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The influence of charge on the structure and dynamics of water encapsulated in reverse micelles† Animesh Patra,‡a Trung Quan Luong,‡b Rajib Kumar Mitra*a and Martina Havenith*b Hydrogen-bonded structure and relaxation dynamics of water entrapped inside reverse micelles (RMs) composed of surfactants with different charged head groups: sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (anionic), didodecyldimethylammonium bromide (DDAB) (cationic) and Igepal CO-520 (Igepal) ½water ). (nonionic) in cyclohexane (Cy) have been studied as a function of hydration (defined by w0 ¼ ½surfactant Sub-diffusive slow (sub-ns) relaxation dynamics of water has been measured by the time resolved fluorescence spectroscopy (TRFS) technique using two fluorophores, namely 8-anilino-1-naphthalenesulfonic acid (ANS) and coumarin-343 (C-343). The hydrogen bonded connectivity network of water confined in these RMs has been investigated by monitoring the hydrogen bond stretching and libration

Received 25th January 2014, Accepted 12th May 2014

bands of water using far-infrared FTIR spectroscopy. In addition, the ultrafast collective relaxation

DOI: 10.1039/c4cp00386a

(0.2–2.0 THz) using THz time domain spectroscopy (THz-TDS). While TRFS measurements establish the

dynamics of water inside these RMs has been determined by dielectric relaxation in the THz region retardation of water dynamics for all the RM systems, FTIR and THz-TDS measurements provide with

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signature of charge specificity.

Introduction Water plays a crucial role in many physical and biological processes; however, in many systems water does not exist in its bulk form, especially in restricted environments with the nm length scale in which the confined water molecules strongly interact with the interface, ions and molecular groups and consequently exhibits retarded dynamics and suffers a perturbed hydrogen bonded network compared to that in pure water. Reverse micelles (RMs) serve as one of the most potential platforms to study the properties of water under nano-confinement. RMs are an isotopic mixture of water, surfactant, and organic solvent in which small aqueous droplets coated with a layer of surfactant molecules are dispersed in a nonpolar solvent.1 The structure and dynamics of confined water in RM systems have been studied in detail using various experimental techniques and MD simulation.2–13 These studies have clearly established the fact that water molecules inside the RMs are structured and a

Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India. E-mail: [email protected] b Department of Physical Chemistry II, Ruhr-University Bochum, 44780 Bochum, Germany. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp00386a ‡ Contributed equally to this work.

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their dynamics are considerably retarded compared to that of pure water. Such perturbation of structure and dynamics is a manifestation of the interaction of water molecules with the RM interface. In general two types of water molecules inside RMs can be distinguished, one which are directly or indirectly interacting with the interface, commonly known as ‘bound water’, and the other which forms the usual hydrogen bonding network with surrounding water molecules and resembles ‘bulk water’. It is debatable whether a third contribution of water which is not bound but retarded in hydration dynamics had to be taken into consideration. The exchange equilibrium between these two types of water molecules is believed to depend upon the nature of the RMs, however, it needs a generalized approach to understand how the chemical, mechanical and electrical properties of the interface govern the influence of the confined water molecules.14 While the chemical and mechanical part has so far extensively been taken care of, relatively less effort has been offered regarding the charge type of the interface. Depending upon the nature of the head group, surfactants are classified into four types: cationic, anionic, zwitterionic and nonionic. Most of the earlier studies on RMs have been carried out using anionic surfactant AOT (sodium bis(2-ethylhexyl) sulfosuccinate) with only handful of reports being available with the other charge types. Due to the double tailed wedge shape AOT stands as a perfect candidate for the formation of RMs in almost all hydrocarbons.15 There have been a few studies

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to establish the influence of the interface charge type (cationic and anionic) on the activity of water inside RMs following solvolysis and/or enzyme catalysis reactions; these studies concluded greater nucleophilicity of water inside anionic RMs while higher hydrogen bond donor capability inside cationic RMs.16–19 It has also been reported20 that the relaxation dynamics of confined water is faster in nonionic (Igepal) RM than the ionic (AOT) one, however, a comprehensive study using different charge types is relatively sparse.21 It is also interesting to note that the penetration of the hydrocarbon into the surfactant monolayer does affect its chemical and mechanical properties.22 Thus, in order to discard this effect, it is important to use a single hydrocarbon oil to prepare all the RM systems. Such a study is strongly demanding since it is important to understand whether the charge type of the interface has any influence on the perturbation of water structure keeping all the factors constant. In this study we have used three surfactants with differently charged head groups, anionic AOT, nonionic Igepal (Igepal CO-520) and cationic DDAB (didodecyldimethylammonium bromide). Cyclohexane (Cy) has been used as the hydrocarbon phase. AOT is soluble in Cy and produces RM systems up to   ½water w0 ¼  20.23,24 Igepal has a head group terminated ½surfactant by an alcohol hydroxyl group and is also capable of forming well defined spherical RMs up to w0 = 20.23 The size of RMs in both the systems increases linearly with w0 with Igepal forming slightly bigger RMs compared to that of AOT.25,26 DDAB in Cy offers a unique class of RM systems; small angle neutron scattering (SANS) studies27,28 of the DDAB–Cy–water system have concluded that at low hydration (2 r w0 r 8), the system consists mainly of aggregated rod-like cylinders having their length in the range of 14–20 nm, and radius varying from 1.5 to 1.6 nm. As hydration is increased (w0 B 10) spherical aggregates are formed with a diameter in the range of B6 nm. The highest solubilization capacity of the DDAB–Cy system has been observed to be B13.24 The comparison of spherical RMs with cylindrical RMs (DDAB at low hydration) might be questioned in view of the role of interfacial geometry in determining the dynamics. However, our previous study29 has unambiguously concluded that slow relaxation dynamics of water inside DDAB RM suffers only a minimal effect of interfacial geometry. In order to determine the restriction on motion upon confinement, the slow relaxation dynamics of water molecules in RMs has been measured using the ps-resolved fluorescence spectroscopic technique using two different fluorophores, namely coumarin 343 (C-343) and 8-anilino-1-naphthalenesulfonic acid (ANS). The choice of the fluorophores lies in the specific location of the probes in the RMs so that the relaxation dynamics of water at different regions of the RMs could be extracted. Three different hydration levels have been chosen, namely w0 = 5, 10 and 15. To underline the dependence of physical properties of water on the nature of the interface we have carried out far-infrared (FIR) FTIR spectroscopic investigation which provides information on the hydrogen bonded connectivity network of water inside these RMs. For FTIR measurements

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we have used several hydration levels, namely w0 = 2, 5, 7.5, 10 and 12.5. As complementary information on the label free estimation of the dynamics of the collective motion of water molecules we carry out the dielectric relaxation study using THz time domain spectroscopy in the 0.2–2.0 THz frequency range using a simple double Debye relaxation as a fitting model. Our study is aimed to underline any possible correlation of the water structure and dynamics in RM systems with the charge type of the interface.

Materials and methods Sodium bis(2-ethylhexyl) sulfosuccinate (AOT), Igepal CO-520 (Igepal), didodecyldimethylammonium bromide (DDAB), cyclohexane (Cy), 8-anilino-1-naphthalenesulfonic acid (ANS) and coumarin-343 (C-343) [Scheme 1] were the best available analytical grade chemicals obtained from Sigma Aldrich. All these chemicals were used without further purification. The concentration of all the surfactants in Cy was kept constant at 0.1 M. Dry DDAB is insoluble in Cy, we therefore added a calculated amount of water in a DDAB–Cy mixture to prepare a RM solution with w0 = 1. The calculated amount of water was further added into it to prepare higher w0 systems. Steady-state emission spectra were recorded on a Jobin Yvon Fluorolog-3 fluorimeter. Time-resolved fluorescence measurements were performed on a commercially available spectrophotometer (LifeSpec-ps) from Edinburgh Instrument, U.K. (excitation wavelength 375 nm and 409 nm, 80 ps instrument response function (IRF)). Fluorescence transients were fitted by using software F900 from Edinburgh Instruments. The details of the time-resolved measurements could be found elsewhere.30 The time-dependent fluorescence Stokes shifts, as estimated from TRES (time-resolved emission spectra), were used to construct

Scheme 1

Molecular structures of AOT, DDAB, Igepal, ANS and C343.

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the normalized spectral shift correlation function or the solvent correlation function, C(t), defined as

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CðtÞ ¼

nðtÞ  nð1Þ nð0Þ  nð1Þ

(1)

where n(0), n(t) and n(N) are the emission maximum frequency (in cm1) at time zero, t and infinity respectively. The C(t) function represents the temporal response of the solvent relaxation process, occurring around the probe following its photo-excitation and the associated change in the dipole moment. For anisotropy, r(t), measurements, emission polarization is adjusted to be parallel or perpendicular to that of the excitation and anisotropy is defined as, rðtÞ ¼

Ik  GI? Ik þ 2GI?

(2)

G, the grating factor is determined following the longtime tail matching technique.31 All the anisotropies were measured at the emission peak. The FIR-FTIR spectra were recorded using a VERTEX 80v FTIR Spectrometer (Bruker Optics) under nitrogen gas flow in the sample compartment. For the FIR region (50 to 650 cm1), a mercury-lamp served as an FIR source, a liquid-helium-cooled silicon bolometer was used as a detector. All the FTIR measurements were carried out using a liquid cell (model A145, Bruker Optics) with diamond windows with a spacer thickness of 52.2  0.3 mm. The FTIR spectra of the samples correspond to the difference absorbance spectra between the measured absorbance of the samples and the measured absorbance of the stock solution. The details of the THz time domain spectroscopic measurements are described elsewhere.32 A pump pulse of 20 fs duration with the 80 MHz repetition rate and 500 mW optical output power is focused on the THz emitter (Tera-SED, Gigaoptics) for generation of THz pulses. The THz pulse was then detected using a ZnTe crystal using the electro optic sampling technique. From the measured time-domain data ETHz(t), a fast Fourier transformation was applied to obtain the frequencydependent power and phase of the transmitted pulse. Subsequently, the frequency-dependent absorption coefficient a(n) (power attenuation), the index of refraction n(n) (delay of the THz pulse)

as well as the complex dielectric constant ^ e(n) = e 0 (n)  ie00 (n) were 33 deduced.

Results and discussion Fluorescence measurements In order to determine the retarded dynamics of the water molecules inside RMs and a possible correlation with the interface, we carried out both steady state and time resolved fluorescence measurements using two probes, namely C-34334–38 and ANS.39,40 C-343 is water soluble and insoluble in Cy. Upon excitation at 409 nm, C-343 produces emission maximum at 490 nm in water and 465 nm in dioxane. The emission maximum of C-343 exhibits a blue shift in RMs compared to that in pure water, the effect being more prominent in the case of Igepal (Table 1). The observed B15–18 nm blue shift in Igepal RM is comparable to that obtained in earlier studies using the same probe in various nonionic surfactant RM systems.35,41 The emission maximum is observed to get progressively red shifted with increasing hydration (w0) as envisaged from the increased population of the bulk type water and hence an increased polarity.42 In DDAB, the peak position does not change much with w0. This is in agreement with our earlier studies that cylindrical (w0 = 5) to spherical (w0 = 10) transition hardly affects the local environment of the probe.29 To study the slow relaxation dynamics of water inside the investigated RM systems we construct TRES and deduce the solvent correlation function C(t).30 A representative fluorescence decay transient of C-343 in AOT RM at w0 = 10 is shown in Fig. S1 (ESI†). It is evident that the blue end (440 nm) shows only decay components, whereas the red end (590 nm) could only be fitted after considering a rise component. This clearly indicates the solvation of the probe.43 The corresponding TRES is shown in the lower panel of Fig. S1 (ESI†). Some representative C(t) curves are shown in Fig. 1a. The C(t) curves are fitted using a t

t

bi-exponential decay function, CðtÞ ¼ a1 e t1 þ a2 e t2 where t1 and t2 represent two characteristic time scales assigned to two different relaxation processes of water molecules: t1 (Bhundreds of ps) stems from the hindered rotation of the water molecules

Table 1 Steady state fluorescence maxima and solvation time correlation parameters for coumarin-343 in AOT, DDAB and Igepal RM at different levels of hydration

w0

lmax (nm)

t1 (ns)

t2 (ns)

hti (ns)

tr1 (ns)

tr2 (ns)

htri (ns)

yw

Dw  108 (s1)

AOT 5 10 15

482 485 485

0.34 (55%) 0.26 (68%) 0.20 (62%)

1.80 (45%) 1.85 (32%) 1.52 (37%)

1.01 0.76 0.69

0.25 (42%) 0.24 (47%) 0.26 (54%)

1.85 (58%) 1.41 (53%) 1.35 (46%)

1.17 0.86 0.76

33.9 36.3 39.8

3.14 3.54 3.71

Igepal 5 10 15

472 477 477

0.25 (48%) 0.23 (53%) 0.26 (51%)

2.65 (52%) 2.12 (47%) 2.14 (49%)

1.51 1.12 1.18

0.4 (45%) 0.38 (48%) 0.36 (50%)

2.34 (55%) 2.01 (52%) 1.91 (50%)

1.46 1.23 1.14

35.3 36.8 37.8

2.02 2.24 2.48

DDAB 5 10

488 490

0.39 (53%) 0.42 (55%)

1.58 (47%) 1.45 (45%)

0.94 0.88

0.29 (43%) 0.24 (46%)

1.9 (57%) 1.62 (54%)

1.21 0.99

34.2 35.8

2.71 3.50

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Fig. 1 Left panel (data obtained using C-343 as the fluoroprobe): (a) a representative C(t) plot for different RM systems with w0 = 10, (b) average time constant (hti) for different RM systems at different w0 values, (c) average rotational anisotropy time constant (htri) for different RM systems at different w0 values, (d) wobbling angle (solid symbols, arrow pointing in the direction of left side) and diffusion coefficient (open symbols, arrow pointing in the direction of rightside) for different RM systems at different w0 values. The corresponding values obtained with ANS as a fluoroprobe are shown in the right panel (Fig. e–f).

at the RM surface, whereas t2 (Bseveral ns) is attributed to the coupled rotational–translational motions.44 It could be noted here that the limited time resolution of our instrument (IRF B 80 ps) refrains us from extracting the ultrafast response of bulk water which is orders of magnitude faster than t1 and t2.45 The loss can be estimated by calculating the zero frequency of the fluorescence maximum, npem(0) using the formula

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np developed by Fee and Maroncelli,46 n pem(0) = npabs  [n np abs  n em], p np np where n abs, n abs, n em are the absorption peak of the fluorophore in polar solvent, the absorption peak in nonpolar solvent and the emission peak in nonpolar solvent, respectively. The absorption maximum of C-343 is 435 nm in dioxane and 425 nm in water and we estimate B46% loss for w0 = 10 of AOT RM. It can be argued that the undetected fluorescence

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signal originates from the ultrafast (sub-ps) relaxation of bulklike water molecules present in the system. Since the present investigation is primarily concerned with the dynamics of interfacial water, the missing contribution from bulk water does not significantly affect the conclusion drawn from the detected slow relaxation data. The solvent correlation time constants for all the RM systems are presented in Table 1. For all the RM systems the   P average time constant hti ¼ ai ti decreases with increasing

where b = S2, and S is the generalized order parameter that describes the degree of restriction on the wobbling-in-cone orientational motion. Its magnitude is considered as a measure of the spatial restriction of the probe and has values ranging from zero (for unrestricted rotation of the probe) to one (for completely restricted motion). The semicone angle yw is related to the ordered parameter as, 1 S ¼ cos yw ð1 þ cos yw Þ 2

(4)

i

w0 (Fig. 1b) which is consistent with earlier studies and is attributed to the increased fraction of fast moving bulk water molecules in the system at elevated hydration.23,29,42 Among all the studied systems Igepal offers the slowest relaxation time relative to the other two systems. In order to explore the geometrical restriction imposed on the fluorophore inside the RM systems, we measured the time resolved rotational anisotropy, r(t). Earlier simulation studies have concluded that the water rotational anisotropy relaxation in RM systems is complex and can be described using a sum of three exponential functions where each relaxation time scale is associated with a type of water: ‘surface layer’ or ‘intermediate layer’ or ‘central layer’.11,47 Since our study is mainly associated with a rather slower dynamics of confined water which essentially originates from the surface and/or the intermediate layer, we fit the decay transients using a   2 P t double exponential model, rðtÞ ¼ r0 þ ari exp  where tr tri i¼1 represents the rotational time constant. A representative double exponential fit of the anisotropy transient is shown in Fig. S2 (ESI†). All the fitting parameters are presented in Table 1. It can be observed that the average rotational time   P constant htr i ¼ ari tri is slowest for the Igepal RM systems, i

while those for AOT and DDAB are comparable (Fig. 1c). For further discussion, it should be taken into account that the location of the probe in a micro-heterogeneous environment is of utmost importance in fluorescence spectroscopy and the information drawn from fluorescence measurements is essentially governed by the nature of the immediate local environment experienced by the probe. We can compare the solvation dynamics of the three RM systems at a fixed hydration level (w0 = 10): the hti values are 0.76, 1.12 and 0.88 ns for AOT, Igepal and DDAB RM systems, respectively. In Igepal RMs, C-343 essentially stays at the interface as evident from the blue shifted fluorescence maximum, whereas in AOT and DDAB the probe resides near the bulk core of the RM. For identical w0, Igepal produces larger droplets than AOT,23 however, offers a slower solvation time scale. Restricted rotation of the probe inside RM can be deduced from the anisotropy measurements. Since the origin of the slower solvation time constants is diffusive in nature we analyze the anisotropy data using a wobbling-in-cone model.42,48–50 According to this model, the rotational anisotropy decay function is defined as,   t t   rðtÞ ¼ r0 be tslow þ ð1  bÞe tfast

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(3)

The diffusion coefficient for wobbling motion Dw can be obtained from the following equation, "     1 x2 ð1 þ xÞ2 1þx 1x þ ln Dw ¼ 2 2 ð1  S2 Þtw 2ð1  xÞ (5)  1  x 2 3 4 þ 6 þ 8x  x  12x  7x 24 1 1 1 ¼  . The tw tfast tslow yw and Dw values obtained from this analysis are summarized in Table 1 and shown in Fig. 1d and h. The wobbling angle as well as the diffusion coefficient increases with increasing hydration, indicating an overall ease of rotation with increasing w0 which is correlated with the observed decrease in htri. The obtained Dw values are of the same order of magnitude as those obtained for AOT RM systems in earlier studies and are lower compared to that of bulk water.42,51 The trend observed in Dw can be related to that in hti, with the later being mostly associated with the diffusive nature of restricted water. Dw of Igepal is low compared to the other two systems (Table 1, Fig. 1d), which enforces a slower solvation timescale for the Igepal system. On the other hand, AOT and DDAB systems offer a more polar environment to the probe, which in turn is manifested by the faster Dw and hti values. In addition, we employed a second probe, ANS, which is also a water soluble dye and produces emission maximum at 515 nm in water;52 in RMs it shows a considerable blue shift to produce the maxima at 480, 482 and 469 nm for AOT, Igepal and DDAB RM respectively at a hydration level of w0 = 10 (Table 2). DDAB is observed to produce the slowest relaxation dynamics. ANS prefers to stay at the bulk core in AOT RM40 to minimize the charge interaction and as a result produces a faster dynamics. On the other hand, the favorable charge interaction tends to keep the probe in the vicinity of the oppositely charged DDAB interface, and consequently it experiences a larger fraction of interfacially bound water molecules which eventually results in a delayed solvation response. The Dw values of DDAB are also small compared to the other two systems (Table 2, Fig. 1e–h). The fluorescence results thus confirm a considerably retarded dynamics of water molecules inside all the three different RM systems owing to their confinement, the information obtained on the extent of retardation is found to be specific of the location of the probe. In order to obtain complementary information on the hydrogen bond dynamics, where x = cos yw and tw is defined as,

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Table 2 Steady state fluorescence maxima and solvation time correlation parameters for ANS in AOT, DDAB and Igepal RM at different levels of hydration

w0

lmax (nm)

t1 (ns)

t2 (ns)

hti (ns)

tr1 (ns)

tr2 (ns)

htri (ns)

yw

Dw  108 (s1)

AOT 5 10 15

476 480 480

0.23 (64%) 0.22 (63%) 0.16 (64%)

1.54 (36%) 1.33 (37%) 0.87 (36%)

0.71 0.63 0.42

0.19 (37%) 0.18 (42%) 0.19 (47%)

2.0 (63%) 1.7 (58%) 1.55 (53%)

1.33 1.06 0.91

31.2 33.7 36.3

3.74 4.48 4.73

Igepal 5 10 15

479 482 484

0.18 (61%) 0.19 (62%) 0.17 (60%)

1.36 (39%) 1.15 (38%) 1.07 (40%)

0.64 0.55 0.53

0.2 (39%) 0.21 (40%) 0.22 (46%)

1.82 (61%) 1.63 (60%) 1.68 (54%)

1.19 1.06 1.01

32.2 32.7 35.8

3.70 3.55 3.95

DDAB 5 10

462 469

0.20 (46%) 0.59 (65%)

2.18 (54%) 2.14 (36%)

1.29 1.14

0.52 (42%) 0.51 (50%)

2.57 (58%) 2.27 (50%)

1.70 1.39

34.0 37.8

1.40 1.67

Fig. 2 (a–c) FIR-FTIR spectra of AOT, DDAB and Igepal RM systems at three different w0 values (2, 5 and 10). The FIR spectrum of pure water in this frequency region is shown as a black broken line in an adjusted scale for comparison. (d) The deconvoluted stretching band (SB) of water in different RM systems at w0 = 10. The broken line represents the SB for pure water. (e) SB of water in AOT RM at different w0 values. (f) Peak position of SB for different RM systems and pure water.

we perform additional FTIR and THz experiments. It is well known now that THz spectroscopy probes distinct hydration dynamics whereas IR spectroscopy provides the structural information. FIR FTIR measurements In order to investigate on the collective dynamics of water molecules encapsulated in the RM systems, we carry out FIR

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FTIR measurements of all the studied systems. Some representative results are depicted in Fig. 2a–c. The FIR spectra of all these systems are found to be considerably changed compared to that of bulk water. In pure water, two characteristic peaks can be found in this frequency range, one in the B200 cm1 and the other one in the B600 cm1 region. The peak at B200 cm1 is essentially an optimized hydrogen bond stretching band (SB) contributed by one symmetric stretching (at B160 cm1) and

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several antisymmetric stretching modes (B220 and 290 cm1) of the hydrogen bonded network at the first solvation shell of water.53 The band at B600 cm1 results from the librational motion of water molecules in a hydrogen bonded connected network, called the ‘‘librational band’’ (LB).32,53,54 The position of the LB depends upon the strength of hydrogen bonds in network structure of water, whereas the SB arises from the longitudinal motion of the hydrogen atom along the hydrogen bond axis (i.e., hydrogen-bond stretching) and therefore characterizes the level of hydrogen bonding between neighboring water molecules. In order to obtain quantitative information about the SB in the RM system the FIR absorbance spectra in the 30–650 cm1 region are fitted as a sum of two Gaussian functions in which the position of the low energy Gaussian (near 200 cm1) is related to the strength of the SB and consequently the extent of the collective hydrogen bonded network structure of water. A representative figure of the deconvoluted AOT RM system can be found in Fig. S3 (ESI†). We plot the deconvoluted Gaussian SB curves for different RM systems at w0 = 10 in Fig. 2d. It is evident from Fig. 2d that in RMs the SB shows a clear red shift compared to that of pure water, the effect being more prominent in the case of AOT RMs. Such a red shift in the SB band indicates that the stretching modes of network water are strongly affected by confinement. A similar red shift in the librational band of water (at B670 cm1) was previously observed in the case of AOT RM.55 It can be observed from the figure that the SB appears at B180 cm1 for DDAB and Igepal at w0 = 10, while for AOT it is further red shifted to B160 cm1. The enhanced red shift in AOT clearly suggests a decrease in collective hydrogen bonding of water inside the RM system as a considerable fraction of water molecules are associated with the interface. Fig. 2e depicts the SB of water in AOT nano-pool with varying hydration (w0). The position of the SB peak as a function of w0 for all the three surfactant systems has been plotted in Fig. 2f. It is observed that at each hydration level, AOT RM offers the strongest red shift compared to DDAB and Igepal with DDAB being little more distorted than Igepal. The SB position remains highly red shifted up to w0 = 5, beyond which a progressive blue shift of the peak is observed with increasing hydration. THz-TDS measurements THz spectroscopy was used to investigate the ultrafast collective dynamics of water in RM systems. We performed the THz-TDS measurements on all the RM systems in the frequency range of 0.2–2.0 THz. Optical parameters of water (absorption coefficient and refractive index) inside various RM systems are shown in Fig. S4 (ESI†). As observed from the figure the absorption coefficient (a) of water in the RM is decreased compared to that of pure water in the low frequency region (0.2 to 0.7 THz), whereas it is increased in the higher frequency region. A similar trend has previously been observed for AOT/heptane RM systems56 and the oil–water interface,57 and can be explained on the basis of a high-frequency shift of the fast water relaxation and a simultaneous low frequency shift of the hydrogen bond stretching mode compared to that of pure water.57 This can be interpreted in terms of a significant exposure of water

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molecules towards a more hydrophobic/hydrophilic environment as has previously been observed during the protein folding/unfolding process.58 In order to obtain the dynamics of entrapped water, the dielectric response of water has been fitted using a two component Debye model as, eðoÞ ¼ e0 ðoÞ  ie00 ðoÞ ¼ e1 þ

De1 De2 þ 1 þ iot1 1 þ iot2

(6)

where ti is the time constant of the ith relaxation mode and Dei is the corresponding relaxation strength, eN is the high frequency dielectric constant. The slower time constant t1 is associated with a cooperative reorientation of an ensemble of molecules (B8 ps for pure water) whereas the faster one, t2, emanates from hydrogen bond formation and dissociation (B200 fs for pure water).33,59 Some representative double Debye fits are shown in Fig. 3a and the corresponding fitting parameters have been plotted as a function of w0 (Fig. 3b–d). It can be observed from the figure that the time constant t1 as well as the corresponding relaxation strength (De1 = es  e2, where es is the static dielectric constant of pure water De1 B 73 for pure water) are considerably smaller in RMs compared to those of bulk water, the effect being more prominent in the case of AOT followed by DDAB and Igepal. A similar decrease in t1 and De1 has previously been reported for AOT/heptane RM systems by Mittleman et al.56 The observed decrease suggests that the confinement strongly perturbs the large-scale collective modes, which are predominantly observed in pure water.53 Even weak confinement has also been reported56 to significantly perturb the collective60 hydrogen bond bending (TA phonon) mode of liquid water which appears at B50 cm1.61 The reduction in the dielectric constant stems from a change in the hydration bond dynamics. As observed from Fig. 3, the extent of the change in t1 and De1 is comparable for all the three RM systems at low hydration (w0 = 5) indicating a highly perturbed structure at low hydration. The perturbation is released at higher hydration, especially to a considerable extent for the Igepal RM. The THz study thus corroborates with the FIR results which concluded the disruption of hydrogen bonded network structure inside RMs as manifested in the observed decrease of the connectivity network and a consequent faster relaxation dynamics. The distortion is found to be governed by the charge type of the interface with the effect being more severe for AOT followed by DDAB and Igepal. In summary, we have carried out time-resolved fluorescence, FIR-FTIR and THz-TDS spectroscopic studies of water present inside anionic, cationic and nonionic RMs. The TRFS study unambiguously confirmed the restricted dynamics of the entrapped water molecules irrespective of the nature of the interface. The slow solvation dynamics of water is also correlated with the change in its diffusive motion as evident from the time resolved anisotropy measurements. However, the extent of water structure perturbation seems not very conclusive from the TRFS study owing to the location specific information obtained from the different fluoroprobes. FIR measurements also support this result by recording a red shift of the hydrogen bond stretching band (SB) when the interface change from nonionic to cationic

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Fig. 3 (a) THz-TDS dielectric relaxation spectra and fitted spectra using the double Debye relaxation model of water inside RM for AOT, DDAB and Igepal RM at w0 = 10. (b) Dielectric strength (De1 = es  e2) as a function of w0 for different RM systems. (c) and (d) show the relaxation times as a function of w0 for all the RM systems.

to anionic. The dielectric relaxation study in the THz region confirms the fact that the collective dynamics is highly perturbed at low hydration in all the RM systems, and the perturbation is released upon increasing hydration, the release being more prominent in Igepal compared to the two ionic surfactants. All these measurements thus suggest that the anionic AOT RM interface has the largest influence on the hydration bonding dynamics, followed by DDAB and Igepal. Our study was primarily aimed to investigate the influence of the charge type of the interface. Our results follow a systematic trend with the anionic surface having a larger effect than the cationic and nonionic surfaces.

Acknowledgements This work is part of the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft (DFG). RKM acknowledges CSIR, India, for a research grant (no. 01(2573)/ 12/EMR-II) and Unit for Nano Science & Technology at S.N. Bose Centre for support.

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The influence of charge on the structure and dynamics of water encapsulated in reverse micelles.

Hydrogen-bonded structure and relaxation dynamics of water entrapped inside reverse micelles (RMs) composed of surfactants with different charged head...
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