquid-crystal phase appears for C14G2 content 40%) with the Wm–H1 phase transition eventually occurring at 54%. Optically isotropic nonbirefriengent viscous liquid formed just below the H1 phase was anticipated to be due to a micellar cubic phase (I1), but its broad SAXS pattern (data not shown) confirmed that the viscous liquid is a micellar phase. With increasing temperature, viscosity gradually decreased, leading eventually to a less viscous, easily flowing solution. Note that contrary to the conventional ethylene oxide (EO)-based nonionic surfactants, which usually undergo phase separation (clouding) upon heating due to dehydration of hydrophilic EO 4
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Figure 2. (a) SAXS patterns of the C12G4/water system recorded at 25 °C at different surfactant weight fractions, WS as typical examples, (b) variation of calculated d-spacing at different concentrations at 25 °C, and (c) variation of d-spacing in the 70% C12G4/water system at different temperatures.
spacings for 70% C12G4 at different temperatures are shown in figure 2(c). The SAXS patterns presented in figure 2(a) clearly show that the H1 phase in surfactant-rich regions incorporates a significant amount of water. The phase boundary was determined by plotting d-spacing as a function of 1/Ws. As can be seen in figure 2(b), d100 increases almost linearly, with the weight fraction of water indicating that the H1 phase swells without variation of the molecular arrangement of cylindrical micelles in their hexagonal packing. The d100 increases from ca. 4.419 to 5.236 nm when Ws decreases from 0.90 to 0.59 and then does not change, demonstrating that the H1 phase becomes saturated with water at Ws = 0.59 and upon further addition of water, the excess water separates out from the H1 phase. Therefore, a constant d is expected in the two-phase region. SAXS data indicate that the H1 phase incorporates around 35% water at 25 °C. The phase diagram indicates clearly that the H1 phase of C12G4 exists over a wide temperature range. We have studied the thermal stability of the H1 phase by measuring SAXS spectra over a wide temperature range. Figure 2(c) shows the d-spacings of 70% C12G4 at different temperatures (25, 35, 45, 55, 65, and 75 °C). Three SAXS peaks in the ratio of 1:1/√3:√2 are observed at all temperatures studied (figure S2). With increasing temperature, the SAXS pattern of the H1 phase shifts towards higher-q region, indicating a decreasing d-spacing, shown in figure 2(c). A monotonous decrease in d-spacing with temperature indicates that the H1 phase does not undergo phase separation over the temperature range studied. Moreover, phase
separation was not observed during careful visual observations with viewing through crossed polarizers.
3.2. Structure of C12G4 micelles in aqueous system
In order to investigate the morphology of C12G4 micelles in the dilute region, a series of SAXS measurements was carried out and the SAXS data were evaluated by the GIFT method. First, we will discuss the form of micelles in the 5% C12G4 system at 25 °C. Figure 3 shows SAXS data as a typical example. We can see that I(q) decays with q, giving a clear indication of the formation of micellar aggregates. In the absence of micellar aggregate structure, I(q) is independent of q due to the lack of electron density fluctuations. The I(q) curve follows approximately q0 behaviour in the low-q region with a local minimum at q ∼ 0.82 nm-1 (figure 3(a)), indicating spherical micelles with a core–shell type structure [59– 61]. Micelles seem to interact only weakly with neighbouring micelles, as indicated by the lack of any SAXS peaks in the lower-q region. As mentioned earlier, GIFT enables calculation of the structure factor and form factor simultaneously without the need to consider any form factor model. However, in order to reduce the influence of interparticle interference scattering on the estimation of p(r), an interaction potential model for S(q) is required for which we chose an averaged structure factor model of HS and PY closure relation to solve the OZ equation [51, 52, 62]. The S(q) curve presented in figure 3(a) indicates relatively weak intermicellar interactions (small peak in S(q) curve at q ∼ 0.534 nm–1), although the 5
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Figure 3. (a) Normalized x-ray scattering intensity I(q) of 5% C12G4/water system on absolute scale (symbol) and calculated static structure factor S(q), considering an average structure model of hard-sphere interaction potential, and (b) the corresponding pair-distance distribution functions p(r). Solid (green) and broken lines (red) in panel (a) represent the GIFT fit and calculated form factor for n particles existing in unit volume nP(q), respectively. The downward arrow in the low- and higher-r side of the p(r) curve in the panel (b) indicates the radius of micellar core Rc and maximum dimension of micelle Dmax, respectively. The cartoon in panel (b) displays a cross-sectional view of the core– shell structure of spherical micelles.
interactions strengthen with the volume fraction of surfactant. We will discuss this feature in the following section. The real-space p(r) function presented in figure 3(b) gives microstructural information with respect to the shape, size, and inhomogeneity of C12G4 micelles. We have already mentioned that the shape of the p(r) curve resembles the shape for micelles and the value of r at which p(r) reaches zero at the higher-r side can be used as an estimate of the maximum dimension Dmax [61]. The shape of the p(r) curve is nearly symmetrical, with a local maximum (at r ∼ 1.17 nm) and minimum (at r ∼ 2.88 nm) in the lower-r side, which confirms the spherical shape of micelles with a core–shell (inhomogeneous) structure with maximum diameter ca. 8.7 nm. Note that the local maximum and minimum in the lower-r side of the p(r) curve result from the respective negative and positive electron densities from the hydrophobic core and hydrophilic shell.
shape and size of the micelles remain constant, as indicated by the similar shape of p(r) curves and the fixed position of Dmax (at 8.7 nm). Figure 4(c) shows the normalized p(r) functions by weight fractions of C12G4, p(r)/Ws, which further confirm that the micelles do not grow (in case of growth, the p (r)/Ws would increase with Ws), with increasing concentration at least until 20% of C12G4. Nevertheless, we cannot avoid the possibility of formation of elongated rodlike micelles at higher concentrations below the phase boundary line. Since the SAXS data analysis for the micellar solution at higher volume fractions of surfactant suffers from a complicated structure factor effect and it is difficult to retrieve reliable structural information, we did not evaluate the micellar structure at higher concentrations before the phase boundary line. Note that the inflection point seen at r ∼ 1.6 nm (indicated by an arrow), which is a semi-quantitative measure of the hydrophobic core radius Rc, also remains unchanged with increasing C12G4 concentration, indicating that concentration does not modulate the internal structures of micelles. Thus, p(r) function provides quantitative information on the internal structure of the micelles without any theoretical modelling. The S(q) curves presented in figure 4(d) show that intermicellar interactions intensify with increasing C12G4 concentration and these are responsible for the growing SAXS peaks in the low-q of the I(q) curves in figure 4(a). The S(q) curves reveal that the structure factor peak, which measures the mean distance between neighbouring micelles, shifts towards higher-q with C12G4 concentration (as indicated by arrows), indicating that an increase in C12G4 concentration decreases the mean distance between neighboring micelles. In figure 4(d) extrapolation of S(q) to zero-q, S(q → 0), indicates the osmotic compressibility of the system and, for HS with monodisperse size distribution, S(q → 0) is determined by the packing fraction of HS [62]. We note that this could be a good approximation for nonionic micelles. A monotonous decrease of S(q → 0) with C12G4 concentration demonstrates the reduced osmotic compressibility of the system. We calculated
3.3. Effect of C12G4 concentration on micellar structure
The effect of C12G4 concentration on micellar structure was investigated by performing SAXS measurements on 5–20% C12G4 systems at 25 °C. Figure 4 shows the dependencies of I (q), p(r), S(q), and the radial electron density profiles Δρ(r) on C14G2 concentration. The scattering intensity increases over the entire q-range with increasing C12G4 concentration from 5–10%, which is expected based on an increase in the number density of micelles in the unit-scattering volume. A small interaction peak that appears in the I(q) curve for 10% C12G4 increases in intensity and shifts towards the higher-q regime, with further increases in C12G4 concentration followed by a decrease in the I(q = 0) value. However, the position of the local minimum persists unchanged at q ∼ 0.82 nm-1. From these observations, it can be concluded that increases in C12G4 concentration cause an increase in the strength of intermicellar interactions, keeping the microstructure of micelles unchanged [59, 61]. The micellar structure can be estimated from the real-space p(r) functions (figure 4(b)), which show that despite the increase in C12G4 concentration, 6
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Figure 4. Effect of surfactant concentration on the micellar structure: (a) Normalized x-ray scattering intensities I(q), (b) pair-distance
distribution function p(r), (c) normalized p(r) by the weight fraction of the surfactant Ws, (d) effective static structure factor S(q) curves, (e) comparison of the S(q → 0) values as obtained by SAXS experiments (filled circles) and those calculated from an extended PY theory (solid curve) for a dispersion of the hard-sphere, and (f) radial electron density profile obtained by the deconvolution of p(r) using DECON program for the 5–20% C12G4/water system at 25 °C. The solid and broken lines in panel (a) represent the GIFT fit and the calculated form factor for n particles existing in unit volume nP(q), respectively. Rc, Dmax, and Rmax in panels (b) and (d) represent core radius, maximum dimension, and maximum radius of micelle, respectively.
values of S(q → 0) using the semiempirical extension of the PY theory derived by Carnahan and Starling [63], which provides an accurate analytical expression for the osmotic pressure of a suspension of monodisperse HS as S (q → 0 ) =
( 1 − φHS )4 1+4φHS + 4φHS 2 + 4φHS 3 + 4φHS 4
microemulsion droplets formed by nonionic surfactants. Using the density of solvent, micellar solutions, and C12G4, we have converted the weight fraction of the surfactant Ws into the surfactant volume fraction ϕs and assuming that the surfactant volume fraction ϕs is virtually identical to the micellar volume fraction ϕmic, we have calculated the concentration dependence of S(q → 0). Figure 4(e) compares S(q → 0) from the results of GIFT and as obtained from extended PY theory. S(q → 0) for the C12G4 micelles follows a similar trend as predicted for an HS system, implying that the osmotic compressibility of the C12G4 micellar system can
(9)
where ϕHS is the volume fraction of HS. This equation has been successfully applied, for example, to aqueous micelles, nonaqueous inverse micelles, and oil-in-water or water-in-oil 7
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be explained in terms of the excluded volume of C12G4 micelles. The structure factor parameters, effective volume fraction (Φeff), polydispersity (σ), and interaction radius (r) obtained from the GIFT evaluation of the SAXS data for the C12G4/ water system at different concentrations 0.05 ⩽ Ws ⩾ 0.2 are supplied in table 1 of the supporting information (available at stacks.iop.org/NANO/26/204002/mmedia). The polydispersity of micelles lies within 5% at all concentrations, demonstrating that C12G4-based spherical micelles are rather monodisperse. Note that calculated Φeff from GIFT is higher than Ws due to extensive hydration of the glyceryl units of the hydrophilic moiety. The radial electron density profile Δρ(r) obtained from deconvolution of the p(r) function is shown in figure 4(f). The negative electron density in the low-r side of the Δρ(r) profile indicates that the micellar core consists of relatively electrondeficient hydrocarbon chains [59]. The Δρ(r) increases as r becomes positive, and eventually reaches zero at the higher-r side. It should be noted that value of r at which Δρ(r) reaches zero in the lower-r side provides a quantitative estimation of micellar core radius Rc. From the radial electron density profile, Rc is ca. 1.59 nm, which is close to the semiquantitative estimation made from the total p(r) function, and the Rc remains unchanged at all concentrations. We note that the experimental Rc calculated from the SAXS data (from the Δρ(r) curve) is slightly smaller than the extended hydrocarbon chain of the C12G4 surfactant, which is ca. 1.66 nm when obtained using Tanford’s equation. These observations reveal that the hydrocarbon chain of the surfactant is not in fully extended form at the interior of micelles. Based on the radial electron density profile, the maximum radius of micelle Rmax is ca. 4.23 nm, which gives a micellar shell thickness of 2.65 nm. This shell thickness is much larger than the extended length of the hydrophilic moiety of the surfactant. The length of one molecule of glycerol is ∼0.4–0.5 nm, which implies that the total length of the hydrophilic moiety of the C12G4 surfactant should be ∼1.60–2.0 nm. Therefore, the experimentally obtained shell thickness (2.65 nm) should be the joint symptoms of hydration of the micelles, the chain distribution of the polyglycerol unit of the surfactant, and position distribution of surfactant molecules normal to the interface. Surfactant molecules are not always fixed at the interface. Sometimes, some molecules pop up from the interface, which also contribute to larger diffused shell thickness. It is well known that macroscopic flow properties of surfactant solutions depend on the microstructure (shape and size of micelles), volume fraction of surfactant, and number density of micelles. In general, viscosity of spherical micelles (just above CMC) is comparable to that of pure solvent (water). An increase in viscosity of micellar solution with the volume fraction of surfactant could either be due to microstructure transformation into rods or to strong intermicellar interactions. Viscosity of rodlike micelles and their network structure (wormlike micelles) is reported to be much higher than the pure solvent and/or spherical micelles [40, 64]. Figure 5(a) shows the measured viscosity η of aqueous binary
mixtures of C12G4 as a function of shear rate at 25 °C at different surfactant weight fractions (0.03–0.4), while figure 5(b) shows the relative zero-shear viscosity (ηR = η0 (micelle)/η0(water), where η0(micelle) and η0(water) represent zeroshear viscosity of micellar solution and pure water, respectively). Steady-shear rheology data confirmed that C12G4 micellar solutions display Newtonian fluidlike behaviour, which can be judged from the shear-independent viscosity. In systems containing long cylindrical micelles with a network structure, a shear-thinning behaviour is observed where viscosity decays after a critical shear rate. The experimental relative viscosity ηR remains unaffected by Ws at lower concentrations below Ws = 0.2. However, the ηR increases steadily, followed by a sharp increase above Ws = 0.3 and the ηR curve deviates significantly from that predicted for the HS as proposed by the Krieger–Dougherty (K–D) equation [65] ⎛ φHS ⎞−[η] φ ⁎ ηR = ⎜ 1 − ⎟ φ⁎ ⎠ ⎝
(10)
where [η] is a fitting parameter based on the shape of particles, often called intrinsic viscosity, and taken equal to 2.5 for the spherical shape of particles, ϕHS is the volume fraction of HS, and ϕ* is the volume fraction of maximum packing (for spherical particles, ϕ* = 0.64). The K-D equation was developed for noninteracting HS dispersions and can be applied to polymer-stabilized colloidal dispersions, provided the thickness of the stabilizing layer is considered to determine the volume fraction of colloidal dispersions [66]. Here we assume that the ηR of C12G4 aqueous micelles can be described by the K-D model until Ws = 0.2, which is in agreement with the SAXS data, namely that the shape and size of C12G4 micelles remain essentially the same over a wide concentration range up to Ws = 0.2. A slight deviation of the ηR until Ws = 0.3 may account for the increased interactions among C12G4 micelles [67]. However, a significant deviation of experimental ηR from the K-D model at Ws = 0.4 cannot be solely explained in terms of strong micellar interactions. Due to the significant deviation from the K-D model, in combination with the fact that the micellar phase transforms into a hexagonal liquid-crystal phase at higher concentrations above Ws = 0.54 (figure 1), we anticipate that elongated rodlike micelles may present just below the phase boundary. 3.4. Effect of temperature on micellar structure
To investigate the thermoresponsive behaviour of C12G4 micelles, SAXS measurements were carried out at a fixed composition (10% C12G4) over a wide temperature range (25–85 °C). Figure 6 shows these SAXS results. Generally, an increase in temperature favours a sphere-to-rod type transition in the micellar structure of conventional EO-based nonionic surfactants in an aqueous system due to dehydration of the surfactant’s hydrophilic headgroup [68]. Dehydration caused by heating reduces the effective cross-sectional area of the surfactant’s headgroup and consequently increases the critical packing parameter (equation (1)) favouring micellar growth in 8
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Figure 5. Rheological properties of C12G4 micellar solutions: (a) viscosity versus shear rate curves for different weight fractions of C12G4 at 25 °C, and (b) the corresponding relative zero-shear viscosity ηR (experimental) and calculated using the Krieger–Dougherty equation for noninteracting hard-spheres.
Figure 6. Effect of temperature on the micellar structure: (a) normalized x-ray scattering intensities I(q) on absolute scales, and (b) pair-
distance distribution function p(r) for the 10% C12G4/water system at different temperatures (25, 50, 75, and 85 °C). The solid and broken lines in panel (a) represent the GIFT fit and the calculated form factor for n particles existing in unit volume nP(q), respectively. Rc, and Dmax in panel (b) represent micellar core radius and maximum dimension of micelle, respectively.
4. Conclusions
aqueous systems. However, the opposite trend is generally observed in reverse micelles in nonaqueous media [40] due to interpenetration of solvent towards the micellar interface. The I(q) curves presented in figure 6(a) show that the effect of temperature on the C12G4 aqueous micelles is not simple. The scattering intensity decreases with increasing temperature, with the position of the local minimum in the low-q region maintained at q ∼ 0.82 nm-1. Nevertheless, the maximum dimension of micelles estimated from the realspace p(r) curves (figure 6(b)) remains at 8.7 nm despite the increase in temperature from 25 to 85 °C. Thus, the decreasing scattering intensity with increasing temperature can be attributed to the different contrast of the system. In general, increases in temperature would lower the contrast (electron density difference of solvent and aggregates) of amphiphilic core–shell systems, which might lead to a critical underestimation of micellar size. Closer observation of the p (r) curve, particularly in the higher-r side, reveals that its shape at higher temperature (85 °C) is not perfectly symmetrical, indicating the possibility of rodlike micelles at temperatures higher than 85 °C.
An aqueous binary phase diagram of glycerol-based nonionic surfactant tetraglycerol lauryl ether (C12G4) was constructed over a wide range of temperatures and compositions, and structural characterization of the self-assembled mesostructures was performed by using the SAXS technique. SAXS data were evaluated using a GIFT method to obtain microstructural information (shape, size, and internal structure) of the C12G4 micelle depending on composition and temperature. Due to its strong amphiphilicity, C12G4 self-assembled into normal micelles (Wm), a hexagonal liquid crystal (H1), and inverse micelles (Om) depending on the composition. The lyotropic H1 phase extended over a wide range of concentration (54–95%) and temperature (beyond 100 °C). In contrast to the conventional nonionic counterparts, C12G4 did not exhibit LCST behaviour, demonstrating the higher thermal stability of C12G4 micelles due to strong hydration effects. Bulky hydrophilic groups result in the formation of spherical core–shell type micelles in the dilute region, whose shape, size, and structures did not vary with increases in concentration up to surfactant weight fraction Ws = 0.2. In the 9
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lower concentration region (Ws < 0.2), the relative viscosity ηR remains unaffected by Ws. However, at higher concentrations (Ws ⩾ 0.3), the ηR increases and deviates significantly from that predicted for the HS as proposed by the K-D equation, demonstrating the presence of interacted micelles. Intermicellar interactions expressed in terms of structure factor obtained by the GIFT method confirmed that micelles interact particularly at higher C12G4 concentration, and this is responsible for the reduced osmotic compressibility of the system. This feature was explained in terms of the excluded volume of C12G4 micelles as proposed by the Carnahan and Starling model for manodisperse HS.
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Acknowledgments LKS thanks the Japan Society for the Promotion of Science (JSPS) for Grant-in-Aid for Young Scientists B (25790021).
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