Langmuir 1992,8, 1354-1359
1354
Molecular Orientation of a Black Soap Film Studied by Polarized Fourier Transform Infrared and Ultraviolet-Visible Absorption Spectroscopy Yongchi Tian Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China Received June 25,1991. In Final Form: November 18, 1991
Polarized Fourier transform infrared and ultraviolet-visible absorption spectra of black soap films withdrawn from the M aqueous solution of cetyltrimethylammoniumchloride (CTAC)containing 10-2 M methyl orange (MO)are recorded below and above the gel-to-liquid crystal phase transition temperature of the film. Quantitative evaluation of the tilt angles from the surface normal is made for the transition moments of the CH2 stretching modes of CTAC and of the YT* absorption of MO by using Hansen's optical formulas for thin films. In the gel film, the tilt angle of the hydrocarbon chain axis of CTAC and that of MO are found to be approximately identical (ca. 39O),indicating that the packing and orientation are similar between the CTAC and MO layers. In the liquid crystalline film, the calculated results of orientation are discussed in connection with order parameters by considering the disorder of the chain conformation and molecular motion. The hydrocarbon chains possess an orientational order parameter of 0.22,suggesting a small order of their orientation, while the MO molecules have an order parameter near zero, implying random orientation. The ultraviolet-visible spectra of both the gel and liquid crystalline films give a T-T* absorption band at 410 nm which is ascribable to the formation of H-aggregate.
Introduction
It is well-known that surfactants in aqueous solutions adsorb and orientate themselves at the interface between the aqueous solution and air, forming a gadliquid interfacial monolayer. Thus, by withdrawing a metal frame from the surface of such a solution to air, a single planar soap film can be easily prepared. At the beginning, the thickness of the film is fairly large, due to the existence of a rather thicker aqueous core between two surfactant monolayers. With time, however, the core water drains downward and the film undergoes a self-organization until an equilibrium is finally achieved. At equilibrium, the film is sufficiently thin (4-50 nm) to appear black by reflected light and thus is called black soap The black soap film has a sandwich structure consisting of a thin layer of aqueous core enclosed between two surfactant mon0layers.2*~*5 One of the crucial characteristics of a black soap film is the molecular orientation in the two surfactant monolayer^.^^ A better understanding of this point will lead to establishment of structural reality of such an ultrathin self-assembling film. On the other hand, the black soap films in air possess, in various respects, physicochemical characteristics which are intrinsically similar to those of biolipid membranesS2Functional processes occurring in the biomembranes are of essential interest and largely dependent on the precise location and orientation of molecules present in the membranes, as attested by an outstanding example of the recent determination of the three-dimensional structure of the bacterial photosynthetic (1)Clunie, J. S.; Goodman, J. F.; Ingram, B. T. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley-Interscience Press: New York, 1971; Vol. 3, p 167. (2) Ivanov, I. B. Thin Liquid Film, Marcel Dekker, Inc.: New York, 1988. (3) Mysels, K. J.; Shinoda, K.; Frankel, S. P. Soap Films, Studies of Their Thinning and a Bibliography, Pergamon: Oxford, 1959.
(4) Corkill, J. M.; Goodman, J. F.; Ogden, C. P.; Tate, J. R. Proc. R. SOC.London, A 1963,273,84. (5) Lyklema, J.; Scholten, P. C.; Mysels, K. J. J . Phys. Chem. 1965,69, 116. (6) Umemura, J.; Mataumoto, M.; Kawai, T.; Takenaka, T. Can. J. Chem. 1986,63, 1713.
reaction centers.' However, no attempt has been made to quantitatively evaluate the molecular orientation in black soap film. Although polarized spectroscopy was well established for the study of oriented thin films, quantitative analysis of orientation based on thin film optics was developed very recently.g10 Hansenll derived the rigorous optical formalisms for stratified layered medium. Umemura et ala6performed an excellent fitting between the calculated and experimentally obtained results of the absorption profile of the aqueous core of a black soap film. Furthermore, t h e P l 0 recently succeeded in quantitatively evaluating the molecular orientation in thin LangmuirBlodgett films by applying the formalisms. In a previous article,12 it was reported that a gel-toliquid crystalline phase transition13J4occurred a t 34 "C (T,) in the black soap film prepared from the M aqueous solution of cetyltrimethylammonium chloride (CTAC) containing M methyl orange (MO).The
thickness of the aqueous core of the film was estimated to be ca. 5 nm above T,and ca. 1 nm below T , by the infrared absorption intensity of the core water band at ca. 3400 cm.-l It was already realized that the CTA+ cations preferentially bind to MO- anions to yield a 1:l interaction product in aqueous solution,1sand the interaction product (7) Deisenhofer, 3.; Michel, H. Angew. Chem.,Int. Ed. Engl. 1989,28, 829. Huber, R. Angew. Chem., Int. Ed. Engl. 1989,28, 848. (8) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J.Phys. Chem. 1990, 94,62. (9) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989,5, 1378. (10) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (11) Hansen, W. N. J. Opt. Soc. Am. 1968,58,380. (12) Tian, Yongchi J. Phys. Chem. 1991,95,9985. (13) The terminology gel-to-liquid crystalline phase transition is adopted here according to ref 14. Virtually, it is probably appropriate
to refer this transition to a liquid-to-liquid crystalline one. (14)Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56. (15)Hiskey, C. F.; Downey, T. A. J. Phys. Chem. 1954,58,835.
0743-7463/92/24O~-1354$03.oo/o0 1992 American Chemical Society
Molecular Orientation in Black Soap Film
formed an adsorbed monolayer a t the oil/water interface.16 The orientation of the chromophoric part (MO) of this monolayer was investigated by resonance Raman spectroscopy.16 However, the orientation of the hydrocarbon chain of CTA+ in the monolayer could not be studied by the Raman method. In the present paper, the molecular orientation in the black soap film was quantitatively examined below and above T, by polarized Fourier transform infrared (FT-IR) and ultraviolet-visible (UVvis) absorption spectra by using Hansen’s optical equations. The structural aspects relating molecular orientation in the black soap film are discussed on the basis of the calculated results.
Model of the Black Soap Film In order to apply Hansen’s optical formalisms to the calculation of molecular orientation in the black soap film examined, it is essential to establish a layered model for the film although the sandwich structure is generally accepted. Previous study on this film12demonstrated that MO- anions bind to CTA+ monolayers in the film. Thus, the problem is the location of MO- in the film. Two fashions are possible. First, the MO- anions incorporate with the hydrocarbon chains of CTA+ in the monolayers, directing their sulfonate groups toward the aqueous core. Second, the MO- anions form a separate layer on the positively charged surface of CTA+ monolayers, binding their sulfonate groups to the monolayer surface, as appeared in the alkylammonium bilayer membranes bound to MO- ani011s.l~ If MO- incorporates with the hydrophobic part of CTA+ in the monolayer, the space between hydrocarbon chains in the tail part becomes much wider than that of close packing and it will increase the population of gauche conformer in the chain as observed in the case of cholesterolls and aromatic compoundslg incorporated within the lipid bilayers, where the antisymmetric frequency of the lipid acyl chain increased with the incorporation. This is certainly not the case in the present gel film as deduced from the CH2 stretching frequencyof 2918 cm-l (seeFigure 2) which is characteristic of the trans-methylene chains.12 On the other hand, from a UV-vis absorption spectroscopic study on MO in the micellar solution of poly((vinylbenzyl)triethylammonium chloride) (PVBTEA),20it was concluded that a monomeric band is observed at 460 nm when MO- anions are located in the hydrophobic region of the micelle of PVBTEA while this band is blue shifted to 410 nm when MO- anions form aggregates on the positively charged surface of the micelle. Furthermore, such a blue shift was observed in MO bound to the surface of a synthetic bilayer membrane, which was undoubtedly attributed to the formation of MO aggregates on the surface.17 It was this aggregationthat facilitates a liquid crystal-to-gelphase transition of the bilayer membrane.17 Since the UV-vis spectra of the black film of the present study show an absorption band a t 410 nm in both gel and liquid crystalline states (see Figure 31, the formation of MO- aggregates in the films will be suggested vide infra. These arguments indicate that the spectroscopic results observed in the present film cannot be interpreted by assuming the incorporation of MO- with the CTA+ (16) Takenaka, T.; Nakanaga, T. J . Phys. Chem. 1976,80,475. (17) Nakaehima, N.; Fukuehima, H.; Kunitake, T. Chem. Lett. 1981, 1555. (18) Umemura, J.; Cameron, D. G.; Mantach, H. H. Biochim. Biophys. Acta 1980, 602, 32. (19) Szalontai, B. Biochem. Biophys. Res. Commun.1976, 70,947. (20) Quadrifoglio, F.: Crescenzi, V. J . Colloid Interface Sci. 1971,35,
447.
Langmuir, Vol. 8, No. 5, 1992 1355
hydrocarbon chains in the monolayer and suggest that MO- form a separate layer on the CTA+monolayers in the film, supporting the second fashion stated above. There fore, a seven-phase plane-bounded system of air/CTA+ layer/MO- layer/water/MO- layer/CTA+ layer/air will be considered for this black soap film. In this model, we have to consider the existence of MO/water interface. Although the methyl group of the amine on MO is hydrophobic,the nitrogen atom of the amine shows obvious electron donor property and fairly strong hydrogen bonding activity.21 In fact, the alkylamine has been exhibited to play a hydrophilic role in water surface monolayer.22 Moreover, it was found that the azo moiety tends to bind to water in a cetyl orange monolayer on the water surface, where the UV-vis absorption maximum is also This suggests that the azo moiety located at 410 nm3.23-25 of MO may contribute an additional hydrophilic effect even in the form of aggregate. Taking all these into account, the above proposed seven-phase model is most likely for the present film.
Calculation of Molecular Orientation Suppose a transition moment is uniaxially oriented with an angle C#J from the axis z (Figure 1) normal to the film surface (the xy plane). The plane of incidence is the x z plane. When the radiation is obliquely incident upon the film surface, the band intensity in the p-polarization transmission spectrum depends upon the contribution of two parts, the x component of the imaginary part of the complex dielectric constant ex” multiplied by that of the mean-square electric field ( E x 2 ) and , e/ multiplied by (Ex2). On the other hand, the band intensity in the spolarized transmission spectrum is proportional to e/ (Fy2). The e X ” ( = c / ) and e / values can be obtained by consideringthe contributions from all transition moments uniformly distributed at an angle around the z axis, Le., by integrating the contributions over the projected angle $ of the transition moment in the xy plane from 0 to 2 ~ . ~ Thus, the ratio between the p- and s-polarizedabsorbances, A, and A,, are related to the orientation angle C#J by
Here, the relation E“ = 2nk and the isotropic assumption of the refractive indices n, = n, = n, are used. Therefore, ifwecanestimatethe ( E x 2 ) / ( E y 2and ) (Ez2)/(Ey2) values, we can get the orientation angle r#J from the observed ratio between A, and A,. Now, the absorbances in the p- and s-polarized spectra of thin isotropic films can be calculated by using Hansen’s equations.” For an N-phase plane-bounded system, the first and Nth phases are regarded as semiinfinite ones, and the thickness, angle of incidence or refraction, and (21) Dewar, M. J. S. The Molecular Orbital Theory of Organic Chemistry: McGraw-Hilk New York, 1969; Chapter 9. (22) Xiao, X.D.; Vogel, V.; Shen, Y. R. J . Chem. Phys. 1991,94,2315. (23) Umemura, J.; Kawai, T.; Takenaka, T. Bull. Inst. Chem. Res., Kyoto Univ. 1990,68,241. (24) Matauda, H.; Kawai, T.; Umemura, J.;Takenata,T. J. Mol. Struct.
1991, 242, 39. (25) Umemura, J.; Matauda, H.; Kawai, T.; Takenaka, T. Reu. Phys. Chem. Jpn. 1990, special issue, 139.
Tian
1356 Longmuir, Vol. 8, No. 5,1992
z
X' Figure 1. Uniaxial orientation model of the transition moment.
the complex refractive index of the jth layer are hj, Oj, and hj = nj + ikj, respectively. Then, the transmittances for the p- and s-polarized light are given by
where r = (Ex2)/(Ez2)can be evaluated from eqs 8 and 9, and the ratio APi/A,' can be calculated by eqs 2 and 3. By substitution of eqs 10 and 11into eq 1,the orientation angle 4 can be obtained. In practice, the calculations of the transmittances and the mean-square electric fields were carried out by a modified computer program based on software developed by Umemura et alS6t8 The calculation of the orientation angles is based on the seven-phase model described in the previous section. The following parameters are used for the gel film: the refractive indices a t 2920 cm-l, nl = n7 = 1.00 (air), n2 = n6 = 1.45 0.01i (CTA+monolayer), n3 = n5 = 1.45 (MOlayer), n4 = 1.42 + 0.016i (water);26and those at 410 nm, nl = n7 = 1.00, n2 = n6 = 1.45, n, = n5 = 1.45 O.Oli, n4 = 1.33; the thicknesses h2 = h6 = 25 A (CTA+monolayer),8 h3 = hg = 16 A (MOlayer), hq = 50 A (water).6J2 For the liquid crystalline film, the thickness of CTA+ monolayer was also found to be ca. 25 and that of the aqueous core is estimated to be 10 A from the infrared measurement.12 These values are used in the calculation. The remaining parameters for the liquid crystalline film are the same as those for the gel film. In practice, the variation of the thickness value of the CTA+ layer in the range of 12.5-25 A hardly affecta the results of the orientation calculation.
+
+
A27128
(3) where qj = COS Ojlnj, pj = nj COS Oj, and mvwis the vwth element of the following 2 X 2 matrices M, = MZPM? ... MN-1'
(4)
M, M,BM,B... MN-t
(5)
with
]
MjP = cos flj -iqj sin /3,
-(i/qj) sin Oj cos Sj
Mi8 I cos 3/, -ipj sin 0,
-(i/pj) sin o j ] cos Sj
(6)
where /3j = 2~uhjnjcos O j and u is the wavenumber of the incident light. Thus, in the case of isotropic film, the transmittance can be evaluated by a set of given optical parameters involved in eqs 2 and 3, and absorbance can be consequently obtained by the relation A = log (To/T). Here T and 2'0 are the transmittances with and without absorption, respectively,of the MO-(forthe UV-vis region) or CTA+ (for the infrared region) layer. Furthermore, the field strength matrix Q(z) can be derived from the above matrices, which consists of first element V ( z )and second element U(z). The mean-square fields (Ex2)and ( E z 2 )for p-polarized light arell (Ex2) =
JlV(Z)l2dz
S
(8)
SInl sin tJlu(z)/nj12dz
(Ez2)=
(9)
Sdz The integrations are performed over the thickness range of the jth layer concerned. Since there are two layers of CTA+ (or M0-)in the film, the mean-square fields are further averaged out for these layers. For the hypothetical isotropic film, it is easy to show that
Experimental Section CTAC and MO samples used in this experiment are the same as those reported previously.12 Water was purified with a modified Mitamura Riken Model PLS-DFR automatic lab still consisting of a reverse osmosis module, an ion-exchangecoli.unn, and a double distiller. Vertical black film was prepared by withdrawing a rectangular platinum frame from the M aqueous solution of CTAC containing M MO in a cylindrical cell. The film frame in the cell was designed to be capable of rotating around ita central axis which is vertical to the surface of the solution so that the angle of incidence can be easily changed. The cell adopted for the infrared measurements is the same as that described by Umemura et The incident infrared beam passes through the sample film and two CaFz windows (12 mm diameter) on the cell wall. A copper tube with inner diameter of 6 mm was coiled up around the cell in order to control temperature. Through the tube thermostated water was circulated by a Neslab RTE-8 bath circulator. For the UV-vis measurements, on the other hand, the cell used was made of glass (35 mm internal diameter and 85 mm high). The incident light passes through the sample film and two quartz windows (10 mm diameter) on the cell wall.The cell was surrounded by an electric band heater for temperature control. Prior to the withdrawal of the film, for both infrared and UV-vis measurements, the cell containing the solution was placed overnight in the sample compartment of the corresponding spectrophotometer, in which temperature was adjusted to a set value to achieve the equilibrium of temperature and vapor pressure inside the cell. Temperature was monitored by a copperconstantan thermocouple inserted inside the cell. The overall accuracy of temperqture control and reading was within fO.l OC. Infrared spectra were recorded on a Nicolet 6000C FT-IR spectrophotometer equipped with an MCT detector. A Hitachi wire grid polarizer was used for polarization measurements. Three thousand interferograms (for the gel film), and five hundred in(26) Downing, H. D; Williams, D. J. Geophys. Res. 1971,80,1666. (27) Ekwall, P.; Mandell, L.; Fontell, K. Acta Chim. Scand. 1968,22, 1543. (28) Fontell, K. Mol. Cryst. Liq. Cryst. 1981, 63,59.
Langmuir, Vol. 8, No.5, 1992 1357
Molecular Orientation in Black Soap Film
dicular to the hydrocarbon chain axis.l0 Thus, the tilt angle y of the chain axis from the surface normal can be obtained from the corresponding angles (aand 8) of the transition moments of the two CH2 stretching modes by the orthogonal relation cos2a + cos2j3 + cos2y = 1 (12) For the liquid crystalline film, however, the y value thus obtained does not mean the orientation angle of the chain axis because of the disorder of chain conformation. But as to a given hydrocarbon chain, the y value is the average of the direction (2') which is perpendicular to both transition moments of the antisymmetric and symmetric CH2 stretching modes of each CH2 segment. Since the chain rapidly changes its conformation, each segmentalters its orientation and it varys moment by moment. On the time scale of the spectroscopic experiment, only an averaged orientation of all z' directions can be obtained. Therefore, the y value, in this case, affords a degree of orientation order by the parameter29 3000 2960 2920 2880 2840 2800
WAVENUMBER / cm" Figure 2. Polarizedinfrared spectra in the 3000-2800 cm-l region M aqueous solution of of the black film formed from the CTAC containing 10-2 M MO. S and P designate s- and p-polarized spectra, respectively, 0 is the angle of incidence. terferograms (for the liquid crystalline film), collected with the maximumoptical retardationof 0.25 cm were coadded, apodized with the Hap-Genzel function, and Fourier transformed with one level of zero filling to yield spectra of a high signal-to-noise ratio with the resolution of 4 cm-1. Polarized UV-vis spectra were measured on a Hitachi U-3400 spectrophotometer equipped with a Frank-Ritter prism.
Results and Discussion The molecular orientation of the film is examined below and above the T,. Before applying the above-described method to the study of the molecular orientation, it is necessary to make sure of the uniaxiality of the molecular orientation of CTA+ and MO- around the surface normal. To do this, a series of polarized infrared and UV-vis transmission spectra of the film were measured by rotating the polarization plane on the film surface. No difference was found among these infrared spectra, as well aa among these UV-vis spectra. These facts reveal that the condition of the uniaxial orientation is fulfilled for both CTAC and ~ 0 . 9
Figure 2 represents the polarized FT-IR spectra in the CH Stretching region of the black film measured with the angle of incidence (0) of 45' at temperatures of 26 and 39 'C. The nonpolarized spectra measured with normal incidence at the respective temperatures are also shown in this figure. The peaks at ca. 2920 and ca. 2850 cm-1 are ascribed to the antisymmetric and symmetric CH2 stretching vibrations of the CTA+ hydrocarbon chain, respect i ~ e l y . ~ * It ~ is J ~seen J ~ that the spectra have been recorded with a high signal-to-noise ratio. This makes the calculation of the molecular orientation based on the observed band intensity reliable. For the gel film, it has been established that the hydrocarbon chain of CTA+ takes the all-trans conformation in the black soap films.12 Therefore, the direction of the transient moment of the antisymmetric CH2 stretching mode is perpendicular to the zigzag plane of the hydrocarbon chain and the symmetric CH2 stretching mode is parallel to the zigzag plane, both being perpen-
s = 1/2(3(c0s27) - 1)
(13)
The observed absorbance values of the two CH2 stretching bands in the p- and s-polarized spectra (A, and A,) of CTA+in the black film at 26 and 39 'C are listed in Table I together with their ratio APIA,and the calculated values of the orientation angles (a,8) by eq 1 and (y) by eq 12. Before discussing the results of these angles, let us consider the change in the s-polarized absorbances with the increase in the angle of incidence. Observed ratios of the absorbances at t9 = 45' and 60° to that at normal incidence (A8(t91)/A8(Oo))are also given in Table I. The s-polarized incident beam only produces the electric field in the y direction in the film. This electric field does not change its projection on the film as the angle of incidence is altered. Therefore, the change in band intensity on changing the angle of incidence is independent of molecular orientation but is dependent upon the change in the optical path length in the film and the effects of the multireflection at the interfaces. Under such considerations, the theoretical values of A,(t91)/As(O0)were calculated by eq 3 to be A,(O0):A,(45'):A,(6O0) = 1:1.45:1.97. The observed values for the two CH2 stretching vibrations and for both gel and liquid crystalline films given in Table I are in good agreement with the calculated values. Note that the simple treatment merely considering the change in the optical length resulted in the ratio A,(0°):A,(45'): A8(600)= 1:1,13:1.24,which is completely different from the observed values. These facts indicate that the model employed in the calculation is reasonable and that the observed values possess a satisfactory precision. In the gel film, the tilt angles of the transition moments of both the antisymmetric and symmetric CH2 stretching bands are found to be ca. 64'. Consequently, the orientation angle of the hydrocarbon chain axis y is calculated from eq 12 to be 39'. This value can be compared with the correspondingvalues in n-dodecylammonium bromide (DAB)and cetyltrimethylammonium bromide (CTAB) crystals.30Jl In these crystals, the long chain molecules are arranged in a bilayer structure with antiparallel fashion, giving rise to a pronounced interdigitation within the bilayer. The tilt angle of the chain axis to the normal direction of the layer plane is 27' for DAB and 25' for (29) Silver, B. L. The Physical Chemistry of Membranes; The Solomon Press: New York, 1985; p 75. (30) Lunden, B. M. Acta Crystallogr. 1974, B30, 1756. (31) Campanelli, A. R.; Scaramuzza, L. Acta Crystallogr. 1986, C42,
1380.
1358 Langmuir, Vol. 8, No. 5, 1992
Tian
Table I. p and s-Polarized FT-IR Absorbances, Their Ratio, and the Tilt Angle of the Transition Moment of the u.(CHz) and v.(CHz) Bands in Gel and Liquid Crystalline Black Films* angle of incidence, deg
A,
A,
vu(CHd A,(61)/A,(0°)
0 45 60
2.6 2.0 1.5
2.6 3.4 4.8
1 1.3 1.9
0 45 60
0.9 0.9 0.7
0.9 1.3 1.8
1 1.4 2.0
A,/A a,deg A, Gel (26"C) 1.00 3.8 0.58 64 2.9 0.31 65 2.2 Liquid Crystal (39 OC) 1.00 1.6 0.74 56 1.4 0.36 58 1.1
A,
va(CHd A,(81)/A,(O0)
APIA,
8, deg
y,b deg
3.8 5.3 7.2
1 1.4 1.9
1.00 0.55 0.31
63 65
39 39
1.6 2.3 3.2
1 1.4 2.0
1.00 0.64 0.37
61 57
46 45
The absorbance values given here should be multiplied by lo3. The tilt angle of the hydrocarbon chain axis.
Table 11. p- and s-Polarized UV Absorbances at 410 nm, Their Ratio, and the Tilt Angle of the T-T* Transition Moment of MO in Gel and Liquid Crystalline Black Films liquid crystal (39"C)
gel (26"C) angle of incidence, deg
0 45 50 55 60 a
A,
lo3 2.4 3.5 3.8 3.8 4.6 X
A,
X
lo3 A,(61)/A,(O0)
2.4 3.8 4.2 4.3 4.7
1 1.6 1.8 1.8 2.0
APIA,
6,' deg
0.92 0.90 0.88 0.98
36 36 35 34
A,
lo3
A,
2.2
103 2.1 3.7
2.2
5.2
h\
T=26"C
X
2.1
X
A,(t9~)lA,(0°) A d A , 1 1.4 0.59
1.9
0.42
6,' deg
55 53
The tilt angle of the w r * transition moment.
CTAB, respectively. However, the molecules in the black soap film cannot provide such a space-filling structure and hence give rise to a larger tilt angle of the hydrocarbon chains. Moreover, since the gelation of the CTA+ monolayer is induced by the counterion MO-instead of bromide ion in the present case,12 the packing and orientation of the CTA+ monolayer should also be affected by those of the MO- layer. In the liquid crystalline film, the tilt angles of the transition momenta of the antisymmetric and symmetric CH2 stretching modes are found to be around 58'. The y value is calculated to be 46O, yielding an order parameter S = 0.22. As is apparent from eq 13,the random orientation of a structural unit corresponds to S = 0 or y = 54.7'. By analogy with the order parameter values (S = 0.2 to 0.4) obtained for typical smectic liquid crystals,29it is suggested that the hydrocarbon chains in the liquid crystalline film still possess a slight orientational order although they are conformationally disordered. For investigating the molecular orientation of MO in the black soap film, polarized UV-vis spectra of the film are also recorded at 26 and 39 OC, respectively. These spectra measured at e = 4 5 O are displayed in Figure 3 together with the nonpolarized spectra for comparison. It is seen that the nonpolarized spectra of both the gel and liquid crystalline films show a broad absorption band at 410 nm. Since these two spectra are measured with incident beam normal to the film, the optical effect on the absorption maximum is not con~idered.3~ This band is attributed to the TU* absorption with transition moment along the long axis of the trans-MO molecule.33 This band appears at 460 nm in the dilute aqueous solution. The blue-shift phenomenon seen here has also been observed for MO in micellar solution20and bilayer membranes,17 and for a black soap film of CTAB containing M0,34and has been ascribed to the formation of MO aggregates as mentioned above. According to Kasha's molecular exciton theory,36the blue shift from the monomer band is (32)Chollet, P.-A. Thin Solid Films 1978,52,343. (33)Beveridge, D.L.; Jaffe, H. H. J. Am. Chem. SOC.1966,88,1948. (34)Jiang, Y.;Tian, Y.; Liang, Y. Sci. Sin. 1991,34,1. (35)McFLae,E. G.;Kaeha, M.PhysicalProcessesinRadiationBiology; Academic Press: New York, 1964;p 23.
I
k 250 300 350 400 450 500 550
WAVELENGTH / nm Figure 3. Polarized UV-vis spectra in t h e 250-550 nm region M aqueous solution of of t h e black film formed from t h e CTAC containing M MO. S and P designate s- and p-polarized spectra, respectively. 0 is t h e angle of incidence.
indicative of a linear chromophore aggregate with the transition momenta parallel to each other and ordered nearly perpendicular to the stacking direction (so-called H-aggregate). Therefore, the above fact suggests that the MO molecules aggregate in H-like fashion in the black soap film in both the gel and liquid crystalline states. It is noted in Figure 3 that the peak maxima in both sand p-polarized spectra observed at B = 4 5 O shift to shorter wavelength (ca. 380 nm) with respect to that in the nonpolarized spectra at normal incidence. Similar peakmaximum shifta and concomitant distortions of band shape under the oblique incidence were also found by Umemura et al.6 for the OH stretching band of aqueous core of a black film of sodium dodecyl sulfate and have been
Molecular Orientation in Black Soap Film
interpreted as due to the effects of reflection and interference at the interfaces. This phenomenon is typical for an absorption band with large extinction coefficient and large anomalous dispersion in refractive index. Since the absorption coefficient of the -* transition band of MO is fairly large (2.2X 104mol-’ L cm-1),20the observed peak maximum shifts by the oblique incidence in Figure 3 can be considered to be the same phenomenon. In this case, therefore, the band intensity should be read at the wavelength of the peak maximum in the nonpolarized spectra observed at the normal incidence, Le., at 410 nm, for the quantitative evaluation of the molecular orientation of MO. In fact, the band intensities thus obtained bring about the absorbance ratio series of the s-polarized spectra at oblique incidence (see Table 11)A,(0°):A,(450):As(600) in close agreement with the calculated value mentioned above. The band intensities (A, and As),their ratios, and the tilt angle, 6 , of the long axis of MO molecule calculated from eq 1, are also given in Table 11. It is found that the tilt angle is ca. 35O in the gel film. This orientation angle is roughly the same as that (39O)of the CTA+ chain axis in the gel film. Considering the proposed model for this black soap film and the formation of a 1:l interaction product between CTA+and MO- in the film,it is reasonable to assume that the packing density of CTA+ monolayer is the same as that of the MO monolayer. Therefore, the similar orientation angle of the CTA+ chain and MO-molecular axis is quite feasible. In the liquid crystalline film, on the other hand, the orientation angle is found to be ca. 54O,resulting in an order parameter nearly equal to zero. This corresponds to random orientation. In the previous paper,12it was reported that the CTA+monolayersexpand their lateral packing, breaking the packing state of the MO-layer in the liquid crystalline film. This will increase the orientational freedom of MO- and allow for penetration of the core water into the MO-layer. In addition, MOwill partly diffuse into the aqueous core.12 Thus, the orientation of MO- becomes random in the liquid crystalline state.
Langmuir, Vol. 8, No. 5, 1992 1359
To sum up, the molecular orientation in the black soap
film composed of CTAC and MO has been quantitatively examined, on the basis of the proposed seven-phasemodel, by using Hansen’s optical formulations. It is understood that the orientation angle of the hydrocarbon chain axis of CTA+ and that of MO- are approximately the same in the gel state. This reflects a structural aspect: the CTA+ monolayer tightly conjugates the MO- monolayer and, consequently,the two layers mutually influence in packing and orientation. In the liquid crystallinefilm, on the other hand, the CTA+ monolayer still possesses a slight orientational order. However, as a result of the expansion of the lateral packing of the CTA+ monolayer and the concomitant mutual diffusion of MO- and core water, the MO- orientation becomes random.
Concluding Remarks The present study demonstrated that the optical methodology associated with the polarized spectroscopic obeervationsoffersthe significant detaiIs of the orientation events in the black soap film. The use of black soap films is advantageous as a step toward understanding biomimetic membranes through spectroscopy. As a perspective, black map film should be employed as not only a typical aidliquid/& interfacial system but also an important ultrathin film to be functionalized. Therefore, the orientation analysisof the film is expected to play an interesting part in this field. Acknowledgment. I express heart-felt thanks to Professor Tohru Takenaka of Kyoto University, Japan, for the constant guidance and encouragement during this work. I am grateful to Dr. Junzo Umemura of Kyoto University for valuable discussion and for the use of computer programs. Thanks also goes to Professor Yingqiu Liang of Jilin University for continuous support. Financial support from Chinese State Education Commission is gratefully acknowledged, which made the author’s stay at Kyoto University possible. %&try NO. CTAC, 112-02-7;MO,547-58-0.
