Article pubs.acs.org/Langmuir
Intercalation of a Surfactant with a Long Polyfluoroalkyl Chain into a Clay Mineral: Unique Orientation of Polyfluoroalkyl Groups in Clay Layers Tatsuto Yui,†,‡,§,∥ Shunsuke Fujii,† Kazuki Matsubara,‡,∥ Ryo Sasai,§,⊥ Hiroshi Tachibana,† Hirohisa Yoshida,† Katsuhiko Takagi,§,# and Haruo Inoue*,† †
Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachiohji-City, Tokyo 192-0397, Japan Department of Materials Science and Technology, Faculty of Engineering and Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi-2, Niigata 950-2181, Japan § Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, 464-8603, Japan ∥ NEXT Program ‡
S Supporting Information *
ABSTRACT: Eight novel polyfluorinated surfactants (CnF2n+1CONH(CH2)2 N+(CH3)2C16H33 Br−; abbreviated as CnF−S, where n = 1, 2, 3, 4, 5, 6, 8, 10) were synthesized and their intercalation into cationexchangeable clay was investigated. All of the polyfluorinated surfactants intercalated in amounts exceeding the cation exchange capacity (CEC) of the clay. The C4F−S and C5F−S surfactants exhibited intercalation up to 480% of the CEC as a saturated adsorption limit. On the basis of X-ray analysis, CnF−S surfactants intercalated between clay nanosheets to form a bilayer structure in which the surfactant molecules tilt at an angle of ∼60° with respect to the clay surface. The saturated adsorption limits and layer distances differed between surfactants with short (n = 1, 2) and long (n = 3−10) perfluoroalkyl chains. For long-chain surfactants, their saturated adsorption limits were independent of the perfluoroalkyl chain length and the layer distances systematically increased with increasing perfluoroalkyl chain length. These results suggest that the microscopic orientation differed between the short and long chains. X-ray analysis showed that the long-chain surfactants orient the perfluoroalkyl chains at a tilt of 41 ± 5° with respect to the clay layer. This value was in good agreement with polarized IR measurements of 42 ± 2° for this angle.
■
INTRODUCTION
Polyfluorinated compounds are of interest due to their unique characteristics of strong lipophobicity, very weak intermolecular interactions, strong hydrophobicity, high solubility of gas molecules, strong resistance to oxidation, and excellent stability in the presence of strong acids or bases.21−27 Upon mixing with water and hydrocarbon solvents, perfluoroalkanes form a distinct third phase due to their very weak intermolecular interactions with both water and organic solvents.27−30 Thus, it is difficult for organic compounds to be solubilized in polyfluorinated solvents or polyfluorinated environments. However, a surfactant with both a short perfluoroalkyl chain and a long hydrocarbon chain is expected to form a molecular assembly with a polyfluorinated environment. In this context, we have reported the synthesis of polyfluorinated surfactants and shown that they form molecular
Organic/inorganic hybrid compounds have been widely investigated due to their unique chemical and physical properties, which include self-assembly, control of molecular orientation, structural and chemical stability, highly functional surface activity, and formation of a microenvironment for chemical reactions, and so forth.1−5 Typically, inorganic layered materials are used as inorganic host materials due to their expandable and two-dimensional wide-layered structure. Clay minerals, layered double hydroxide, and layered semiconductors are used as typical layered host materials.4−6 Saponite is a representative clay mineral that is able to intercalate cationic molecules in its interlayer spaces due to its layered structure with cation-exchange properties.4−7 These characteristics enable fabrication of organic−inorganic hybrid materials that can accommodate functional molecules in their interlayers.4−6 In particular, surfactant/clay hybrids4−6,8−13 are utilized in a variety of applications as adsorbents,14 artificial membranes,15 catalysts and supports,16−18 and for other functions.19,20 © XXXX American Chemical Society
Received: May 21, 2013 Revised: July 15, 2013
A
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
assemblies such as micelles,31 vesicles,32 and ion pair films33 with unique microstructures. We have investigated the intercalation of polyfluorinated surfactants34−37 (CnF2n+1CONH(CH2)2N+(CH3)2C16H33 Br−; abbreviated as CnF−S) with short perfluoroalkyl chains (n = 1, 2, and 3) into clay layers. These surfactants readily intercalated into the clay interlayer space in amounts exceeding the cation exchange capacity (CEC) of the clay.34 The C3F−S surfactant exhibited intercalation up to 440% of the CEC as the saturated adsorption limit. These results for the saturated intercalation levels and interlayer distances suggest that CnF−S (n = 1−3) intercalated in clay form a bilayer structure in which the surfactant molecules tilt with respect to the clay surface.34 However, different microscopic orientations of CnF−S surfactants, microscopic environments, and aggregation behavior of cointercalated metalloporphyrins35−37 were observed for CnF−S (n = 1−3) surfactant and clay hybrids. The effect of the perfluoroalkyl groups in the surfactants on the intercalation phenomena and the microscopic orientation of the surfactants in clay layers have not been clarified. We are interested in determining what would happen with polyfluorinated surfactants with longer perfluoroalkyl chains (n > 3); and how the perfluoroalkyl chain, long alkyl chain, and carbonyl group would orient. In this work, we report syntheses of polyfluorinated surfactants with longer perfluoroalkyl chains (n = 4, 5, 6, 8, and 10), and describe the intercalation of the surfactants into saponite clay and the microscopic orientation of the surfactants in the clay interlayers.
■
Energy Accelerator Research Organization, Tsukuba, Japan. The wavelength of the monochromatic X-ray was 0.1488 nm and scattered X-rays were detected by a one-dimensional position-sensitive proportional counter (PSPC). The distance between the specimen and the PSPC was 680 mm, so that the scattering vector range covered 1.25 nm < [S−1 = ((2/λ) sin θ)−1] < 200 nm, where 2θ is the scattering angle, and λ is the X-ray wavelength. Dichroic polarized Fourier transform infrared spectra (FT-IR) were measured on a FT/IR-610 spectrometer (JASCO) equipped with a GHP-560 polarizer unit (JASCO). FT-IR spectra were measured for cast films of surfactant/clay hybrids. These films were prepared by casting a benzene dispersion of the hybrid onto a fluorite (CaF2) plate and air drying to remove the benzene.35−37 The XRD profiles for the surfactant/clay films cast from a dispersion mixture in benzene were the same as those for the solid surfactant/clay before dispersion in benzene, which indicates that the structures of the surfactant/clay hybrids were unaffected by the benzene dispersion.35−37 The parallel orientation of the hybrid layer with respect to the substrate was confirmed by XRD analysis and SEM images.
■
RESULTS AND DISCUSSION Formation of Polyfluorinated Surfactant/Clay Hybrids. An aqueous solution of surfactant was poured into the clay aqueous dispersion. This resulted in rapid formation of a white precipitate due to intercalation of the surfactant into the clay interlayer. The amounts of surfactant intercalated into the clay for various loading levels of C3F−S to C8F−S are shown in Figure 1 and their saturated adsorption limits are given in
EXPERIMENTAL SECTION
Synthesis of Polyfluorinated Surfactants. Eight novel polyfluorinated cationic ammonium surfactants (CnF2n+1CONH(CH2)2 N+(CH3)2C16H33 Br−; CnF−S) with various chain lengths (n) were synthesized. The symbol n denotes the number of carbon atoms in the acyl group, F indicates a polyfluorinated surfactant, and S denotes a surfactant with a single long alkyl chain.31,32 The syntheses of C1F−S, C2F−S, and C3F−S have been reported elsewhere.31 C4F−S, C5F−S, C6F−S, C8F−S, and C10F−S were newly synthesized using the procedures described in the Supporting Information (SI). Materials. Cation-exchangeable clay (Sumecton SA) was kindly provided by Kunimine Industries and was used as received. Sumecton SA is a sodium saponite that is synthesized via a hydrothermal reaction and has a theoretical surface area calculated to be 750 m2 g−1 and a cation exchange capacity (CEC) of 0.997 meq g−1.34,38,39 Deionized water (conductivity 6.5 nm for any of the polyfluorinated surfactant/clay hybrids. A clear d(001) diffraction peak at S = 0.018−0.028, which is due to the layered structure, was observed for every SAXS profile, and the estimated layer distances from the SAXS profiles were the same
Figure 2. Small-angle X-ray scattering profiles of polyfluorinated surfactant/clay hybrids. (a) C1F−S = 200% CEC, (b) C2F−S = 240% CEC, (c) C3F−S = 440% CEC, (d) C4F−S = 320% CEC, (e) C5F−S = 270% CEC, (f) C6F−S = 290% CEC, (g) C8F−S = 400% CEC, and (h) C10F−S, adsorbed amount could not be estimated (see text).
as those observed from the powder XRD analyses. The d(001) peaks for the short-chain surfactants were shifted to higher Svalues with increasing fluorocarbon chain length, indicating a decrease of the layer distance. In contrast, the d(001) peaks for long-chain surfactants were shifted systematically to lower Svalues (i.e., increasing layer distance) with increasing fluorocarbon chain length. Molecular Orientation of the Intercalated Polyfluorinated Surfactants in the Clay Layer. The structures of the CnF−S polyfluorinated surfactants were optimized using the DFT B3LYP/6-31G(d) method in Gaussian 09.45 The structure of C10F−S is shown in Figure 3, as a typical
Figure 3. Molecular structure and three-dimensional model of C10F− S, a representative CnF−S surfactant. C
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
nm was observed. This value corresponds to the vicinity of the ammonium ion of C3F−S and is thought to be due to the electron-dense bromide counterion of the surfactant, which indicates that the cationic C3F−S molecules exceeding CEC were neutralized by bromide anions.34 Similar phenomena have been described by Emerson et al.48 The maximum at X = 1.3− 2.3 nm corresponds to the aluminosilicate layer of the clay itself. The experimental results for saturated intercalation levels, occupied areas, CLS, and electron density profile all indicate that the polyfluorinated surfactants form a rigidly packed bilayer structure. Microscopic Molecular Orientation of the Intercalated Polyfluorinated Surfactants in the Clay Layer. Our next goal was to understand the structure of the rigidly packed bilayer of polyfluorinated surfactants within the clay interlayer, in terms of how the surfactant, perfluoroalkyl chain, and long alkyl chain are oriented in the interlayer. The thickness of a surfactant monolayer (one side of the bilayer) can be expressed as half of the clearance space (CLS/2). However, the surfactant has a semitriangular structure and its molecular length is defined as L1 (the side opposite the obtuse angle in the optimized structure). On the basis of CLS/2 and L1, the tilt angle of each surfactants (Θ1) was calculated (Table 2). Schematic images of the orientations of a surfactant from different viewpoints are shown in Figure 5. All surfactants tilt at about 60° on the clay surface, as shown in Figure 5(b).
example. The molecular geometries of all the surfactants are given in the Supporting Information. The polyfluorinated surfactant has a contorted semitriangular structure, with the molecule bending at the amide/ammonium (N+) spacer group. The length of the side opposite the obtuse angle of the triangular structure is defined as the molecular length (L1) (see Figure 3 and Table 2). The distances between layers46 Table 2. Structural Data for Polyfluorinated Surfactant/Clay Hybrids C1F−S C2F−S C3F−S C4F−S C5F−S C6F−S C8F−S C10F−S
CLSa(nm)
L1b(nm)
L2c(nm)
Θ1d(deg)
3.16 2.99 2.81 3.01 3.23 3.43 3.64 4.25
2.97 3.07 3.18 3.31 3.46 3.56 3.85 4.07
0.45 0.55 0.69 0.81 0.95 1.07 1.33 1.59
58 61 64 63 62 61 62 59
a
The clearance space was calculated by subtraction of the aluminosilicate layer thickness of the clay sheet (0.96 nm) from the layer distance of hybrids determined by SAXS analysis. bLength of the side opposite the obtuse angle in the optimized structure in vacuo. c Length of the CnF2n+1 chain. dTilt angle of the side opposite of the surfactant.
(clearance space, hereafter CLS) of the CnF−S/clay hybrids and L1 of the CnF−S surfactants are listed in Table 2. A simple comparison of these L1 lengths with the almost similar CLS values might lead to a conclusion that the surfactants are oriented perpendicular to the clay surface in a zigzag monolayer.4,5,8,47 If the surfactants orient in a monolayer, then the occupied area of one molecule should be twice as large as the cross-sectional area of the surfactant (∼ 0.26 × 2 nm2). However, the occupied areas of the long-chain surfactants were all below 0.31 nm2 (see Table 1), indicating that these molecules form a bilayer, rather than a monolayer, within the clay interlayer space.48−50 The one-dimensional electron density profile of the C3F−S/ clay hybrids in the c-axis direction (normal to the clay surface) calculated from the XRD profile is shown in Figure 4.50 X = 0 nm indicates the center of the hybrid. The electron density profile has a minimum at X = 0 nm, and the profile is symmetrical at X = 0 nm, strongly indicating that C3F−S molecules orient in a bilayer structure in the clay layer with a well-aligned, nonstaggered alignment. A maximum at X ≈ 0.5
Figure 5. Schematic drawing of the orientation of C8F−S adsorbed on the clay surface and corresponding tilt angles: (a) top view; (b) side view parallel to the side (L1) opposite the bend angle; (c) relative to the perfluoroalkyl chain; and (d) relative to the long alkyl chain. Atomic colors are yellow (carbon with attached fluorine atoms); blue (nitrogen); red (oxygen); and sky-blue (carbon in the hydrocarbon chain); the fluorine and hydrogen atoms are omitted for clarity.
Figure 4. Electron density profile of the C3F−S/clay hybrid in the caxis direction and a corresponding schematic drawing of C3F−S in the clay layer. D
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
is governed by a delicate balance among (1) electrostatic attractive/repulsive force between the ammonium group and the negatively charged sites on the clay, (2) strong hydrophobic and lipophobic interactions of the perfluorinated alkyl chain, (3) hydrophobic interactions of the hydrocarbon long alkyl chain, and (4) hydrogen bonding between the amide carbonyl and amide-NH bonds in adjacent molecules. The number of carbon atoms in the perfluorinated headgroup obviously has a magic number of n = 3 which looks to be critical as seen in Figure 6. The similar tendencies were also observed in the cases of micelles17 and vescilces formations.18 Since the bilayer structures in the clay interlayer were all observed for C1F−S to C10F−S, the striking difference of the microscopic orientation structure should be mostly derived from different nature of the perfluorinated headgroup rather than the long alkyl group. It should be very curious what are the key factors for the differences. Very interesting data on melting points and boiling points should be noted here. A systematic study on a series of perfluoroalkanes (perfluorometane (n = 1) to perfluorodecane (n = 10)) indicates that perfluoropropane (n = 3) has the lowest melting point and the boiling points have different tendency between the short chain ones (n = 1, 2) and the long chain ones (n > 3) when compared with those of corresponding hydrocarbon series.51 The difference should be composed of both enthalpy and entropy factors between the packed or condensed phase and mutually separated environment. Regrettably, thermodynamic parameters have not been reported so far for short chain perfluorinated surfactants. Since perfluoroalkanes are not miscible with water and hydrocarbons,27−30 the perfluoroalkyl head groups of CnF−S surfactans would avoid a free mixing with hydrocarbon moiety within the surfactants themselves or adjacent molecules. Even in the case, however, for short-chain surfactants (C1F−S, C2F−S), the cation−anion stabilization might overcome the hydrophobic interactions between the fluorocarbon chains to force the unfavorable mixing of the headgroup with the hydrocarbon moiety, while the strong interactions between the perfluoroalkyl chains dominate for long-chain surfactants, leading to the unique standing orientation of the polyfluorinated alkyl group. Thus, C1F−S and C2F−S might orient in a bulky-type structure, i.e., exhibiting a “footprint”.36 As a result, the long alkyl chains only weakly interact in the clay layer. Although sufficient rationalization based on thermodynamic parameters have still not necessarily being obtained, these results would surely provide very interesting examples as the unique behavior among different chain length of perfluoroalkyl group. Circular Dichroism Polarized IR Measurements. To obtain more information on the tilt angles of the perfluoroalkyl, hydrocarbon, and carbonyl groups, dichroic polarized FT-IR measurements were carried out. The relationship between the rotation angle (α) of the substrate (or incident angle) and the dichroic ratio (R) for uniaxial orientation is expressed by eq 2.33,37,52−55
The tilt angles of the perfluoroalkyl groups were estimated based on the correlation between the fluorocarbon chain length and clearance space from SAXS measurements,. The polyfluorinated alkyl chains extend their arms straightforwardly due to their high rigid conformation.17,18 This chain is thus defined as a “head-group,” with length designated as L2 (see Figure 3 and Table 2). The relationship between L2 and CLS/2 from SAXS analysis is shown in Figure 6. The plot indicates that the
Figure 6. Relationship between the half width of the clearance space (CLS/2) and the length of the headgroup of short chain (open circles) and long-chain (solid circles) polyfluorinated surfactants. The solid line drawn through the data represents the linear fit given by eqs 1 or 1′.
short-chain (n = 1 and 2) and long-chain (n = 3, 4, 5, 6, 8, and 10) series again have different tendencies. The long-chain surfactants exhibit a systematic increase of CLS/2, with a good linear relationship between the length of the headgroup of these surfactants and the CLS/2 values. The solid line drawn through the data represents the fit to the values for the longchain surfactants shown in eq 1. CLS/2 = A × L 2 + constant
(1)
Equation 1 provides important and interesting insights into the orientation of the polyfluorinated surfactants within the clay interlayer. The first term, A × L2, indicates a systematic increase of CLS/2 with an increase in the number of carbon atom in the rigid polyfluorinated chain; while the second term, constant, is independent of the polyfluorinated chain length. The relationship strongly suggests that the long-chain surfactants all have the same microscopic orientation in the clay interlayer. Assuming the microscopic structure in Figure 5, the equation can be rewritten as eq 1′. CLS/2 = L 2 cos Θ2 + L3 cos Θ3(constant)
(1′)
, where Θ2, L3, and Θ3 denote the tilt angle of the headgroup, the length of the side of the triangular structure including the long alkyl chain, as defined in Figure 3, and its tilt angle, respectively. The slope in Figure 6 corresponds to cos Θ2 = 0.75, and thus the tilt angle of the headgroup (Θ2) was estimated to be 41 ± 5°. The intercept in Figure 6 corresponds to L3 cos Θ3 = 0.89 nm. The tilt angle of the side of the triangular structure including the long alkyl chain (Θ3) in the surfactant was calculated to be 71°, based on L3 = 2.8 nm. In contrast, the CLS/2 values for the short-chain surfactants decreased with increasing headgroup length (L2). These results again reflect a difference in the microscopic orientation of the short-chain surfactants in the clay layer. As discussed for micelles,17 vesicles,18 and intercalation into clay,34 the molecular organization of polyfluorinated cationic surfactants
R yx = A y /Ax = {2[sin 2 θ + sin 2 α(3 cos2 θ − 1)] − (3 sin 2 α − 1)(3 cos2 θ − 1)sin 2 γ } /[2 sin 2 θ + (2 − 3 sin 2 θ )sin 2 γ ]
(2)
where, Ax, Ay, θ, and γ represent the intensities for the horizontally and vertically polarized incident light, the angle of the molecular axis to the optical transition moment, and the tilt E
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
absorption νas(CH2) and νs(CH2) bands for the microcrystal of CnF−S surfactants, however, were also observed by KBr pellete method at the very similar wavenumbers (cm−1) at 2927, 2855 for C4F−S, 2927, 2856 for C5F−S, 2927, 2856 for C6F−S, 2927, 2858 for C8F−S, and 2929, 2859 for C10F−S, respectively. The long alkyl chains in crystal are generally packed in all-trans conformation and the νas(CH2) band for the all-trans form shifts to the higher frequency in clathrate environment than in neat crystal.62 A bilayer structure has been reported to have much smaller translational disorder than a monolayer one in hybrid compounds.63 The optimized structure of C3F−S by a DFT calculation clearly indicates that the long alkyl chain has all-trans conformation, though the disorder in gauche and trans conformers might be involved in the ethylene group in the spacer moiety. These results, thus, indicate that a straightforward speculation from the position of the wavenumber for the νas(CH2) band on a disorder in gauche and trans conformers in the long alkyl chains hybrid layer is rather questionable. Though a clear conclusion is not obtained through these considerations, the very similar positions of νas(CH2) and νs(CH2) bands between those in microcrystal and the hybrid compounds strongly suggest that the long alkyl chains in the clay hybrid systems are in all-trans conformation. The structures in Figures 3 and 5 are thus illustrated on the basis of these considerations. The tilt angle of the long alkyl chains of C8F−S were, thus, not be determined from polarized IR analysis to avoid an enexpected confusion. The carbonyl stretching region (1600−1800 cm−1) was also analyzed by FT-IR. Two types of vibrational bands were observed for δ(CO), at 1713 and 1686 cm−1 for free and hydrogen bonded CO, respectively. However, it was difficult to obtain the tilt angle from the δ(CO) bands because the results could not be fitted with eq 2, probably due to the coexistence of the two different types of carbonyl groups. Summary. Eight novel polyfluorinated surfactants (CnF−S, where n = 1, 2, 3, 4, 5, 6, 8, and 10) were synthesized and their intercalation into synthetic clay (Sumecton SA) was investigated. All of the surfactants intercalated in amounts exceeding the CEC of the clay. All of the experimental results, including the saturated intercalation levels, occupied areas, layer distance measurements, and electron density profiles, indicated that the CnF−S surfactants form a rigidly packed bilayer structure in the clay interlayer. Intercalation behavior, such as saturated intercalation levels and layer distances, differed between the long-chain (n = 3−10) and short-chain (n = 1 and 2) surfactants. The short-chain surfactants have large occupied areas, i.e., large “footprints” on the clay layer.34,36 The orientation of the long-chain surfactants was independent of the fluorocarbon chain length and all appeared to have similar oriented structures within the clay layers. All of the CnF−S surfactants tilt at an angle of about 60° relative to the clay layer. SAXS analysis and dichroic IR measurements revealed tilt angles of the perfluoro headgroup (Θ2) in the long-chain surfactants of about 41° with respect to the clay surface. On the basis of the values for CLS/2, Θ2, and the alkyl chain length (2.8 nm), the tilt angle of the long alkyl chain (Θ3) of the longchain surfactants was calculated to be 71°. These measurements permit an understanding of the microscopic orientation of the surfactants adsorbed on the clay surface, as shown schematically for C8F−S in Figure 5.
angle of the molecule to the substrate normal, respectively. The polarized FT-IR measurement is well suited to measurements for longer linear molecules, but in the case of the C10F−S hybrid, the longest surfactant molecule used in the present work, this measurement could not be performed because of the coexistence of the three different types of structures in the material. Thus, the C8F−S/clay hybrid film cast from a benzene dispersion was used for the polarized IR measurement. The spectrum was monitored at 1150−1160 cm−1 for the CF2 stretching band (ν(CF2)), at 2800−3000 cm−1 for CH2 symmetry and asymmetry stretching bands (νs(CH2) and νas(CH2)), and at 1600−1800 cm−1 for the carbonyl stretching band (δ(CO)). It is well-known that the perfluoroalkyl chain has an all-trans conformation due to the high rigidity of the fluorocarbon chain, and thus estimation of exact tilt angles is possible using polarized IR. The angle between the molecular axis of the perfluorocarbon chain and the optical transition moment (θ) of ν(CF2) is taken to be 90°. The molecular axis of the headgroup is defined as the line through the carbon chain of the perfluoroalkyl group to the carbonyl carbon. The experimental correlation between α and R for C8F−S at ν(CF2) in the C8F− S/clay hybrid cast film (C8F−S = 400% CEC) is shown in Figure 7. R showed a dependence on α and had a maximum at α = 0°. This indicates that the C8F−S molecules are oriented in the clay hybrids in the film aligned parallel to the glass plate.
Figure 7. Experimental relationship between the dichroic ratio (R) and the incident angle (α) for the C8F−S/clay hybrid cast film, based on the CF2 symmetrical stretch in the IR spectrum (1150−1160 cm−1). The solid line is calculated using eq 2.
The tilt angle (Θ2) of the headgroup in C8F−S was estimated by fitting of eq 2 to the correlations between R, α, and θ, to give the solid line in Figure 7. The experimental data were well fitted by eq 2 and the tilt angle of the headgroup in C8F−S was estimated as γ = 42 ± 2°. Thus, the headgroup of C8F−S orients at 42 ± 2° relative to the clay layer. This value is in good agreement with the angle obtained from the SAXS analyses (Θ2 = 41 ± 5°) described above. Dichroic polarized IR measurements were also carried out for the CH2 stretching bands. Methylene chains in an all-trans state (ordered) exhibit νas(CH2) and νs(CH2) bands at 2915 and 2845 cm−1, respectively, and these move to higher frequencies as the number of gauche conformations along the hydrocarbon chain (disordered) increases.56−61 Strong aliphatic absorption νas(CH2) and νs(CH2) bands at 2926 and 2856 cm−1, respectively, were observed for the C8F−S/clay hybrid cast film. The higher shifts of the νas(CH2) and νs(CH2) bands may indicate that the methylene chains exist as a mixture of gauche and trans conformers in the hybrid layer. The aliphatic F
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
■
Article
(16) Shimazu, S.; Hirano, T.; Uematsu, T. Shape Selective Hydrogenation by Ruthenium-Hectorite Catalysts with Various Interlayer Distances. Appl. Catal 1987, 34, 255−261. (17) Shimazu, S.; Ishida, T.; Uematsu, T. Catalytic Behaviour of Interlayer-Supported Palladium(Ii) Complexes on Lithium Hectorite. J. Mol. Catal. 1989, 55, 353−360. (18) Sento, T.; Shimazu, S.; Ichikuni, N.; Uematsu, T. Asymmetric Hydrogenation of Itaconates by Hectorite-Intercalated Rh-DIOP Complex. J. Mol. Catal. A: Chem. 1999, 137, 263−267. (19) Bondarenko, S. V.; Zhukova, A. I.; Tarasevich, Y. I. Evaluation of the Selectivity of Some Organo-Substituted Layer Silicates. J. Chromatogr. A 1982, 241, 281−286. (20) Lee, T. W.; Park, O.; Kim, J.; Hong, J.; Kim, Y. Efficient Photoluminescence and Electroluminescence from Environmentally Stable Polymer/Clay Nanocomposites. Chem. Mater. 2001, 13, 2217− 2222. (21) Briks, J. B.; Christophorou, L. G. Photophysics of Aromatic Molecules; John Wiley & Sons Ltd.: New York, 1969; p Chapter 4. (22) Lawson, C. W.; Hirayama, F.; Lipsky, S. Effect of Solvent Perturbation on the S3 → S1 Internal Conversion Efficiency of Benzene, Toluene, and p-Xylene. J. Chem. Phys. 1969, 51, 1590−1597. (23) Banks, R. E. Organofluorine Chemicals and their Industrial Applications; Ellis Horwood Ltd.: Chichester, U.K., 1979. (24) Kitazume, T.; Ishihara, T.; Taguchi, T. Chemistry of Fluorine; Kodansha Scientific: Tokyo, 1993. (25) Myers, K. E.; Kumar, K. Fluorophobic Acceleration of DielsAlder Reactions. J. Am. Chem. Soc. 2000, 122, 12025−12026. (26) Negishi, A., Chemistry of Fluorine. Mruzen: Tokyo, 1988. (27) Horváth, I. T.; Rábai, J. Facile Catalyst Separation without Water: Fluorous Biphase Hydroformylation of Olefins. Science 1994, 266, 72−75. (28) Scott, R. L. The Solubility of Fluorocarbons 1. J. Am. Chem. Soc. 1948, 70, 4090−4093. (29) Scott, R. L. The Anomalous Behavior of Fluorocarbon Solutions. J. Phys. Chem. 1958, 62, 136−145. (30) Dorset, D. L. Binary Phase Behavior of Perfluoroalkanes. Macromolecules 1990, 23, 894−901. (31) Kameo, Y.; Takahashi, S.; Krieg-Kowald, M.; Ohmachi, T.; Takagi, S.; Inoue, H. Surface Polyfluorinated Micelles. J. Phys. Chem. B 1999, 103, 9562−9568. (32) Kusaka, H.; Uno, M.; Krieg-Kowald, M.; Ohmachi, T.; Kidokoro, S.; Yui, T.; Takagi, S.; Inoue, H. Surface Polyfluorinated Cationic Vesicles. Phys. Chem. Chem. Phys. 1999, 1, 3135−3140. (33) Ohtani, O.; Furukawa, T.; Sasai, R.; Hayashi, E.; Shichi, T.; Yui, T.; Takagi, K. Effect of Fluorinated Ammonium Counterions upon the Reversibility in E-Z Photoisomerization of Azobenzene Ion Pair Films. J. Mater. Chem. 2004, 14, 196−200. (34) Yui, T.; Yoshida, H.; Tachibana, H.; Tryk, D. A.; Inoue, H. Intercalation of Polyfluorinated Surfactants into Clay Minerals and the Characterization of the Hybrid Compounds. Langmuir 2002, 18, 891− 896. (35) Matsuoka, R.; Yui, T.; Sasai, R.; Takagi, K.; Inoue, H. Enhanced Aggregation of Tin(IV)porphyrins in a Polyfluorinated Surfactant-Clay Hybrid Environment. Mol. Cryst. Liq. Cryst. 2000, 341, 333−338. (36) Yui, T.; Uppili, S. R.; Shimada, T.; Tryk, D. A.; Yoshida, H.; Inoue, H. Microscopic Structure and Microscopic Environment of a Polyfluorinated Surfactant/Clay Hybrid Compound: Photochemical Studies of Rose Bengal. Langmuir 2002, 18, 4232−4239. (37) Lucia, L. A.; Yui, T.; Sasai, R.; Takagi, S.; Takagi, K.; Yoshida, H.; Whitten, D. G.; Inoue, H. Enhanced Aggregation Behavior of Antimony(V) Porphyrins in Polyfluorinated Surfactant/Clay Hybrid Microenvironment. J. Phys. Chem. B 2003, 107, 3789−3797. (38) Takagi, S.; Shimada, T.; Eguchi, M.; Yui, T.; Yoshida, H.; Tryk, D. A.; Inoue, H. High-Density Adsorption of Cationic Porphyrins on Clay Layer Surfaces without Aggregation: The Size-Matching Effect. Langmuir 2002, 18, 2265−2272. (39) Takagi, S.; Shimada, T.; Ishida, Y.; Fujimura, T.; Masui, D.; Tachibana, H.; Eguchi, M.; Inoue, H. Size-Matching Effect on Inorganic Nanosheets: Control of Distance, Alignment, and
ASSOCIATED CONTENT
S Supporting Information *
Additional experimental details, a scheme, figures, and a table. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Present Addresses ⊥
Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu-cho, Matsue 690-8504, Japan. # Kanagawa Academy of Science and Technology (KAST), KSP W6F, 3-2-1 Sakato, Takatsu, Kawasaki, Kanagawa 213-0012, Japan. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Kapoor, M. P.; Inagaki, S., Hierarchically Organized Hybrid Organic-Inorganic Mesoporous Nanocomposite Materials. In Bottomup Nanofabrication; Ariga, K., Nalwa, H. S., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2009; Vol. 6, pp 255−281. (2) Rao, K. V.; Datta, K. K. R.; Eswaramoorthy, M.; George, S. J. Light-Harvesting Hybrid Assemblies. Chem.Eur. J. 2012, 18, 2184− 2194. (3) Yui, T.; Kameyama, T.; Sasaki, T.; Torimoto, T.; Takagi, K. Pyrene-to-Porphyrin Excited Singlet Energy Transfer in LBLDeposited LDH Nanosheets. J. Porphyr. Phthalocyanines 2007, 11, 428−433. (4) Ogawa, M.; Kuroda, K. Photofunctions of Intercalation Compounds. Chem. Rev. 1995, 95, 399−438. (5) Sasai, R.; Yui, T.; Takagi, K. Photochemistry of Dyes and Layered Inorganic Materials. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Valencia CA, 2011; Vol. 24, pp 303−361. (6) Handbook of Clays and Clay Minerals, 3rd ed.; Gihodou Shuppan: Tokyo, 2009. (7) Velde, B. Introduction to Clay Minerals; Chapman & Hall: London, 1992. (8) Lagaly, G. Characterization of Clay by Organic-Compounds. Clay Minerals 1981, 16, 1−21. (9) Xu, S. H.; Boyd, S. A. Cationic Surfactant Adsorption by Swelling and Nonsewlling Layer Silicates. Langmuir 1995, 11, 2508−2514. (10) Gelfer, M.; Burger, C.; Fadeev, A.; Sics, I.; Chu, B.; Hsiao, B. S.; Heintz, A.; Hsu, S. L.; Kojo, K.; Si, M.; Rafailovich, M. Thermally Induced Phase Transitions and Morphological Changes in Organoclays. Langmuir 2004, 20, 3746−3758. (11) Bisio, C.; Carniato, F.; Paul, G.; Gatti, G.; Boccaleri, E.; Marchese, L. One-Pot Synthesis and Physicochemical Properties of an Organo-Modified Saponite Clay. Langmuir 2011, 27, 7250−7257. (12) Zhao, Q.; Burns, S. E. Microstructure of Single Chain Quaternary Ammonium Cations Intercalated into Montmorillonite: A Molecular Dynamics Study. Langmuir 2012, 28, 16393−16400. (13) Zhang, J.; Li, L.; Wang, J.; Xu, J.; Sun, D. Phase Inversion of Emulsions Containing a Lipophilic Surfactant Induced by Clay Concentration. Langmuir 2013, 29 (12), 3889−3894. (14) Boyd, S. A.; Lee, J.-F.; Mortland, M. M. Attenuating Organic Contaminant Mobility by Soil Modification. Nature 1988, 333, 345− 347. (15) Okahata, Y.; Shimizu, A. Preparation of Bilayer-Intercalated Clay Films and Permeation Control Responding to Temperature, Electric Field, and Ambient pH Changes. Langmuir 1989, 5, 954−959. G
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
(62) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G. Carbon-Hydrogen Stretching Modes and the Structure of n-Alkyl Chains. 2. Long, Alltrans Chains. J. Phys. Chem. 1984, 88, 334−341. (63) Naik, V. V.; Vasudevan, S. Effect of Alkyl Chain Arrangement on Conformation and Dynamics in a Surfactant Intercalated Layered Double Hydroxide: Spectroscopic Measurements and MD Simulations. J. Phys. Chem. C 2011, 115, 8221−8232.
Orientation of Molecular Adsorption as a Bottom-Up Methodology for Nanomaterials. Langmuir 2013, 29, 2108−2119. (40) Li, B. Y.; Fujii, M.; Fukada, K.; Kato, T.; Seimiya, T. Time Dependent Anchoring of Adsorbed Cationic Surfactant Molecules at Mice/Solution Interface. J. Colloid Interface Sci. 1999, 209, 25−30. (41) Fujii, M.; Li, B. Y.; Fukada, K.; Kato, T.; Seimiya, T. TwoDimensional Arrangements of Adsorbed Alkylammonium Halides on Cleaved Mica Surface. Langmuir 2001, 17, 1138−1142. (42) Tsao, Y. H.; Yang, S. X.; Evans, D. F.; Wennerstrom, H. Interaction beteen Hydrophobic SurfacesDependence on Temperature and Alkyl Chain-Length. Langmuir 1991, 7, 3154−3159. (43) Ogawa, M.; Aono, T.; Kuroda, K.; Kato, C. Photophysical Probe study of Alkylammonium Montmorillonites. Langmuir 1993, 9, 1529− 1533. (44) Ogawa, M.; Wada, T.; Kuroda, K. Intercalation of Pyrene into Alkylammonium-Exchanged Swelling Layered Silicates: The Effects of the Arrangements of the Interlayer Alkylammonium Ions on the States of Adsorbates. Langmuir 1995, 11, 4598−4600. (45) In the previous report (ref 34), PM3MO method was used for structural optimization. (46) Estimated from subtraction of the layer thickness of clay (9.6 Å) from the d(001) values obtained from X-ray analyses. (47) Weiss, A. Die Innerkristalline Quellung als Allgemeines Modell fur Quellungsvorganege. Chem. Ber. Recl. 1958, 91, 487−502. (48) Slade, P. G.; Raupach, M.; Emerson, W. W. Ordering of Cetylpyridinium Bromide on Vermiculite. Clays Clay Miner. 1978, 26, 125−134. (49) Itoh, T.; Shichi, T.; Yui, T.; Takagi, K. Odd-Even Effect of the Methylene Chain Number in the Template Polymerization of α,βDiacetylenecarboxylates Incorporated in Layered Double Hydroxide Clay. Langmuir 2005, 21, 3217−3220. (50) Itoh, T.; Ohta, N.; Shichi, T.; Yui, T.; Takagi, K. The SelfAssembling Properties of Stearate Ions in Hydrotalcite Clay Composites. Langmuir 2003, 19, 9120−9126. (51) Kissa, E. Fluorinated Surfactsns and Repellents, 2nd ed.; Mercel Dekker, Inc.: New York, 2001. (52) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VCH: New York, 1986. (53) Lopez Arbeloa, F.; Martinez Martinez, V.; Arbeloa, T.; Lopez Arbeloa, I. Photoresponse and Anisotropy of Rhodamine Dye Intercalated in Ordered Clay Layered Films. J. Photochem. Photobiol. C: Photochem. Rev. 2007, 8, 85−108. (54) Iyi, N.; Sasai, R.; Fujita, T.; Deguchi, T.; Sota, T.; Lopez Arbeloa, F.; Kitamura, K. Orientation and Aggregation of Cationic Laser Dyes in a Fluoromica: Polarized Spectrometry Studies. Appl. Clay Sci. 2002, 22, 125−136. (55) Sasai, R.; Shin’ya, N.; Shichi, T.; Takagi, K.; Gekko, K. Molecular Alignment and Photodimerization of 4′-Chloro-4-stilbenecarboxylic Acid in Hydrotalcite Clays: Bilayer Formation in the Interlayers. Langmuir 1999, 15, 413−418. (56) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Interlayer Structure and Molecular Environment of Alkylammonium Layered Silicates. Chem. Mater. 1994, 6, 1017−1022. (57) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. Carbon-Hydrogen Stretching Modes and the Structure of n-Alkyl Chains. 1. Long, Disordered Chains. J. Phys. Chem. 1982, 86, 5145−5150. (58) Wang, W.; Li, L.; Xi, S. A Fourier Transform Infrared Study of the Coagel to Micelle Transition of Cetyltrimethylammonium Bromide. J. Colloid Interface Sci. 1993, 155, 369−373. (59) Yamaguchi, N.; Shimazu, S.; Ichikuni, N.; Uematsu, T. Synthesis of Novel Nano-Structured Clays: Unique Conformation of Pillar Complexes. Chem. Lett. 2004, 33, 208−209. (60) Theng, B. K. G.; Newman, R. H.; Whitton, J. S. Characterization of an Alkylammonium-Montmorillonite-Phenanthrene Intercalation Complex by Carbon-13 Nuclear Magnetic Resonance Spectroscopy. Clay Minerals 1998, 33, 221−229. (61) Pratum, T. K. A Solid-State Carbon-13 NMR Study of Tetraalkylammonium/Clay Complexes. J. Phys. Chem. 1992, 96, 4567−4571. H
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Intercalation of a Surfactant with a Long Polyfluoroalkyl Chain into a Clay Mineral: Unique Orientation of Polyfluoroalkyl Groups in Clay Layers Tatsuto Yui,†,‡,§,∥ Shunsuke Fujii,† Kazuki Matsubara,‡,∥ Ryo Sasai,§,⊥ Hiroshi Tachibana,† Hirohisa Yoshida,† Katsuhiko Takagi,§,# and Haruo Inoue*,† †
Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachiohji-City, Tokyo 192-0397, Japan Department of Materials Science and Technology, Faculty of Engineering and Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi-2, Niigata 950-2181, Japan § Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, 464-8603, Japan ∥ NEXT Program ‡
S Supporting Information *
ABSTRACT: Eight novel polyfluorinated surfactants (CnF2n+1CONH(CH2)2 N+(CH3)2C16H33 Br−; abbreviated as CnF−S, where n = 1, 2, 3, 4, 5, 6, 8, 10) were synthesized and their intercalation into cationexchangeable clay was investigated. All of the polyfluorinated surfactants intercalated in amounts exceeding the cation exchange capacity (CEC) of the clay. The C4F−S and C5F−S surfactants exhibited intercalation up to 480% of the CEC as a saturated adsorption limit. On the basis of X-ray analysis, CnF−S surfactants intercalated between clay nanosheets to form a bilayer structure in which the surfactant molecules tilt at an angle of ∼60° with respect to the clay surface. The saturated adsorption limits and layer distances differed between surfactants with short (n = 1, 2) and long (n = 3−10) perfluoroalkyl chains. For long-chain surfactants, their saturated adsorption limits were independent of the perfluoroalkyl chain length and the layer distances systematically increased with increasing perfluoroalkyl chain length. These results suggest that the microscopic orientation differed between the short and long chains. X-ray analysis showed that the long-chain surfactants orient the perfluoroalkyl chains at a tilt of 41 ± 5° with respect to the clay layer. This value was in good agreement with polarized IR measurements of 42 ± 2° for this angle.
■
INTRODUCTION
Polyfluorinated compounds are of interest due to their unique characteristics of strong lipophobicity, very weak intermolecular interactions, strong hydrophobicity, high solubility of gas molecules, strong resistance to oxidation, and excellent stability in the presence of strong acids or bases.21−27 Upon mixing with water and hydrocarbon solvents, perfluoroalkanes form a distinct third phase due to their very weak intermolecular interactions with both water and organic solvents.27−30 Thus, it is difficult for organic compounds to be solubilized in polyfluorinated solvents or polyfluorinated environments. However, a surfactant with both a short perfluoroalkyl chain and a long hydrocarbon chain is expected to form a molecular assembly with a polyfluorinated environment. In this context, we have reported the synthesis of polyfluorinated surfactants and shown that they form molecular
Organic/inorganic hybrid compounds have been widely investigated due to their unique chemical and physical properties, which include self-assembly, control of molecular orientation, structural and chemical stability, highly functional surface activity, and formation of a microenvironment for chemical reactions, and so forth.1−5 Typically, inorganic layered materials are used as inorganic host materials due to their expandable and two-dimensional wide-layered structure. Clay minerals, layered double hydroxide, and layered semiconductors are used as typical layered host materials.4−6 Saponite is a representative clay mineral that is able to intercalate cationic molecules in its interlayer spaces due to its layered structure with cation-exchange properties.4−7 These characteristics enable fabrication of organic−inorganic hybrid materials that can accommodate functional molecules in their interlayers.4−6 In particular, surfactant/clay hybrids4−6,8−13 are utilized in a variety of applications as adsorbents,14 artificial membranes,15 catalysts and supports,16−18 and for other functions.19,20 © XXXX American Chemical Society
Received: May 21, 2013 Revised: July 15, 2013
A
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
assemblies such as micelles,31 vesicles,32 and ion pair films33 with unique microstructures. We have investigated the intercalation of polyfluorinated surfactants34−37 (CnF2n+1CONH(CH2)2N+(CH3)2C16H33 Br−; abbreviated as CnF−S) with short perfluoroalkyl chains (n = 1, 2, and 3) into clay layers. These surfactants readily intercalated into the clay interlayer space in amounts exceeding the cation exchange capacity (CEC) of the clay.34 The C3F−S surfactant exhibited intercalation up to 440% of the CEC as the saturated adsorption limit. These results for the saturated intercalation levels and interlayer distances suggest that CnF−S (n = 1−3) intercalated in clay form a bilayer structure in which the surfactant molecules tilt with respect to the clay surface.34 However, different microscopic orientations of CnF−S surfactants, microscopic environments, and aggregation behavior of cointercalated metalloporphyrins35−37 were observed for CnF−S (n = 1−3) surfactant and clay hybrids. The effect of the perfluoroalkyl groups in the surfactants on the intercalation phenomena and the microscopic orientation of the surfactants in clay layers have not been clarified. We are interested in determining what would happen with polyfluorinated surfactants with longer perfluoroalkyl chains (n > 3); and how the perfluoroalkyl chain, long alkyl chain, and carbonyl group would orient. In this work, we report syntheses of polyfluorinated surfactants with longer perfluoroalkyl chains (n = 4, 5, 6, 8, and 10), and describe the intercalation of the surfactants into saponite clay and the microscopic orientation of the surfactants in the clay interlayers.
■
Energy Accelerator Research Organization, Tsukuba, Japan. The wavelength of the monochromatic X-ray was 0.1488 nm and scattered X-rays were detected by a one-dimensional position-sensitive proportional counter (PSPC). The distance between the specimen and the PSPC was 680 mm, so that the scattering vector range covered 1.25 nm < [S−1 = ((2/λ) sin θ)−1] < 200 nm, where 2θ is the scattering angle, and λ is the X-ray wavelength. Dichroic polarized Fourier transform infrared spectra (FT-IR) were measured on a FT/IR-610 spectrometer (JASCO) equipped with a GHP-560 polarizer unit (JASCO). FT-IR spectra were measured for cast films of surfactant/clay hybrids. These films were prepared by casting a benzene dispersion of the hybrid onto a fluorite (CaF2) plate and air drying to remove the benzene.35−37 The XRD profiles for the surfactant/clay films cast from a dispersion mixture in benzene were the same as those for the solid surfactant/clay before dispersion in benzene, which indicates that the structures of the surfactant/clay hybrids were unaffected by the benzene dispersion.35−37 The parallel orientation of the hybrid layer with respect to the substrate was confirmed by XRD analysis and SEM images.
■
RESULTS AND DISCUSSION Formation of Polyfluorinated Surfactant/Clay Hybrids. An aqueous solution of surfactant was poured into the clay aqueous dispersion. This resulted in rapid formation of a white precipitate due to intercalation of the surfactant into the clay interlayer. The amounts of surfactant intercalated into the clay for various loading levels of C3F−S to C8F−S are shown in Figure 1 and their saturated adsorption limits are given in
EXPERIMENTAL SECTION
Synthesis of Polyfluorinated Surfactants. Eight novel polyfluorinated cationic ammonium surfactants (CnF2n+1CONH(CH2)2 N+(CH3)2C16H33 Br−; CnF−S) with various chain lengths (n) were synthesized. The symbol n denotes the number of carbon atoms in the acyl group, F indicates a polyfluorinated surfactant, and S denotes a surfactant with a single long alkyl chain.31,32 The syntheses of C1F−S, C2F−S, and C3F−S have been reported elsewhere.31 C4F−S, C5F−S, C6F−S, C8F−S, and C10F−S were newly synthesized using the procedures described in the Supporting Information (SI). Materials. Cation-exchangeable clay (Sumecton SA) was kindly provided by Kunimine Industries and was used as received. Sumecton SA is a sodium saponite that is synthesized via a hydrothermal reaction and has a theoretical surface area calculated to be 750 m2 g−1 and a cation exchange capacity (CEC) of 0.997 meq g−1.34,38,39 Deionized water (conductivity 6.5 nm for any of the polyfluorinated surfactant/clay hybrids. A clear d(001) diffraction peak at S = 0.018−0.028, which is due to the layered structure, was observed for every SAXS profile, and the estimated layer distances from the SAXS profiles were the same
Figure 2. Small-angle X-ray scattering profiles of polyfluorinated surfactant/clay hybrids. (a) C1F−S = 200% CEC, (b) C2F−S = 240% CEC, (c) C3F−S = 440% CEC, (d) C4F−S = 320% CEC, (e) C5F−S = 270% CEC, (f) C6F−S = 290% CEC, (g) C8F−S = 400% CEC, and (h) C10F−S, adsorbed amount could not be estimated (see text).
as those observed from the powder XRD analyses. The d(001) peaks for the short-chain surfactants were shifted to higher Svalues with increasing fluorocarbon chain length, indicating a decrease of the layer distance. In contrast, the d(001) peaks for long-chain surfactants were shifted systematically to lower Svalues (i.e., increasing layer distance) with increasing fluorocarbon chain length. Molecular Orientation of the Intercalated Polyfluorinated Surfactants in the Clay Layer. The structures of the CnF−S polyfluorinated surfactants were optimized using the DFT B3LYP/6-31G(d) method in Gaussian 09.45 The structure of C10F−S is shown in Figure 3, as a typical
Figure 3. Molecular structure and three-dimensional model of C10F− S, a representative CnF−S surfactant. C
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
nm was observed. This value corresponds to the vicinity of the ammonium ion of C3F−S and is thought to be due to the electron-dense bromide counterion of the surfactant, which indicates that the cationic C3F−S molecules exceeding CEC were neutralized by bromide anions.34 Similar phenomena have been described by Emerson et al.48 The maximum at X = 1.3− 2.3 nm corresponds to the aluminosilicate layer of the clay itself. The experimental results for saturated intercalation levels, occupied areas, CLS, and electron density profile all indicate that the polyfluorinated surfactants form a rigidly packed bilayer structure. Microscopic Molecular Orientation of the Intercalated Polyfluorinated Surfactants in the Clay Layer. Our next goal was to understand the structure of the rigidly packed bilayer of polyfluorinated surfactants within the clay interlayer, in terms of how the surfactant, perfluoroalkyl chain, and long alkyl chain are oriented in the interlayer. The thickness of a surfactant monolayer (one side of the bilayer) can be expressed as half of the clearance space (CLS/2). However, the surfactant has a semitriangular structure and its molecular length is defined as L1 (the side opposite the obtuse angle in the optimized structure). On the basis of CLS/2 and L1, the tilt angle of each surfactants (Θ1) was calculated (Table 2). Schematic images of the orientations of a surfactant from different viewpoints are shown in Figure 5. All surfactants tilt at about 60° on the clay surface, as shown in Figure 5(b).
example. The molecular geometries of all the surfactants are given in the Supporting Information. The polyfluorinated surfactant has a contorted semitriangular structure, with the molecule bending at the amide/ammonium (N+) spacer group. The length of the side opposite the obtuse angle of the triangular structure is defined as the molecular length (L1) (see Figure 3 and Table 2). The distances between layers46 Table 2. Structural Data for Polyfluorinated Surfactant/Clay Hybrids C1F−S C2F−S C3F−S C4F−S C5F−S C6F−S C8F−S C10F−S
CLSa(nm)
L1b(nm)
L2c(nm)
Θ1d(deg)
3.16 2.99 2.81 3.01 3.23 3.43 3.64 4.25
2.97 3.07 3.18 3.31 3.46 3.56 3.85 4.07
0.45 0.55 0.69 0.81 0.95 1.07 1.33 1.59
58 61 64 63 62 61 62 59
a
The clearance space was calculated by subtraction of the aluminosilicate layer thickness of the clay sheet (0.96 nm) from the layer distance of hybrids determined by SAXS analysis. bLength of the side opposite the obtuse angle in the optimized structure in vacuo. c Length of the CnF2n+1 chain. dTilt angle of the side opposite of the surfactant.
(clearance space, hereafter CLS) of the CnF−S/clay hybrids and L1 of the CnF−S surfactants are listed in Table 2. A simple comparison of these L1 lengths with the almost similar CLS values might lead to a conclusion that the surfactants are oriented perpendicular to the clay surface in a zigzag monolayer.4,5,8,47 If the surfactants orient in a monolayer, then the occupied area of one molecule should be twice as large as the cross-sectional area of the surfactant (∼ 0.26 × 2 nm2). However, the occupied areas of the long-chain surfactants were all below 0.31 nm2 (see Table 1), indicating that these molecules form a bilayer, rather than a monolayer, within the clay interlayer space.48−50 The one-dimensional electron density profile of the C3F−S/ clay hybrids in the c-axis direction (normal to the clay surface) calculated from the XRD profile is shown in Figure 4.50 X = 0 nm indicates the center of the hybrid. The electron density profile has a minimum at X = 0 nm, and the profile is symmetrical at X = 0 nm, strongly indicating that C3F−S molecules orient in a bilayer structure in the clay layer with a well-aligned, nonstaggered alignment. A maximum at X ≈ 0.5
Figure 5. Schematic drawing of the orientation of C8F−S adsorbed on the clay surface and corresponding tilt angles: (a) top view; (b) side view parallel to the side (L1) opposite the bend angle; (c) relative to the perfluoroalkyl chain; and (d) relative to the long alkyl chain. Atomic colors are yellow (carbon with attached fluorine atoms); blue (nitrogen); red (oxygen); and sky-blue (carbon in the hydrocarbon chain); the fluorine and hydrogen atoms are omitted for clarity.
Figure 4. Electron density profile of the C3F−S/clay hybrid in the caxis direction and a corresponding schematic drawing of C3F−S in the clay layer. D
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
is governed by a delicate balance among (1) electrostatic attractive/repulsive force between the ammonium group and the negatively charged sites on the clay, (2) strong hydrophobic and lipophobic interactions of the perfluorinated alkyl chain, (3) hydrophobic interactions of the hydrocarbon long alkyl chain, and (4) hydrogen bonding between the amide carbonyl and amide-NH bonds in adjacent molecules. The number of carbon atoms in the perfluorinated headgroup obviously has a magic number of n = 3 which looks to be critical as seen in Figure 6. The similar tendencies were also observed in the cases of micelles17 and vescilces formations.18 Since the bilayer structures in the clay interlayer were all observed for C1F−S to C10F−S, the striking difference of the microscopic orientation structure should be mostly derived from different nature of the perfluorinated headgroup rather than the long alkyl group. It should be very curious what are the key factors for the differences. Very interesting data on melting points and boiling points should be noted here. A systematic study on a series of perfluoroalkanes (perfluorometane (n = 1) to perfluorodecane (n = 10)) indicates that perfluoropropane (n = 3) has the lowest melting point and the boiling points have different tendency between the short chain ones (n = 1, 2) and the long chain ones (n > 3) when compared with those of corresponding hydrocarbon series.51 The difference should be composed of both enthalpy and entropy factors between the packed or condensed phase and mutually separated environment. Regrettably, thermodynamic parameters have not been reported so far for short chain perfluorinated surfactants. Since perfluoroalkanes are not miscible with water and hydrocarbons,27−30 the perfluoroalkyl head groups of CnF−S surfactans would avoid a free mixing with hydrocarbon moiety within the surfactants themselves or adjacent molecules. Even in the case, however, for short-chain surfactants (C1F−S, C2F−S), the cation−anion stabilization might overcome the hydrophobic interactions between the fluorocarbon chains to force the unfavorable mixing of the headgroup with the hydrocarbon moiety, while the strong interactions between the perfluoroalkyl chains dominate for long-chain surfactants, leading to the unique standing orientation of the polyfluorinated alkyl group. Thus, C1F−S and C2F−S might orient in a bulky-type structure, i.e., exhibiting a “footprint”.36 As a result, the long alkyl chains only weakly interact in the clay layer. Although sufficient rationalization based on thermodynamic parameters have still not necessarily being obtained, these results would surely provide very interesting examples as the unique behavior among different chain length of perfluoroalkyl group. Circular Dichroism Polarized IR Measurements. To obtain more information on the tilt angles of the perfluoroalkyl, hydrocarbon, and carbonyl groups, dichroic polarized FT-IR measurements were carried out. The relationship between the rotation angle (α) of the substrate (or incident angle) and the dichroic ratio (R) for uniaxial orientation is expressed by eq 2.33,37,52−55
The tilt angles of the perfluoroalkyl groups were estimated based on the correlation between the fluorocarbon chain length and clearance space from SAXS measurements,. The polyfluorinated alkyl chains extend their arms straightforwardly due to their high rigid conformation.17,18 This chain is thus defined as a “head-group,” with length designated as L2 (see Figure 3 and Table 2). The relationship between L2 and CLS/2 from SAXS analysis is shown in Figure 6. The plot indicates that the
Figure 6. Relationship between the half width of the clearance space (CLS/2) and the length of the headgroup of short chain (open circles) and long-chain (solid circles) polyfluorinated surfactants. The solid line drawn through the data represents the linear fit given by eqs 1 or 1′.
short-chain (n = 1 and 2) and long-chain (n = 3, 4, 5, 6, 8, and 10) series again have different tendencies. The long-chain surfactants exhibit a systematic increase of CLS/2, with a good linear relationship between the length of the headgroup of these surfactants and the CLS/2 values. The solid line drawn through the data represents the fit to the values for the longchain surfactants shown in eq 1. CLS/2 = A × L 2 + constant
(1)
Equation 1 provides important and interesting insights into the orientation of the polyfluorinated surfactants within the clay interlayer. The first term, A × L2, indicates a systematic increase of CLS/2 with an increase in the number of carbon atom in the rigid polyfluorinated chain; while the second term, constant, is independent of the polyfluorinated chain length. The relationship strongly suggests that the long-chain surfactants all have the same microscopic orientation in the clay interlayer. Assuming the microscopic structure in Figure 5, the equation can be rewritten as eq 1′. CLS/2 = L 2 cos Θ2 + L3 cos Θ3(constant)
(1′)
, where Θ2, L3, and Θ3 denote the tilt angle of the headgroup, the length of the side of the triangular structure including the long alkyl chain, as defined in Figure 3, and its tilt angle, respectively. The slope in Figure 6 corresponds to cos Θ2 = 0.75, and thus the tilt angle of the headgroup (Θ2) was estimated to be 41 ± 5°. The intercept in Figure 6 corresponds to L3 cos Θ3 = 0.89 nm. The tilt angle of the side of the triangular structure including the long alkyl chain (Θ3) in the surfactant was calculated to be 71°, based on L3 = 2.8 nm. In contrast, the CLS/2 values for the short-chain surfactants decreased with increasing headgroup length (L2). These results again reflect a difference in the microscopic orientation of the short-chain surfactants in the clay layer. As discussed for micelles,17 vesicles,18 and intercalation into clay,34 the molecular organization of polyfluorinated cationic surfactants
R yx = A y /Ax = {2[sin 2 θ + sin 2 α(3 cos2 θ − 1)] − (3 sin 2 α − 1)(3 cos2 θ − 1)sin 2 γ } /[2 sin 2 θ + (2 − 3 sin 2 θ )sin 2 γ ]
(2)
where, Ax, Ay, θ, and γ represent the intensities for the horizontally and vertically polarized incident light, the angle of the molecular axis to the optical transition moment, and the tilt E
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
absorption νas(CH2) and νs(CH2) bands for the microcrystal of CnF−S surfactants, however, were also observed by KBr pellete method at the very similar wavenumbers (cm−1) at 2927, 2855 for C4F−S, 2927, 2856 for C5F−S, 2927, 2856 for C6F−S, 2927, 2858 for C8F−S, and 2929, 2859 for C10F−S, respectively. The long alkyl chains in crystal are generally packed in all-trans conformation and the νas(CH2) band for the all-trans form shifts to the higher frequency in clathrate environment than in neat crystal.62 A bilayer structure has been reported to have much smaller translational disorder than a monolayer one in hybrid compounds.63 The optimized structure of C3F−S by a DFT calculation clearly indicates that the long alkyl chain has all-trans conformation, though the disorder in gauche and trans conformers might be involved in the ethylene group in the spacer moiety. These results, thus, indicate that a straightforward speculation from the position of the wavenumber for the νas(CH2) band on a disorder in gauche and trans conformers in the long alkyl chains hybrid layer is rather questionable. Though a clear conclusion is not obtained through these considerations, the very similar positions of νas(CH2) and νs(CH2) bands between those in microcrystal and the hybrid compounds strongly suggest that the long alkyl chains in the clay hybrid systems are in all-trans conformation. The structures in Figures 3 and 5 are thus illustrated on the basis of these considerations. The tilt angle of the long alkyl chains of C8F−S were, thus, not be determined from polarized IR analysis to avoid an enexpected confusion. The carbonyl stretching region (1600−1800 cm−1) was also analyzed by FT-IR. Two types of vibrational bands were observed for δ(CO), at 1713 and 1686 cm−1 for free and hydrogen bonded CO, respectively. However, it was difficult to obtain the tilt angle from the δ(CO) bands because the results could not be fitted with eq 2, probably due to the coexistence of the two different types of carbonyl groups. Summary. Eight novel polyfluorinated surfactants (CnF−S, where n = 1, 2, 3, 4, 5, 6, 8, and 10) were synthesized and their intercalation into synthetic clay (Sumecton SA) was investigated. All of the surfactants intercalated in amounts exceeding the CEC of the clay. All of the experimental results, including the saturated intercalation levels, occupied areas, layer distance measurements, and electron density profiles, indicated that the CnF−S surfactants form a rigidly packed bilayer structure in the clay interlayer. Intercalation behavior, such as saturated intercalation levels and layer distances, differed between the long-chain (n = 3−10) and short-chain (n = 1 and 2) surfactants. The short-chain surfactants have large occupied areas, i.e., large “footprints” on the clay layer.34,36 The orientation of the long-chain surfactants was independent of the fluorocarbon chain length and all appeared to have similar oriented structures within the clay layers. All of the CnF−S surfactants tilt at an angle of about 60° relative to the clay layer. SAXS analysis and dichroic IR measurements revealed tilt angles of the perfluoro headgroup (Θ2) in the long-chain surfactants of about 41° with respect to the clay surface. On the basis of the values for CLS/2, Θ2, and the alkyl chain length (2.8 nm), the tilt angle of the long alkyl chain (Θ3) of the longchain surfactants was calculated to be 71°. These measurements permit an understanding of the microscopic orientation of the surfactants adsorbed on the clay surface, as shown schematically for C8F−S in Figure 5.
angle of the molecule to the substrate normal, respectively. The polarized FT-IR measurement is well suited to measurements for longer linear molecules, but in the case of the C10F−S hybrid, the longest surfactant molecule used in the present work, this measurement could not be performed because of the coexistence of the three different types of structures in the material. Thus, the C8F−S/clay hybrid film cast from a benzene dispersion was used for the polarized IR measurement. The spectrum was monitored at 1150−1160 cm−1 for the CF2 stretching band (ν(CF2)), at 2800−3000 cm−1 for CH2 symmetry and asymmetry stretching bands (νs(CH2) and νas(CH2)), and at 1600−1800 cm−1 for the carbonyl stretching band (δ(CO)). It is well-known that the perfluoroalkyl chain has an all-trans conformation due to the high rigidity of the fluorocarbon chain, and thus estimation of exact tilt angles is possible using polarized IR. The angle between the molecular axis of the perfluorocarbon chain and the optical transition moment (θ) of ν(CF2) is taken to be 90°. The molecular axis of the headgroup is defined as the line through the carbon chain of the perfluoroalkyl group to the carbonyl carbon. The experimental correlation between α and R for C8F−S at ν(CF2) in the C8F− S/clay hybrid cast film (C8F−S = 400% CEC) is shown in Figure 7. R showed a dependence on α and had a maximum at α = 0°. This indicates that the C8F−S molecules are oriented in the clay hybrids in the film aligned parallel to the glass plate.
Figure 7. Experimental relationship between the dichroic ratio (R) and the incident angle (α) for the C8F−S/clay hybrid cast film, based on the CF2 symmetrical stretch in the IR spectrum (1150−1160 cm−1). The solid line is calculated using eq 2.
The tilt angle (Θ2) of the headgroup in C8F−S was estimated by fitting of eq 2 to the correlations between R, α, and θ, to give the solid line in Figure 7. The experimental data were well fitted by eq 2 and the tilt angle of the headgroup in C8F−S was estimated as γ = 42 ± 2°. Thus, the headgroup of C8F−S orients at 42 ± 2° relative to the clay layer. This value is in good agreement with the angle obtained from the SAXS analyses (Θ2 = 41 ± 5°) described above. Dichroic polarized IR measurements were also carried out for the CH2 stretching bands. Methylene chains in an all-trans state (ordered) exhibit νas(CH2) and νs(CH2) bands at 2915 and 2845 cm−1, respectively, and these move to higher frequencies as the number of gauche conformations along the hydrocarbon chain (disordered) increases.56−61 Strong aliphatic absorption νas(CH2) and νs(CH2) bands at 2926 and 2856 cm−1, respectively, were observed for the C8F−S/clay hybrid cast film. The higher shifts of the νas(CH2) and νs(CH2) bands may indicate that the methylene chains exist as a mixture of gauche and trans conformers in the hybrid layer. The aliphatic F
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
■
Article
(16) Shimazu, S.; Hirano, T.; Uematsu, T. Shape Selective Hydrogenation by Ruthenium-Hectorite Catalysts with Various Interlayer Distances. Appl. Catal 1987, 34, 255−261. (17) Shimazu, S.; Ishida, T.; Uematsu, T. Catalytic Behaviour of Interlayer-Supported Palladium(Ii) Complexes on Lithium Hectorite. J. Mol. Catal. 1989, 55, 353−360. (18) Sento, T.; Shimazu, S.; Ichikuni, N.; Uematsu, T. Asymmetric Hydrogenation of Itaconates by Hectorite-Intercalated Rh-DIOP Complex. J. Mol. Catal. A: Chem. 1999, 137, 263−267. (19) Bondarenko, S. V.; Zhukova, A. I.; Tarasevich, Y. I. Evaluation of the Selectivity of Some Organo-Substituted Layer Silicates. J. Chromatogr. A 1982, 241, 281−286. (20) Lee, T. W.; Park, O.; Kim, J.; Hong, J.; Kim, Y. Efficient Photoluminescence and Electroluminescence from Environmentally Stable Polymer/Clay Nanocomposites. Chem. Mater. 2001, 13, 2217− 2222. (21) Briks, J. B.; Christophorou, L. G. Photophysics of Aromatic Molecules; John Wiley & Sons Ltd.: New York, 1969; p Chapter 4. (22) Lawson, C. W.; Hirayama, F.; Lipsky, S. Effect of Solvent Perturbation on the S3 → S1 Internal Conversion Efficiency of Benzene, Toluene, and p-Xylene. J. Chem. Phys. 1969, 51, 1590−1597. (23) Banks, R. E. Organofluorine Chemicals and their Industrial Applications; Ellis Horwood Ltd.: Chichester, U.K., 1979. (24) Kitazume, T.; Ishihara, T.; Taguchi, T. Chemistry of Fluorine; Kodansha Scientific: Tokyo, 1993. (25) Myers, K. E.; Kumar, K. Fluorophobic Acceleration of DielsAlder Reactions. J. Am. Chem. Soc. 2000, 122, 12025−12026. (26) Negishi, A., Chemistry of Fluorine. Mruzen: Tokyo, 1988. (27) Horváth, I. T.; Rábai, J. Facile Catalyst Separation without Water: Fluorous Biphase Hydroformylation of Olefins. Science 1994, 266, 72−75. (28) Scott, R. L. The Solubility of Fluorocarbons 1. J. Am. Chem. Soc. 1948, 70, 4090−4093. (29) Scott, R. L. The Anomalous Behavior of Fluorocarbon Solutions. J. Phys. Chem. 1958, 62, 136−145. (30) Dorset, D. L. Binary Phase Behavior of Perfluoroalkanes. Macromolecules 1990, 23, 894−901. (31) Kameo, Y.; Takahashi, S.; Krieg-Kowald, M.; Ohmachi, T.; Takagi, S.; Inoue, H. Surface Polyfluorinated Micelles. J. Phys. Chem. B 1999, 103, 9562−9568. (32) Kusaka, H.; Uno, M.; Krieg-Kowald, M.; Ohmachi, T.; Kidokoro, S.; Yui, T.; Takagi, S.; Inoue, H. Surface Polyfluorinated Cationic Vesicles. Phys. Chem. Chem. Phys. 1999, 1, 3135−3140. (33) Ohtani, O.; Furukawa, T.; Sasai, R.; Hayashi, E.; Shichi, T.; Yui, T.; Takagi, K. Effect of Fluorinated Ammonium Counterions upon the Reversibility in E-Z Photoisomerization of Azobenzene Ion Pair Films. J. Mater. Chem. 2004, 14, 196−200. (34) Yui, T.; Yoshida, H.; Tachibana, H.; Tryk, D. A.; Inoue, H. Intercalation of Polyfluorinated Surfactants into Clay Minerals and the Characterization of the Hybrid Compounds. Langmuir 2002, 18, 891− 896. (35) Matsuoka, R.; Yui, T.; Sasai, R.; Takagi, K.; Inoue, H. Enhanced Aggregation of Tin(IV)porphyrins in a Polyfluorinated Surfactant-Clay Hybrid Environment. Mol. Cryst. Liq. Cryst. 2000, 341, 333−338. (36) Yui, T.; Uppili, S. R.; Shimada, T.; Tryk, D. A.; Yoshida, H.; Inoue, H. Microscopic Structure and Microscopic Environment of a Polyfluorinated Surfactant/Clay Hybrid Compound: Photochemical Studies of Rose Bengal. Langmuir 2002, 18, 4232−4239. (37) Lucia, L. A.; Yui, T.; Sasai, R.; Takagi, S.; Takagi, K.; Yoshida, H.; Whitten, D. G.; Inoue, H. Enhanced Aggregation Behavior of Antimony(V) Porphyrins in Polyfluorinated Surfactant/Clay Hybrid Microenvironment. J. Phys. Chem. B 2003, 107, 3789−3797. (38) Takagi, S.; Shimada, T.; Eguchi, M.; Yui, T.; Yoshida, H.; Tryk, D. A.; Inoue, H. High-Density Adsorption of Cationic Porphyrins on Clay Layer Surfaces without Aggregation: The Size-Matching Effect. Langmuir 2002, 18, 2265−2272. (39) Takagi, S.; Shimada, T.; Ishida, Y.; Fujimura, T.; Masui, D.; Tachibana, H.; Eguchi, M.; Inoue, H. Size-Matching Effect on Inorganic Nanosheets: Control of Distance, Alignment, and
ASSOCIATED CONTENT
S Supporting Information *
Additional experimental details, a scheme, figures, and a table. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Present Addresses ⊥
Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu-cho, Matsue 690-8504, Japan. # Kanagawa Academy of Science and Technology (KAST), KSP W6F, 3-2-1 Sakato, Takatsu, Kawasaki, Kanagawa 213-0012, Japan. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Kapoor, M. P.; Inagaki, S., Hierarchically Organized Hybrid Organic-Inorganic Mesoporous Nanocomposite Materials. In Bottomup Nanofabrication; Ariga, K., Nalwa, H. S., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2009; Vol. 6, pp 255−281. (2) Rao, K. V.; Datta, K. K. R.; Eswaramoorthy, M.; George, S. J. Light-Harvesting Hybrid Assemblies. Chem.Eur. J. 2012, 18, 2184− 2194. (3) Yui, T.; Kameyama, T.; Sasaki, T.; Torimoto, T.; Takagi, K. Pyrene-to-Porphyrin Excited Singlet Energy Transfer in LBLDeposited LDH Nanosheets. J. Porphyr. Phthalocyanines 2007, 11, 428−433. (4) Ogawa, M.; Kuroda, K. Photofunctions of Intercalation Compounds. Chem. Rev. 1995, 95, 399−438. (5) Sasai, R.; Yui, T.; Takagi, K. Photochemistry of Dyes and Layered Inorganic Materials. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Valencia CA, 2011; Vol. 24, pp 303−361. (6) Handbook of Clays and Clay Minerals, 3rd ed.; Gihodou Shuppan: Tokyo, 2009. (7) Velde, B. Introduction to Clay Minerals; Chapman & Hall: London, 1992. (8) Lagaly, G. Characterization of Clay by Organic-Compounds. Clay Minerals 1981, 16, 1−21. (9) Xu, S. H.; Boyd, S. A. Cationic Surfactant Adsorption by Swelling and Nonsewlling Layer Silicates. Langmuir 1995, 11, 2508−2514. (10) Gelfer, M.; Burger, C.; Fadeev, A.; Sics, I.; Chu, B.; Hsiao, B. S.; Heintz, A.; Hsu, S. L.; Kojo, K.; Si, M.; Rafailovich, M. Thermally Induced Phase Transitions and Morphological Changes in Organoclays. Langmuir 2004, 20, 3746−3758. (11) Bisio, C.; Carniato, F.; Paul, G.; Gatti, G.; Boccaleri, E.; Marchese, L. One-Pot Synthesis and Physicochemical Properties of an Organo-Modified Saponite Clay. Langmuir 2011, 27, 7250−7257. (12) Zhao, Q.; Burns, S. E. Microstructure of Single Chain Quaternary Ammonium Cations Intercalated into Montmorillonite: A Molecular Dynamics Study. Langmuir 2012, 28, 16393−16400. (13) Zhang, J.; Li, L.; Wang, J.; Xu, J.; Sun, D. Phase Inversion of Emulsions Containing a Lipophilic Surfactant Induced by Clay Concentration. Langmuir 2013, 29 (12), 3889−3894. (14) Boyd, S. A.; Lee, J.-F.; Mortland, M. M. Attenuating Organic Contaminant Mobility by Soil Modification. Nature 1988, 333, 345− 347. (15) Okahata, Y.; Shimizu, A. Preparation of Bilayer-Intercalated Clay Films and Permeation Control Responding to Temperature, Electric Field, and Ambient pH Changes. Langmuir 1989, 5, 954−959. G
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
(62) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G. Carbon-Hydrogen Stretching Modes and the Structure of n-Alkyl Chains. 2. Long, Alltrans Chains. J. Phys. Chem. 1984, 88, 334−341. (63) Naik, V. V.; Vasudevan, S. Effect of Alkyl Chain Arrangement on Conformation and Dynamics in a Surfactant Intercalated Layered Double Hydroxide: Spectroscopic Measurements and MD Simulations. J. Phys. Chem. C 2011, 115, 8221−8232.
Orientation of Molecular Adsorption as a Bottom-Up Methodology for Nanomaterials. Langmuir 2013, 29, 2108−2119. (40) Li, B. Y.; Fujii, M.; Fukada, K.; Kato, T.; Seimiya, T. Time Dependent Anchoring of Adsorbed Cationic Surfactant Molecules at Mice/Solution Interface. J. Colloid Interface Sci. 1999, 209, 25−30. (41) Fujii, M.; Li, B. Y.; Fukada, K.; Kato, T.; Seimiya, T. TwoDimensional Arrangements of Adsorbed Alkylammonium Halides on Cleaved Mica Surface. Langmuir 2001, 17, 1138−1142. (42) Tsao, Y. H.; Yang, S. X.; Evans, D. F.; Wennerstrom, H. Interaction beteen Hydrophobic SurfacesDependence on Temperature and Alkyl Chain-Length. Langmuir 1991, 7, 3154−3159. (43) Ogawa, M.; Aono, T.; Kuroda, K.; Kato, C. Photophysical Probe study of Alkylammonium Montmorillonites. Langmuir 1993, 9, 1529− 1533. (44) Ogawa, M.; Wada, T.; Kuroda, K. Intercalation of Pyrene into Alkylammonium-Exchanged Swelling Layered Silicates: The Effects of the Arrangements of the Interlayer Alkylammonium Ions on the States of Adsorbates. Langmuir 1995, 11, 4598−4600. (45) In the previous report (ref 34), PM3MO method was used for structural optimization. (46) Estimated from subtraction of the layer thickness of clay (9.6 Å) from the d(001) values obtained from X-ray analyses. (47) Weiss, A. Die Innerkristalline Quellung als Allgemeines Modell fur Quellungsvorganege. Chem. Ber. Recl. 1958, 91, 487−502. (48) Slade, P. G.; Raupach, M.; Emerson, W. W. Ordering of Cetylpyridinium Bromide on Vermiculite. Clays Clay Miner. 1978, 26, 125−134. (49) Itoh, T.; Shichi, T.; Yui, T.; Takagi, K. Odd-Even Effect of the Methylene Chain Number in the Template Polymerization of α,βDiacetylenecarboxylates Incorporated in Layered Double Hydroxide Clay. Langmuir 2005, 21, 3217−3220. (50) Itoh, T.; Ohta, N.; Shichi, T.; Yui, T.; Takagi, K. The SelfAssembling Properties of Stearate Ions in Hydrotalcite Clay Composites. Langmuir 2003, 19, 9120−9126. (51) Kissa, E. Fluorinated Surfactsns and Repellents, 2nd ed.; Mercel Dekker, Inc.: New York, 2001. (52) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VCH: New York, 1986. (53) Lopez Arbeloa, F.; Martinez Martinez, V.; Arbeloa, T.; Lopez Arbeloa, I. Photoresponse and Anisotropy of Rhodamine Dye Intercalated in Ordered Clay Layered Films. J. Photochem. Photobiol. C: Photochem. Rev. 2007, 8, 85−108. (54) Iyi, N.; Sasai, R.; Fujita, T.; Deguchi, T.; Sota, T.; Lopez Arbeloa, F.; Kitamura, K. Orientation and Aggregation of Cationic Laser Dyes in a Fluoromica: Polarized Spectrometry Studies. Appl. Clay Sci. 2002, 22, 125−136. (55) Sasai, R.; Shin’ya, N.; Shichi, T.; Takagi, K.; Gekko, K. Molecular Alignment and Photodimerization of 4′-Chloro-4-stilbenecarboxylic Acid in Hydrotalcite Clays: Bilayer Formation in the Interlayers. Langmuir 1999, 15, 413−418. (56) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Interlayer Structure and Molecular Environment of Alkylammonium Layered Silicates. Chem. Mater. 1994, 6, 1017−1022. (57) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. Carbon-Hydrogen Stretching Modes and the Structure of n-Alkyl Chains. 1. Long, Disordered Chains. J. Phys. Chem. 1982, 86, 5145−5150. (58) Wang, W.; Li, L.; Xi, S. A Fourier Transform Infrared Study of the Coagel to Micelle Transition of Cetyltrimethylammonium Bromide. J. Colloid Interface Sci. 1993, 155, 369−373. (59) Yamaguchi, N.; Shimazu, S.; Ichikuni, N.; Uematsu, T. Synthesis of Novel Nano-Structured Clays: Unique Conformation of Pillar Complexes. Chem. Lett. 2004, 33, 208−209. (60) Theng, B. K. G.; Newman, R. H.; Whitton, J. S. Characterization of an Alkylammonium-Montmorillonite-Phenanthrene Intercalation Complex by Carbon-13 Nuclear Magnetic Resonance Spectroscopy. Clay Minerals 1998, 33, 221−229. (61) Pratum, T. K. A Solid-State Carbon-13 NMR Study of Tetraalkylammonium/Clay Complexes. J. Phys. Chem. 1992, 96, 4567−4571. H
dx.doi.org/10.1021/la4019212 | Langmuir XXXX, XXX, XXX−XXX
