Langmuir 1993,9, 3649-3655
3649
Structural Characterization of Surfactant and Clay-Surfactant Films of Micrometer Thickness by FT-IR Spectroscopy Kosaku Sugat and James F. Rusling' Department of Chemistry (U-60), University of Connecticut, Storrs, Connecticut 06269-3060 Received September 8, 199F Reflection absorption IR (RAIR) spectroscopy was used for structural studies of films cast from tetraalkylammonium bromides and their composites with colloidal clay. The dependence of p-polarized spectra on the angle of incidence of source radiation was used to find reference transition dipoles. Orientations of the symmetric Si04 stretch of clay in composite films and the symmetric C-N stretch in hexadecyltrimethylammoniumbromide films were found to be nearly normal to their film planes. These bands were used as references to estimate orientations of other transition dipoles. Hydrocarbon chains were found to tilt 20-40° to the normal. Frequencies for CH stretching and bending were sensitive to surfactant hydrocarbon chain conformations, which control the thermotropic phase of these films. Band positions for surfactant films in solidlike and liquid crystal phases were similar to reported values for these respective phases for a lamellarwater-dialkyldimethylammoniumsystem. Phase-sensitiveband frequencies for surfactant in compositefilms gave poor correspondencewith the latter system. Water interacts mainly with head groups in the pure surfactant films, but did not influence RAIR bands in composite films. Results are consistent with a previously proposed multibilayer structure of the films, but clearly reveal the tilt of the surfactant hydrocarbon tails.
Introduction Ordered films of water-insoluble Surfactants' can be prepared by casting their solutions or dispersions onto a solid support and evaporating the solvent. Films of surfactant intercalated between layers of clay platelets2 or linear ionic polymers3g4 have also been prepared by casting. Stable, ordered surfactant films have many possible applications, including controlled permeability membranes,- and coatings for sensors.s We recently incorporated redox catalysts in films of tetraalkylammonium surfactants and clay-surfactant composites. When coated on electrodes, these films in liquid crystal states provided excellent charge transport and catalysis for reductions of organohalide pollutants.88 Proteins have also been incorporated into cast surfactant films.ld-' + On leave from the Department of Biomolecular Engineering, Tokyo Institute of Technology, Yokohama 227, Japan. @Abstractpubliehed in Advance ACS Abstracts, November 1, 1993. (1)(a)Nakashima, N.; Ando,R.; Kunitake,T. Chem.Lett. 1983,15771580. (b) Kunitake, T.; Shimomura, M.; Knjiyama, T.; Harada, A.; Okuyama, K.; Taknyanagi, M. Thin Solid Films 1984,121,L89-91. (c) Ishikawa, Y.; Kunitake, T. J. Am. Chem. SOC.1986,108,8300-8302. (d) Hamachi, I.; Noda, S.; Kunitake, T. J. Am. Chem. SOC.1990,112,67446745. (e) Hamachi, I.; Honda, T.; Noda, S.; Kunitake, T. Chem. Lett. 1991,1121-1124. (0Hamachi, I.; Noda, 5.;Kunitake, T. J. Am. Chem. SOC.1991,113,9625-9630. (g) Ishikawa, Y.; Kunitake, T. J. Am. Chem. SOC.1991,113,621630. (2) Okahata,Y.; Shimizu, A. Langmuir 1989,5,954-959. (3)(a) Shimomura, M.; Kunitake, T. Polym. J. 1984,16,187-190.(b) Kunitake, T.;Teuge, A.; Nakashima, N. Chem. Lett. 1984,1783-1786.(c) Nakashima, N.; Kunitake, M.; Kunitake, T.; Tone, S.; Kajiyama, T. Macromolecules 1986, 18, 1515-1516. (d) Higaahi, N.; Kajiyama, T.; Kunitake, T.; Praas, W.; Ringedorf, H.; Takahara, A. Macromolecules 1987,20,29-33.(e) Naakaahima, N.;Eda, H.; Kunitake, M.; Manabe, 0.; Nakano, K. J. Chem. SOC.,Chem. Commun. 1990,443-444. (0Kunitake, T. Polym. J. 1991,23,613-618. (4)(a)Okahata,Y.; Enna, G.; Taguchi, K.; Seki, T. J.Am. Chem. SOC. 198S,107,5300-53001. (b) Okahata, Y.; Enna, G. J. Phys. Chem. 1988, 92,4546-4551. (c) Okahata,Y.; Enna, G.; Takenouchi, K. J. Chem. SOC., Perkin Tram. 2 1989,835-843. (5)Okahata,Y.; Ebato, H. Anal. Chem. 1991,63,203-207. (6)Rualiig, J. F.; Zhang, H. Langmuir 1991,7,1791-1796. (7)Rusling, J. F.; Hu, N.; Zhang, H.; Howe, D.; Miaw, C.-L. Couture, E. In Electrochemktry in Microheterogeneous Fluids; Mackay, R. A., Texter, J., Eds.; VCH Publiehere: New York, 1992;pp 303-318. (8)Hu, N.; Rusling, J. F. Anal. Chem. 1991,63,2163-2168.
Results of X-ray diffraction, electron microscopy, and comparative phase transition studies have generally supported the proposal that surfactants are ordered in bilayer structures resembling biomembranes in these multilayer films.lb In the composite films, surfactant bilayers are thought to intercalate between clay or polymer layers. Reflection absorption FT-IR (RAIR) spectroscopy can provide considerable information about molecular orientation of thin films on solid surfaces.s18 For ultrathin films containing oriented amphiphilic molecules, spectra from grazing angle RAIR combined with transmission or internal reflectance IR were used1'-" to obtain the molecular tilt angles to the normal to the underlying surface. Also, frequencies of IR bands of methylene groups of amphiphiles arranged in bilayers reflect molecular conformations characteristic of the highly ordered solidlike gel state, or the liquid crystal state.18J9 Thus, we felt that considerable information about molecular orientations of components of cast surfactant and surfactant-composite filmscould be obtained by using RAIR. In this paper, we estimate molecular tilt angles and explore the influence of the phase state and interactions with water for films of dialkyldimethylammonium bromides and clay-dialkyldimethylammonium composites. Because these films can be prepared in micrometer thicknesses, we were able to observe changes in the RAIR (9)Allara, D. L.;Baca, A.; Pryde, C. A. Macrmolecules 1978,11,12151220. (10)Allara, D.L.; Swalen, J. D. J. Phys. Chem. 1982,86,2700-2704. (11)Allala, D. L.;Nuzzo, R. G.Langmuir 1986,1,45-52, 52-68. (12)Porter, M. D.; Bright, T. B.; Allara, D. L.;Chideey, C. E.D. J. Am. Chem. SOC.1987,109,3559-3568. (13)Troughton, E.B.; Bain, C. D.; Whitesidee, G. M.; Nuzzo, R. G.; Allara, D.L.;Porter, M. D. Langmuir 1988,4,365-385. (14)Umemura, J.; Hishiro, Y.; Kawai, T.; Takenara, T.; Gotoh, Y.; Fujihira, M. Thin Solid Alma 1989,178,281-287. (15)Nuzzo, R. G.;Dubois, L. H.; Allara, D. L. J.Am. Chem. SOC.1990, 112,558-569. (16)Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6,682-691. (17)Song, Y. P.; Petty, M. C.; Yanvood, J.; Feaat, W. J.; Teibouklia, J.; Mukherjee, S. Langmuir 1992,8,257-261. (18)Caaal, H. L.; Mantsch, H. H. Biochim. Biophys. Acta 1984,779, 381-401. (19)Umemura, J.; Kawai, T.; Takenaka, T.; Kodama, M.; Ogawa, Y.; Seki, S. Mol. Cryst. Liq. Cryst. 1984,112,293-309.
0143-1463/93/2409-3649$04.00/00 1993 American Chemical Society
Suga and Ruling
3650 Langmuir, Vol. 9, No. 12,1993
For two dipole transition momenta with orientation angles 81 and 02, respectively (Figure l), the ratio of absorption coefficients due to p1 and p2 with grazing angle p-polarized light is
Z
The absorption coefficient for an unpolarized transmission spectrum of a randomly oriented sample, such as a KBr pellet containing film componenta, does not depend on the molecular orientation, and is expressed by Figure 1. Schematic illustration of orientation angles of transition dipoles in a cast f i i . A uniaxial distribution is assumed. normal = z-axis
amr a E2p? I( mlQ# ) l2
(4)
The ratio of absorption coefficients due to the two dipole transition momenta is given by aKB,(q)/aKB,(Ccz)= c(cL32/(P2)2
(5)
Division of eq 3 by eq 5 gives 2
-+ Film plane”
Figure 2. Electric field vectors, their components, and their
relation to the surface and angle of incidence in RAIR using p-polarized light. spectra as a function of the angle of incidence of the source radiation. This allowed the choice of reference transition dipoles with near normal orientations to the surface which could be used in absorbance ratios to obtain tilt angles for the other oscillators in the films. Estimation of Tilt Angles. The systemsto be analyzed by RAIR consist of three phases: surfactant f i b , reflecting substrate, and surrounding gas. Application of a threephase model poses the problem of estimating optical parameters for the surfactant f i i as a function of incident beam energy. Many approximations would be required for such estimates, leading to large uncertainties in their accuracy. Thus, we chose to apply a two-phase model, using absorbance ratios to estimate orientations. This avoids requirements for explicit values of optical constanta. A similar approach was recently used for analysis of organic thin films based on absorbance ratio data from RAIR and attenuated total reflectance (ATR).17 Here, we substitute transmission IR for ATR spectra. The infrared absorption coefficient (a) for vibrational mode i can be expressed by Q O a ~ (E*(WdQi>12 I(mlQilk)12 (1) where E is the electric field vector and p is the transition dipole moment vector associated with the normal coordinate Qj. The initial state k, final state m, and Qi determine the integral ( mlQilk ) . In RA with grazing angle p-polarized incident light, the absorption coefficient is related to the average angle Bpv between dp/dQi (=pi) and the surface normal (Figure 1) by
aMa E2p:1(m(Q#)2 cos2Biav (2) The component of the electric field vector normal to the surface, E,, depends on the angle q5 that the incident light makes with the sample. E, increases with increasing 4 (Figure 2). At grazing angles (4 2 8 5 O ) the direction of the electric field vector is almost parallel to the surface normal. As a result, vibrational modes with dipole transition momenta perpendicular to the surface absorb most strongly.
av
2
av
aM(p1)/aM(r2)= ( U ~ , ( ~ , ) / U ~ ~ ( ~ , ) I ( C OelS /COSe2 I Equation 6 shows that if the orientation angle of one dipole transition moment is known,orientation angles of other transition dipoles in the film can be estimated from the absorbance ratios of p-polarized grazing angle RAIR spectra and those of the transmission spectra of a KBr pellet containing the film components.
Experimental Section Didodecyldimethylammoniumbromide (DDAB) and dioctadecyldimethylammoniumbromide (DODAB)were from Eastman Kodak (99+% ) and used as received. Cetyltrimethylammonium bromide (CTAB, 99.8%)was from Fisher Scientific. Bentonite clay (Bentolite H)was from Southern Clay Products and has a cationexchangecapacity of 80 mequiv/100g. Surfactant(DDAB or DODAB) intercalated clay colloid particles were prepared as described previously.28 Films of surfactants or clay-surfactants were prepared by casting a known volume of organic or aqueous dispersion of these compounds on metal underlayers such as aluminum foil fiied onto a glass slideby double stick tape or a vacuum vapor-deposited aluminum f i i on a glass slide. Spectra were similar on both types of aluminum surfaces. Most of the spectra reported in this paper were collected for f i i on vapor-deposited aluminum. Clay-DDAEand clay-DODAB f i b s were cast from dispersions containing ca. 2.2 mg of clay-surfactant powder in 10 mL of chloroform.8 A 100-pLsample of dispersion was spread evenly onto an Al-coated elide (2 cm?, and the chloroformwas evaporated in air. The approximatethickness of these f i b s estimated from the amount of material used8 was 0.1-0.2 pm. Pure surfactant f i i s were prepared from both aqueous and chloroform dispersions. Thick films(2-5pm) were prepared from 100p L of 10 mM dispersions, and thinner f i i (0.5-1 pm) were prepared by using 200 p L of 1mM dispersionson 2 cml Al slides. Thick DDAB f i i s were also prepared by spin coating. In this method, 500 p L of 10 mM surfactant in chloroform was applied dropwise onto an Al slide rotating at 1800 rpm. Although this latter method seemedto give the most uniform f i i , the thickness (5-25 pm) is quite uncertain since some of the dispersion is lost during rotation of the slide. RAIR and transmission spectra were obtained by using a Mattson Galaxy 6020 FT-IRspectrometer at 4cm-1 resolution. RAIR spectrawere obtained by usinga SPECAC variableincident Spectra of angle accessory with a wire grid polarizer of -5. the aluminum underlayer were subtracted as background. A DTGS detector or a liquid nitrogen cooled MCT detector was used. Enhanced FIRST software was used for the control of the spectrometer and analysis of the data. Results and Discussion Spectra of Clay-Surfactant Films. Figure 3 shows normalized RAIR spectra of a clay-DDAB film cast onto
Langmuir, Vol. 9, No.12,1993 3651
Surfactant and Clay-Surfactant Films
Q
For p-polarized light, components of the electric field vector are
= 30'
E, = Ecos&, Ey = 0 , and E, = E s i n &
--
0 n 4
IC
(8) The angle that the transition dipole moment of vibrational mode i makes with the surface normal is 8i. Components of the transition dipole moment vector of mode i are
-
0 6 = 70'
pi,
Q = 80'
4000 Q = 85'
3000
2000
Wavenumbers
I
1000
= pi sin Bi cos b,
Table I. Absorbance Ratios and Orientation Angles of Transition Dipoles of Vibrational Modes in Clay-DDAB Films
0.026 0.515 0.051 77 3636 0.270 1.432 0.188 64 2920 0.178 0.973 0.183 65 2845 0.460 47 0.257 0.559 1468 3.511 -1 4.118 -0 1099 1.105 6.044 0.183 65 1003 631 1.000 1.000 1.000 (0) 59 3.000 0.263 521 0.789 0.743 4.470 0.166 66 465 0 Symbola for all tables: Y, stretching; v,, asymmetric stretch, or antisymmetric for CHa; v,, eymmetic stretch 8, bending; y,rocking. b Grazing angle, 6 = Eo. 0 Peaks not assigned.
vapor-deposited aluminum from a dispersion in chloroform. The spectra depended strongly on the angle of incidence of the p-polarized light. Frequencies and probable assignments of each peak of the spectra of the clay-DDAB film are shown in Table I. The most intense group of bands are in the 1000-llOO-cm-l region and consist of two or three peaks. These are assigned to Si-0 stretching modes in the clay layers.20 Relative intensities of these peaks change drastically with the angle of incidence. The relative intensity of the 1099-cm-l peak is largest at & = 85', but becomes very small at & = 10'. On the contrary, the relative intensity of the 1003-cm-l peak is largest at & = 10' and is only a small shoulder at & = 85'. The relative intensity of the 631-cm-l peak assigned to Si-0 bending varied with & similar to the 1099-cm-l peak. Peaks at 2920 and 2845 cm-l are assigned to the antisymmetric and symmetric stretching modes of the methylene groups of the surfactant. Their relative intensities are very small at & = 85' and increase gradually with decreasing 4. The 3636-cm-' peak assigned to the clay OH stretch behaved like the CH2 stretching modes, but the relative intensity at 85' was smaller than those of CH2 Stretching modes. From eq 2, the absorption coefficient for vibrational mode i is proportional to the square of the scalar product of the electric field vector E and the transition dipole moment vector pi. (20) Stubican, V.; Roy, R. J. Am. Ceram. SOC.1961,44,625-627.
sin B1 sin, b,
pi,
= pi cos Bi
(9) where b is the angle between the x axis and the projected vector of the transition dipole moment vector pi'. From eqs 7-9, the absorption coefficient for mode i is given by aob a E2p:{sin2 0, cos2& cos2b + cos2Bi sin2&] (10)
cm-I
Figure3. ppolarized RAIR absorbance spectraof a clay-DDAB f i at various angles of incidence (normalized to the height of the largest peak in each spectrum).
piy =
These transition dipoles are randomly distributed in the x-y plane (Figure 1). Integration of eq 10 (0 to 274 with respect to b and multiplication by (2s)-l yields the average absorption coefficient aob a E2p:{(1/2) sin2Bi cos2 & + cos2Bi sin2$1 (11) In RA spectroscopy, the path length 1 of the incident light through the sample is determined by the film thickness d and the angle of incidence, and its approximately21 1 = dlcos & (12) Therefore, the absorbance (Aob)of mode i for RA with p-polarized light follows the relation AOb
a
dE2p~{(1/2)sin2 Bi cos & + cos' Bi sin2 &/cos&]
(13) At conductive surfaces in RA, there are standing waves of light just on the surface with an electric field vector component of zero parallel to the surface and a maximum component normal to the surface.21 The magnitude of E is a periodic function of the distance (z)from the surface. The periodicity is on the order of the wavelength of the IR light and depends on the angle of incidence. If we use films that are very much thinner than the wave length, eq 13 reduces to eq 14, because E, = 0 on the metal surface.
Aob a dE2p: cos' Bi sin2&/cos& However, the thicknesses of films used in this study are roughly 0.1-5 pm, while the wavelength of 4OOO-cm-l light is 2.5 pm. Thus, we cannot neglect the contribution of the x component of transition dipole moments to the absorbance. Therefore, the absorbance is represented by
A,,
0:
dE2p:{cos2 Bi sin2 &/cos& + C(d) sin2Bi cos &I
where C(d) is a constant which depends on the film thickness and optical properties of the film and the metal underlayer. Absorbances of peaks in the clay-DDAB RAIR spectra vs & were fit with theoretical curves according to eq 15 using the best fit parameters. Absorbances for 1099- and 631-cm-* peaks were fit well with curves (Figure 4) (21)McIntire, J. D. E. Specular Reflection Spectroecopy of the Electrcde-SolutionInterphase. In Advance8 in Electrochemistry and Electrochemical Engineering; Delahay, P., Tobias, C. W., Ed&;Wiley: New York, 1973; Vol. 9, pp 61-166.
Suga and Rusling
3652 Langmuir, Vol. 9, No. 12, 1993 1.oo
0.50
0.80
0.40
F
d) C
0.60
P
k.
r
C
= 30'
1 -E
-E 8
0.3 0
9 = 10'
h
-a
0.40
A
0.20
4
a
0.20
0.1 0
0.00 90
0.00
30
0
60
0
9 = 70'
v)
4: J2
r I
-
angle of incidence +
2928 om- 1
A
lOS9 om- 1
9 = 80'
851 om- 1
Figure 4. Influence of the angle of incidence on the absorbance of a clay-DDAB film.Theoretical lines computed by using eq 15 in the following forms: 631 cm-l, A = 0.03 sin2 COS 4; 1099 cm-l, A = 0.13 sin24/cos 4; 2969 cm-l, A = 0.01 sin2 C COS 4 + 0.02 cos 4.
4000
3000
2000 Wavenumbers
/
1000
cm-l
Figure 6. p-polarized RAIR spectra of CTAB films at various angles of incidence (normalizedto the height of the largest peak in each spectrum).
h
a6
0.40
0.10 0.00
1
/I I
-
A
0
Si -
0 0 (0; OH '..a
-
Figure 5. Layered structure of a clay platelet.
calculated on the assumption of 8i = 0, i.e., using only the first term on the right-hand side of eq 15. These results suggest that the transition dipole moments for 1099- and 631-cm-1peaks are nearly normal to the underlying surface. Transition dipole moments of symmetric and antisymmetric stretching of CH2 are both perpendicular to the long axis of an all-trans methylene chain. Absorbances of films for these modes at 2847 and 2926 cm-l (Figure 4) could not be fit by the expressionwith 8i = go", but required both terms in eq 15 for adequate fits. This suggests that 8i is different from 90" for these methylene stretching modes and that the methylene chains tilt away from the surface normal. X-ray diffraction analysis also suggested chain tilt in these composite films.2 Films made from clay colloids consist of stacked long aluminosilicate platelets, which are about 10 A thick (Figure 5) and of micrometer-range lengths. Neutron diffraction analysis showed that 90% of the platelets are oriented parallel to the underlying surface.22 A similar orientation was proposed for clay-surfactant films? Thus, the symmetric stretch of the average tetrahedral Si04 should have a transition dipole moment normal to the surface, while the asymmetric stretching mode is not normal. This structural picture agrees with the assignment of the 1099-cm-1 peak to the symmetric stretch of Si04 normal to the underlying surface. Spectra of Surfactant Films. Figure 6 shows p-polarized RAIR spectra of a CTAB film at several angles of incidence. The absorbanceof the 912-cm-1peak increased with 4. The behavior of this peak (Figure 7), assigned to (22) Cebula,D.J.; Thomas,R. K.; Middleton,S.;Ottewill, R. H.;White, J. W . Clay Clay Miner. 1979,27,39-52.
I
.
.
. . .
I
.
. . . .
60
30
90
angle of incidence +
912 cm- 1
2850 cm- 1
A
1481 om- 1
Figure 7. Influenceof the angle of incidence on the absorbance of a CTAB film. Theoretical lines computed by using eq 15 in the following forms: 912 cm-l, A = 0.005 sin2 C COS 4; 1481 cm-l, A = 0.0025sin2@/cos4 + 0.008 cos 4; 2850 cm-l, A = 0.0027 sin2 4/cos 4 + 0.027 cos 4.
the C-N stretch of the head groups, is similar to the 1099cm-l peak in clay-surfactant films. The theoretical curve for 8i = 0 fits these data well. These results suggest that the transition dipole moment of the stretching mode of this C-N bond is almost normal to the metal surface. On the other hand, absorbance vs 4 data for other peaks in this spectrum are fit well only with theoretical curves for a nonzero 8i. Figure 8 shows the dependence of absorbances from p-polarized RAIR spectra of a thick DDAB film on 4. Absorbances at 2853 and 2927 cm-l vs 4 gave reasonably good fits to eq 15, but results suggested that 8i for these dipoles are not greatly different from 90". The absorbance of the 891-cm-1 peak assigned to the C-N stretch increased with increasing angle of incidence, but the data do not fit well at large 4 for 8i = 0. This suggests that the average orientation of the C-N stretch transition dipole deviates significantly from zero. Similar results were found for DODAB films. Molecular Orientationsin Surfactant-ClayFilms. Reasonably good fits of absorbances to eq 15 suggest the approximate validity of the two-phase model for the systems analyzed. Average orientation angles of transition dipole moments can now be estimated from the p-polarized grazing angle RAIR spectrum of the film along with the transmission spectrum of the randomly oriented sample (cf. eq 6). According to results discussed above, the
Surfactant and Clay-Surfactant F i l m
Langmuir, Vol. 9, No. 12, 1993 3663
0.50
1.00
0.40
0.80
Table 111. Orientation Angles of Transition Dipoles in CTAB Films*
I W - 0
0.30
0.60
0.20
0.40
0.1 0
0.20
0.00
2
n
$
f
0.00
30
0
90
60
angle of incidence +
2827
om- 1
0
zssa
801
om- 1
om- 1
Figure 8. Influence of the angle of incidence on the absorbance of a DDAB f i i . Theoretical lines computed by using eq 15 in the following forms: 891 cm-l, A = 0.013 sin2$/cos 4; 2853 cm-l,
A = 0.0025 sin2$/cos $ + 0.30 cos $; 2969 cm-l, A = 0.004 sin2 $/cos $ + 0.47 cos 4. Table 11. Orientation of Transition Dipoles of Vibrational Modes in Clay-Surfactant Films orientation anale (dex) c~Y-DDAB c~~Y-DODAB frequency new agedo new agedo (cm-1) mode 77 69 3636 v(OH) 71 62 70 2920 vi(CH2) 64 64 66 65 70 2845 vdCH2) 48 46 56 56 1468 WH2) -0 -0 -0 -0 1099 v,(Si-0) 1003 v.(Si-0) 65 62 50 631 b(Si-0) (0) (0) (0) (0) 59 59 62 57 521 6 6 6 4 50 65 465 37 28 tilt angle of 40 32 methylene chain 0
Several weeke in air.
transition dipoles of the 631-cm-l (Si04 symmetric bend) and the 1099-cm-l (Si04 stretch) bands for clay-DDAB and clay-DODAB films are normal to the surface. Since the 1099-cm-1 peak is overlapped by another peak, we used the 631-cm-l band as the standard. Average orientation angles of other vibrational modes in these films were estimated from eq 16,obtained from eq 6 by replacing absorption coefficients with absorbance A
where denotes the transition dipole for the reference vibration with Bi = 0. Ratios of absorbances and estimated values of cos2Bi and 8:' are listed in Table I. The value of cos2B p ' for the 1099-cm-1peak is close to 1,confirmingthat the transition dipole moment for this vibrational mode is almost normal to the surface. Table I1 shows the average orientation angles of transition dipole momenta for vibrational modes in freshly prepared and aged clay-DDAB and clay-DODAB. Transition dipole momenta for symmetric and antisymmetric stretching modes of CH2 are normal to the long axis of an all-trans methylene chain.= If we denote the relevant angles from the surface normal, a(va, CH2), Ob,, CH2), (23)(a) Hydrocarbon chains in DODAB bilayers are known to be in the all-trans gel state at ambient temperature? DDAB films are liquid crystals at ambient temperature,%'and aome fraction of the C-C bonda will be in gauche ~ ~ n f ~ r m a t iHowever, ~ ~ . a b the majorityconformation of the cheina is still trans. The fraction of gauche bonda in DDAB films is not expectedto introduce significant error intothe tilt angle eatimates, which have estimated errorsl?of + 4 O . (b) Nagle, J. F. Annu. Rev. Phys. Chem. 1980,31,157-195.
2917 2874 2851 1481 1462 1436 1383 912 718 0
0.0883
8.950 1.521 6.754 1.485 1.982 0.478 0.335 1.OOO 0.822
73 71 73 51 70 67 56
0.1060 0.0824 0.3961 0.1162 0.1477 0.3009
1.oooO
(0)
0.1819
65
Thin film cast from aqueoussolution. b Grazing angle, $t = So.
Table IV. Orientation Anglee of Transition Dipoles of Vibrational Modes and Tilt Angles of Methylene Chains in CTAB Films caet from CHCh cast from water frequency thin0 thicke t h i n 0 thicke (cm-9 mode 2917 va(CH2) 74 73 73 60 2851 dCHd 73 73 73 63 8.(CHrN+) 50 48 51 38 1481 1462 W32) 69 70 64 1436 UCHS-N+) 67 912 vs(C-N) (0) (0) (0) (0) 718 r(CH2) 66 65 48 tilt angle of 23 24 24 42 methylene chain 0.5-1 pm. 2-5 pm. Table V. Orientation of Transition Dipoles of Vibrational Modes in DDAB and DODAB Films* frequency (cm-l) 2927 2853 1468 721 tilt angle of methylene chain 0
orientation angle/degree DDAB cast from DODAB cast from mode CHCh va(CH2) 75 vdCH2) 77 b(CH.2) 69 r(CHd 20
Ha0
CHCh
Hz0
76 78 69
75 76 63 56 21
79 79 70 68 16
19
Reference band WBB CTAB 912 cm-1; see text for detaile.
and methylene chain tilt angle y, we have
+
+
cos2a cos2p cos2 y = 1 (17) Knowing a and 8, y can be found from eq 17. Results show (Table 11) that hydrocarbon chains in freshly prepared films of clay-DDAB and clay-DODAB have larger tilt angles than aged films. Errors are estimated" at f10% or f 4 O . Okahata and Shimizu2estimated tilt angles of methylene chains in several surfactant-intercalated clay films from the clay interlayer separation determined by X-ray diffraction, assuming that intercalated surfactant molecules form bilayers. Our resulta are in reasonably good agreement with their estimated values of 30° for clay-DDAB and 39O for clay-DODAB films. Molecular Orientations in Surfactant Films. The dependence of RAIR spectra of CTAB f i s on C$shows that the orientation angle of the transition dipole moment for the 912-cm-l C-N symmetric stretch is almost normal to the surface. Thus, we estimated the orientation for several vibrational modes in CTAB films on the assumption of B(912 cm-l) = 0. Table I11 shows orientation angles for several vibrational modes for films of different thicknesses. The orientation angle of the transition dipole for the 1436-cm-1 band for symmetric bending of methyl groups
3654 Langmuir, Vol. 9, No. 12,1993
Suga and Rualing
Table VI. Comparison of Band Frequencies for DDAB Films with the 21% Water-DODAC System in Different Phases 21 % water-DODACO DDAB filmsb assignment coagel gel liquid crystal thinc spin coated thickd thickdr + water vn(CHdN+)) 2979.5 2980.7 2979.5 2998 (?) 2998 (?) 3003 (?) va(CHs(N+)) 2953.6 2953.0 2954.5 2955 (IC) 2949 (?) 2947 (?) 2957 (?) v&Hd 2916.1 2920.3 2923.3 2922 (IC) 2920 (g) 2922 (IC) 2922 (IC) vJCHZ) 2851.1 2850.9 2853.5 2853 (IC) 2853 (IC) 2854 (IC) 2853 (IC) WHz) 1471.7 1469.7 146M 1464 (IC?) 1466 (IC) 1468 (IC) 1466 (IC) b(CHdN+)) 1419.1 1423.2 1422.4 1408 (?) 1410 (?) 1410 (?) 1420 (?) v(C-N) 909.9 914.1 919 891 (?) 891 (?) 891 (?) 924 (IC) r(CHz) 717.8 723.2 722 721 (IC) 721 (IC) 721 (IC) 0 Data from ref 19. b Abbreviations give correspondence to 21 % water-DODAC bands assigned on the basis of phase: (c) coagel; (g) gel; (IC) liquid crystal. e 0.5-1 jim cast from CHCls. d 2-5 pm. * Soaked in water for 20 min.
attached to the head group nitrogen is 67O. Since the symmetric C-N stretch is almost normal to the surface, this suggests that three methyl groups in a given head group are all in a plane parallel to the film surface. The direction of the transition dipoles of these methyl groups coincides with that of the C-N bonds in the head group. The four C-N bonds in the head group have almost tetrahedral symmetry, with an expected 70" angle of the transition dipole for the methyl symmetric bend, in good agreement with our experimental result. CTAB films cast from CHC13 gave virtually identical spectral band shapes and dipole orientations for films of different thicknesses (Table IV). Similarly, reproducible spectra were obtained for DDAB films of different thicknesses. These results suggest that band distortione for thicker surfactant films is not a significant problem for the type of analysis applied here. However, thicker films cast from aqueous dispersions led to the estimation of somewhat different tilt angles for the hydrocarbon chains. These films had a physical appearance suggestive of polycrystallinity. Thus, films of these surfactants cast from water may have structural differences from those cast from chloroform. Since we did not find an appropriate reference peak in DDAB and DODAB films, we used the 912-cm-l peak of CTAB films as an external reference. We measured the glancing angle RAIR spectrum of the sample and that of a reference CTAB film under the same conditions. We estimated Amr(pi)/Amr(h)and Am(p(i)/Am(h)for the desired peak in the sample film with the 912-cm-l peak as the reference in the CTAB film. Knowing the weights of surfactants in the sample ( W i , f ) and reference films (wo,f) and in the KBr pellets (Wi,p and W O , ~ )0:", was estimated from
Table VII. Comparison of Band Frequencies for DODAB Films with Those of the 21% water-DODAC System in Different Phases 21% water-DODACC DODAB f i h b liquid from from CHCh aqdisp coagel gel crystal 2979.5 2980.7 2979.5 2980 (c, g) 2953.6 2953.0 2954.5 2953 (9) 2955 (IC) 2916.1 2851.1 1471.7 1419.1 909.9 717.8 a Data
2920.3 2850.9 1469.7 1423.2 914.1 723.2
2923.3 2853.5 1467.2 1422.4 919 722
2915 (c) 2849 (g) 1470 (g) 1418 (c) 718 (c)
2926 (?) 2849 (g) 1470 (SI 1420 (c) 910 (c) 719 (c)
from ref 19. 2-5 pm thick; abbreviations as in Table VI.
and DODAB, 44 "C. Thus, a t ambient temperature of the IR experiments, f i b s containing DDAB are lamellar liquid crystals, while those containing DODAB are most probably in the gel state. We wished to see how closelythe band positions assigned to gel and liquid crystal phases of the lamellar 21% waterDODAC system agreed with those of the films in gel and liquid crystal states. Table VI lists the characteristic frequencies for main peaks of 21% water-DODAC in various thermotropic phases with prior assignments of vibrational modes.lg Table VI also lists frequencies of several DDAB films and denotes the phase that would be derived by comparison with the 21% water-DODAC assignments. Table VI1 lists similar results for DODAB films. Although some small differences in frequencies are evident, peaks due to vibrational modes of CH2 in the DDAB films are similar to those assigned to the liquid crystal phase of water-DODAC, while those for DODAB films are similar to bands for the gel or coagel phases of cos2By = [(Am(pi)/Wi,f)/(ARA(po)/WOf)j/((AKBr(pi)/Wi,p)/ water-DODAC. Results show that the IR spectra of CH bands can be used to indicate whether the cast films are (Amrbo)/~ 0 , 1~ ()18) in the liquid crystal or gel phase. Orientation angles of vibrational modes in DDAB and The positions of bands due to head group vibrations in DODAB films estimated in this way (Table V) suggest DDAB films were shifted significantly from the reported that methylene chains tilt a t angles similar to those found characteristic peak frequencies for 21 % waterDODAC. for CTAB films. However, those for DODAB films agree almost invariably Influence of the Thermotropic Phase. Umemura et with assignment to an ordered, solidlike phase expected al. showed that the frequency and width of the peaks of at ambient temperature. This result may be related to IR spectra of dioctadecyldimethylammonium chloride the fact that the IR spectra for DODAB films indicate (DODAC)containing 21 % water change characteristically significant water present, as shown by the band a t about upon phase transitions from coagel to gel and gel to liquid 3400 cm-l, while spectra for DDAB films suggest much crystal.lB Coagel and gel are ordered phases characterized less water with no band in the 3400-cm-1 region. The by all-transmethylene chains in bilayers, while the lamellar differences of these peak positions for DDAB films, liquid crystal phase contains a fraction of gauche C-C bonds in the chains.18 Characteristic temperatures (T,) therefore, may be attributed to smaller interactions of DDAB head groups with water, since very little water is for gel to liquid crystal phase transitions have been present in these films. In agreement with this idea, observed by differential scanning calorimetry for all of frequencies of the head group bands of DDAB shift when the films studied in the present work. Their values are2"3 the films are treated with water (Table VI). clay-DDAB, 5 "C; clay-DODAB, 54 OC; DDAB, 10-12 OC;
Surfactant and Clay-Surfactant Films
Langmuir, Vol. 9, No. 12, 1993 3655
Table VIII. Comparison of Band Frequencies for Clay-Surfactant Films with the 21% water-DODAC System in Different Phases 21% water-DODAC' aeeimment coaael
liquid gel 2920.3 2870.4 2850.9 1469.7 1419.7
clay-DDABb clav-DODAFP 2924 (IC) 2888 (?) 2849 (9) 1468 (IC) 1418 (c)
crystal
2923.3 2926 (IC?) 2916.1 v.(cH~) u,(CHp-R) 2870.5 2870.2 2886 (?) v,(CH~) 2851.1 2863.5 2849 (g) 1467.2 1468 (IC) G(CH2) 1471.7 1423.2 1418 (c) a,(CH,R) 1419.1 a Data from ref 19. Abbreviated as in Table VI.
0 C
5 L
VI 0
n 4
2800
2900
3000
Wavenumbers f cm-l
I
I I
I
&
(b)
Figure 10. Simplified illustrations representing surfactant orientationsin smallportions of (a)doublechain surfactantf i i s and (b)clay-surfactant films. solution results in the permeation of water molecules into the film and the strong interaction of water with the head groups in the films. Water permeation can also be inferred from the observation of film swelling in water.24 Furthermore, permeation of water molecules may cause a rearrangement of film structure, especially in the region of the head groups. A further indication of a structural difference upon adding water is the large difference in the gel to liquid crystal phase transition temperature reported for dry and wet DDAB films.24 On the other hand, treatment of a DODAB film with 0.1 M KBr solution did not cause significant changes of the RAIR spectrum. This is probably because the DODAB films in the gel state already contained water molecules as described above. Soaking of clay-DDAB and clay-DODAB films in aqueous 0.1 M KBr for 5 days did not cause significant changes in the RAIR spectra. This suggests that water molecules do not greatly influence the composite film spectra in the presence of KBr. Conclusions
I
I\
I
ce 3
N c
Dry F i l m
1400
1200 Wavenumbers
1000
/
800
cm-l
Figure 9. Effect of water on RAIR of a DDAB film in two frequency regions. Table VI11 shows comparisonsof the CH frequencies of clay-surfactant films compared to the 217% water-DODAC system. For these composite films, agreement with DODAC phase assignments differs from peak to peak. This suggests that the influence of thermotropic phases on the frequencies of methylene chains in these intercalated films are different from those in the pure surfactant films. This may reflect strong interactions of surfactant molecules with clay. Effect of Water. Catalytic applications of surfactant films take place in aqueous solutions, so we briefly investigated the effects of water on the vibrational spectra. Figure 9 shows the p-polarized RAIR spectra (4 = 60') of a DDAB film before and after soaking in aqueous 0.1 M KBr for 20 min. Characteristic changes after treatment with water include (1)the broadening of almost all peaks, (2) the shift of wavenumbers for some peaks for the vibrational modes of head groups (cf. Table VI), and (3) the remarkable decrease of the intensity of peaks assigned to symmetric deformation of N-methyls in the head group and symmetric stretching of C-N bonds. These results suggest that soaking of a cast DDAB film in an aqueous
The combination of RAIR and transmission IR spectra provided information about molecular orientations, waterhead group interactions, and phases of films cast in micrometer thicknesses from cationic surfactants and surfactant-clay composites. Orientations of hydrocarbon chains in all the films are consistent with surfactant molecules residing in lamellar bilayer structures, as proposed earlier.lG In addition to wavy stacks of bilayers, a fraction of regions which appeared disordered and may or may not contain surfactant bilayers was observed in the micrometer size range by scanningelectron microscopy (SEM)of cross sections of DDAB films.26 Thus, the general structural picture of the pure surfactant fiis that emerges from the accumulated data features bilayer regions parallel to the film plane, coexisting with some regions having a different appearance by SEM. RAIR results showed that hydrocarbon chains in all the films have an average tilt to the surface normal (Figure lo),as suggested previously for clay-surfactant composites on the basis of X-ray diffraction measurements of clay interlayer spacings.2tM In contrast to such alternatives, RAIR spectra allow a more convenient and rapid estimate of the average tilt angle of the hydrocarbon chains. Acknowledgment. This work was supported by US. PHS Grant No. ES03154awarded by the National Institute of Environmental Health Sciences. (24)Hu, N.;Howe, D. J.; Ahmadi, M. F.; Rueling,J. F. AM^. Chem. 1992,64,3180-3186. (26)Rueling, J. F.;Ahmadi, M. F.; Hu, N. Langmuir 1992,8,24562460.
