J. Phys. Chem. 1991, 95, 9985-9988
9985
Surface-Charge- and Temperature-Induced Phase Transitions in Black Soap Films As Studied by FT-IR Spectroscopy Yongchi Tian Institute of Theoretical Chemistry, Jilin University. Changchun, 130023, China (Received: April 4, 1991; In Final Form: July 9, 1991)
Fourier transform infrared spectroscopy has been applied to study phase transitions, induced by surface charge and temperature, in black soap films in air withdrawn from M aqueous solutions of cetyltrimethylammonium chloride (CTAC) with the addition of up to IC2M methyl orange (MO) as a counterion. A liquid crystal to gel phase transition in the film was observed M, which is attributed to a change in the lateral packing density of the as the bulk concentration of MO increased to CTAC hydrocarbon chain triggered by the counterion MO. The thickness of the aqueous core decreased with increasing bulk concentration of MO, in a common black film range. A thermotropic gel to liquid crystal phase transition occurred M aqueous solution of CTAC containing M MO. The thickness of at 34 OC (T,)in the film formed from the this film decreased with increasing temperature and the film became Newton type above T,. The alteration of the thickness of aqueous core on increasing the temperature is also discussed by considering the shift of balance between attractive and repulsive contributions of hydration force.
Introduction It is accepted that black soap films have a sandwich structure consisting of a thin sheet of aqueous core between two surfactant monolayers.’-* This kind of ultrathin film possesses, in various aspects, physicochemical characteristics that are intrinsically similar to those of biolipid membranes. The physical state of biomembranes is essentially determined by the gel to liquid crystal phase transition and by phase separation, and the former is directly related to the membrane fluidity. However, the gel to liquid crystal phase transition has never been reported for black soap films, although rigid and mobile films were already recognized as the two main categories of the films with different On the other hand, the black soap film has extensively been studied as a good model to examine surface forces of van der Waals type’ and of electrical double layers.8 But these were exclusively concerned with short-range force such as the hydration one. Umemura et ai? conducted a structural study on black soap films using FT-IR spectroscopy and demonstrated that FT-IR spectroscopy is a powerful tool to explore the nature of black soap films. On the other hand, cetyltrimethylammonium chloride (CTAC) and methyl orange (MO) have been shown to form 1:1 interaction product in aqueous solution by the electrical attraction between the negatively charged sulfonate group of MO and the positively charged alkylammonium group of CTAC.l0 This interaction product was found to be adsorbed at the oil/water interface with well oriented order.” In the present study, FT-IR spectroscopy is applied to examine the surface-charge- and temperature-induced phase transitions in black soap films withdrawn from the aqueous solutions of CTAC containing MO as a counterion. The alterations of hydrogen bonding and hydration force in the black films on the phase transitions are also discussed. (1) Clunie, J. S.; Goodman, J . F.; Ingram. B. T. In Surface and Colloid Science; Matijwic, E., Ed.; Wiley-Interscience Press: New York, 1971; Vol. 3, p 167. (2) Ivanov, 1. B. Thin Liquid Film; Marcel Dekker: New York, 1988. (3) Mysels, K. J.; Shinoda, K.; Frankel, S.P.Soap Films, Studies of Their Thinning and a Bibliography; Pergamon: Oxford, U K , 1959. (4)Corkill, J. M.; Goodman, J. F.;Ogden, C. P.;Tate, J. R.froc. R. Soc. London, A 1963, 273, 84. (5) Clunie, J. S.;Goodman, J. F.; Symons, P.C. Nature 1967, 216,. 1203. (6) Lyklema, J.; Scholten, P. C.; Mysels, K. J. J . fhys. Chem. 1965, 69, 116. (7) Scheludko, A. Adv. Colloid Inrerface Sei. 1967, 1, 391. (8) Mysels, K. J.; Jones, M . N. Discuss. Faraday SOC.1966, 42, 42. (9) Umemura, J.; Matsumoto, M.; Kawai, T.; Takenaka, T. Can. J. Chem. 1985, 63, I7 13. ( I O ) Hiskey, C. F.; Downey, T. A. J . fhys. Chem. 1954, 58, 835. ( I I ) Takenaka. T.; Nakanaga, T. J . Phys. Chem. 1976, 80, 475.
Experimental Section CTAC was purchased from Wako Pure Chemical Industries, Ltd., Japan. MO was from Nakarai Chemicals, Ltd., Japan, and was recrystallized from water three times. Doubly distilled water was used for the preparation of aqueous solutions. The sample cell adopted in this work is the same as that described by Umemura et ala9 A vertical black film was prepared by withdrawing a rectangular platinum frame from an aqueous solution of CTAC and MO in the cell. The infrared beam normally passed through the soap film and two CaF2 windows (12 mm diameter) on the cell wall. In order to control temperature, a copper tube with an inner diameter of 6 mm was coiled up around the cylindrical cell. Through the tube thermostated water was circulated by a Neslab RTE-8 bath circulator. For achieving the equilibrium of temperature and vapor pressure inside the cell, prior to the withdrawal of the film, the cell containing the solution was placed overnight in the sample chamber where temperature was adjusted to a set value, with continuous infrared irradiation. Temperature was monitored by a copper-constantan thermocouple inserted inside the cell. The overall accuracy of temperature control and reading was within fO.l OC. Infrared spectra were measured on a Nicolet 6000 FT-IR spectrophotometer equipped with an MCT detector. Typically, 500-3000 interferograms collected with the maximum optical retardation of 0.25 cm were coadded, apodized with the HappGenzel 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-I. The spectrophotometer is accurate and reproducible in frequency to within f0.005 cm-I. The actual frequency accuracies determined by the signal-to-noise ratio and bandwidths were within f0.3 cm-I for the C H stretching bands of CTAC and f3 cm-’ for the OH stretching band of water. Results and Discussion The critical micelle concentration (cmc) of CTAC is 1.3 to 1.5 X M.12 Addition of counterionic dye, in general, lowers the cmc, as demonstrated by some earlier researches.”*14 In the present experiment, the CTAC concentration of the bulk solution M which is far larger than the cmc. Addition of MO is obviously affects the drainage behaviors of the films. When the metal frame is withdrawn from an aqueous CTAC solution without (12) Mukerjee, P.;Mysels, K. J. Critical Micelle Concenrrations of Aqueous Surfactant Systems; U S Government Printing Office: Washington, DC,1971;NSRDS-NBS 36. (13) Mukerjee, P.; Mysels, K. J. J . Am. Chem. SOC.1955, 77, 2937. Kawasaki, M.; Kasatani, K.;Kusumoto, Y.;Nakashima, N. (14) Sato, H.; Chem. Lett. 1980, 1529.
0022-3654/91/2095-9985%02.50/0 0 1991 American Chemical Society
9986 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
WAVENUMBER /cni' Figure 1. Infrared spectra in the 3800-2800-cm-' region of black films M aqueous solutions of CTAC'containing varying formed from amount of MO. Concentrations of MO are 0 (A), IO-' (B), 5 X IO-' (C), and IOv2 M (D). The arrows indicate absorption maxima. *
L. c.
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. I
lo-' lo-' CONCENTRATION OF MO / M
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2918
Figure 2. Changes of the band area and peak wavenumber of the CH2 antisymmetric vibration band with the bulk concentration of MO.
MO, the liquid film shows turbulent motion along the Plateau border where the film meets the frame and gives a succession of horizontal interference fringes indicative of a regular film profile. After about 30 min all of the film becomes black. These phenomena are typical of the so-called mobile film.1*336When the film is lifted from a CTAC solution containing M MO, however, the film takes ca. 7 h to drain completely, exhibits little motion, and gives an irregular display of interference fringes. These show a drainage behavior typical of the so-called rigid film.1*3*6 The differences in drainage behavior should be associated with the structural characteristics of surface monolayer that is present in the films. Figure 1 shows the FT-IR spectra in the 3800-2800-cm-' region of the black soap films which are withdrawn from the IO-* M aqueous solutions of CTAC containing various concentrations of MO. These give the spectral patterns in the O H and C H stretching regions similar to the corresponding ~ peaks at ca. 2920 and ca. 2850 cm-' ones of the l i t e r a t ~ r e .The are assigned to the antisymmetric and symmetric CH2 stretching The vibrations of the CTAC hydrocarbon chain, broad band at ca. 3400 cm-' is undoubtedly ascribed to the OH stretching vibration of the core ~ a t e r . ~Considerable ,~~ changes (IS) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Eiochim. Eiophys. Acta 1980, 602, 32. (16) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J . Colloid Interface Sci. 1985, 103, 56. ( I 7) Kawai, T.; Umemura, J.; Takenaka, T. Bull. Inst. Chem. Res., Kyoto Uniu. 1983. 61. 314. (18) Kawai, T.;Umemura. J.; Takenaka, T.; Gotou, M.; Sunamoto, J . bngmuir 1988,4,449. (19) Hartman, Jr., K. A. J . Phys. Chem. 1966, 70, 270.
Tian in the spectral feature with the bulk concentration of MO are evident, implying the structural alteration of the black soap films. In Figure 2, the wavenumber and band area of the antisymmetric C H 2 stretching band are plotted as the functions of the bulk concentration of MO. It has been known that the CH2 stretching band feature of hydrated surfactants provides a sensitive monitor of the degree of conformational order in the methylene chains.'+l* It is Seen in this figure that the wavenumber keeps a constant value of about 2924 cm-' in the MO concentration range lower than 5X M, which is identical with that observed in liquid crystal spectra of some hydrated s~rfactants,l~*~' and drastically decreases to ca. 2918 cm-l as the MO concentration reaches to M, which is identical with that obtained in gel spectra of these surfactants.'6'8 The band area shows a concomitant but opposite alteration, increasing by more than 3 times, a t the same concentration. It has been established that the value of 2918 cm-' for the CH2 antisymmetric stretching band is characteristic of the trans-methylene chain. Therefore, the above-mentioned finding indicates that the degree of conformational order of the CTAC chain in the black film suddenly improves to the transmethylene chain as the M O concentration increases to M. On the other hand, since the rigid black film prepared from the concentrated M O solutions still retains the double-layer structure of CTAC hydrocarbon chains,',6 the increase in band area arises partly from the increase in chain packing density and partly from the change of the orientation of the transition moment of the CH2 stretching mode. If the CTAC hydrocarbon chain is ordered and tends to orient normally to the film surface upon the formation of the rigid black film, the transition moment of the antisymmetric CH2 stretching mode becomes parallel to the film surface, which results in the increased intensity of the band. These arguments indicate that a liquid crystal to gel phase transition takes place in the film as the counterionic dye, MO, increases its concentration to match that of CTAC in the bulk solution. Clearly, this phase transition is caused by the addition of MO ions and may be explained in terms of changes in CTA+ hydrocarbon chain lateral packing density and the consequent effect on chain orientation20 induced by the counterionic dye, MO- anion. Surface-charge-induced phase transition was first observed by Eibl and Trauble2' and by Verkleij et aLn for charged lipid bilayer membranes in aqueous media. This kind of phase transition has been interpreted by means of changes in the lateral packing density of lipid chain triggered by the external charges.23 In the present case, if it is assumed that the negatively charged sulfonate group of MO- bonds to the positively charged CTA+ monolayer, the head-group Coulomb repulsion in the monolayer will be reduced, which results in a tighter chain packing. Similar effect is clearly demonstrated in monolayer studies.24 Also, an explanation for this phase transition was given on the basis of Gouy-ChapmanOverbeek theory of electric double l a ~ e r . ~ I -This ~ ) theory supposes a strong interaction between the membrane surface charge and the counterion. Taking these into account, the above fact of the phase transition suggests that MO- anions interact preferentially with CTA' monomers, in good agreement with the property of the interaction between MO and CTAC in aqueous solutionlo and in adsorbed interfacial monolayer.'' Moreover, the induction of the disorder-order transition by the introduction of MO in the black film supports a proposed role of the counterions as film stabilizer. In fact, the gel (ordered) black film survived for weeks, in contrast to the relatively short lifetime of liquid crystalline (disordered) black films. As to the location of MO- in the gel phase, two possibilities are considered. One is that the MO- anions incorporate with the hydrocarbon chains of CTA' in the monolayer, directing their sulfonate groups toward the aqueous core, and the other is that the MO- anions form a separate layer on the positively charged (20) Tian. Yongchi. Submitted for publication in Langmuir. (21) Trauble, H.; Eibl, H. J. Proc. Natl. Acad. Sci. U.S.A. 1972, 7/, 214. (22) Verkleij, A. J.; DeKruyff, B.; Ververgaert, P. H. J.; Tocanne, J. F.; Van Deenen, L. L. M. Eiochim. Biophys. Acta 1974, 339, 432. (23) Jahnig, K. Habilitationsschift, Gottingen, 1911. (24) Albrecht, 0.;Gruler, H.; Sackman, E. J . Phys. 1978, 39, 301.
Phase Transitions in Black Soap Films
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9987
3420 3410
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WAVENUMBER /cm-' Figure 4. Temperature-dependent infrared spectra in the 3800-2800cm-l region of the black films drawn from IO-* M aqueous solution of CTAC containing M MO.
centration, which leads to the reduction of the thickness. This also conforms to the favorite binding of the MO- to the counter-charged CTA+ surface monolayer. Considering that the 1: 1 interaction product can be formed between the CTA+ cation and the MO- anion,1° one can expect that the CTA+ monolayers become electrically neutralized as the bulk concentration of MO matches that of CTAC. Thus, the electrostatic repulsion can be ignored in the gel film, and the operative repulsion between the monolayers arises predominantly from the hydration force. On the other hand, the shift of the O H stretching band reflects the structure change of water in the aqueous core on the increase of the MO concentration and on the phase transition. It is noted in Figure 3 that the band maximum approximately displays a linear increase with the bulk concentration of MO in the liquid crystal phase region, while it goes down suddenly as the gel phase is achieved. Considering the change in water structure by the water-MO- interaction, we have to take into account several possible mechanisms which interfere competitively in a complex manner. The first is the interaction between the hydrophobic group of MO-, Le., the methyl group, and water. Such interaction is known to make iceberg or structured water around the hydrophobic group.29 The effect of this interaction with increasing the MO- concentration should decrease the OH stretch frequency. However, this is the opposite direction to what we observe in the liquid crystalline phase, and hence it cannot be the main origin of the observed frequency shift in Figure 3. The second is the hydrogen bonding between the amine nitrogen atom of MO- which has the electron donor property30 and the water hydrogen atom. The strength of the amine-water hydrogen bonding is known to be weaker than the water-water hydrogen bonding.)' Thus, this is in the same direction as the observed O H stretching frequency shift with M O concentration in the liquid crystalline phase. However, the third, and most probable possibility is the strong ion-water interaction, since MO- has the SO3- group. Since it has been pointed out that the shift in O H stretching frequency is sensitive not to cation species but to anion speciesI9 and very similar trends were observed for the aqueous solution of sodium n-alkanes~lfonates,~~ it is reasonable to deduce that the sulfonate group in MO- has a similar effect on hydrogen bonding. This is to say that the band at higher frequency is probably due to the weaker hydrogen bonding between the sulfonate group and the water molecule. In other words, MO- behaves as a hydrogen bond (25) Lunden, B. M.Acta Crystallogr., Sect. B 1974, 30, 1756. (26) Campanelli, A. R.;Scaramuzza, L. Acta Crystallogr., Sect. C 1986, 12, 1380. (27) Taga, T.; Machida, K.; Kimura, N.; Hayashi, S.; Umemura, J.; Takenaka, T. Acta Crystallogr., Sect. C 1987. 43, 1204. (28) Princen, H. M.; Mason, S. G. J . Colloid Sei. 1965, 29, 156.
(29) Franks, F. Water--A Comprehenriue Treatise; Plenum Press: New York, 1975; Vol. 4. (30) Xiao, X. D.; Vogel, V.; Shen, Y. R. J . Chem. Phys. 1991,94,2315. (31) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding, Marcel Dekker: New York, 1974; p 52. (32) Umemura, J.; Takenaka, T. Private communication.
9988 The Journal of Physical Chemistry, Vol. 95, No. 24, I991
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Temperature dependences of the band area and the peak wavenumber of CH2antisymmetric band of the black film drawn from IO-* M aqueous solution of CTAC containing IO-* M MO. Figure 5.
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breaker. In the gel spectra, however, the absorption maximum of the OH stretching band shifts to a lower frequency. This suggests that the water molecules in the aqueous core of the gel film are little influenced by the sulfonate group due to the specific adsorption of this group to the CTA+ monolayers. In other words, the water of hydration to the sulfonate group in the liquid crystal phase is removed in the gel phase, since the sulfonate group binds preferably with the alkylammonium group in the gel phase. Further, the temperature-induced phase transition was also examined for the black film prepared from the M aqueous solution of CTAC containing M MO. Figure 4 gives the temperature-dependent infrared spectra of the black films in the 3800-2800-cm-' region. Remarkable changes in spectral feature are also seen for both CH2 and O H stretching bands. The temperature dependences of the peak wavenumber and the band area of the CH2 antisymmetric stretching vibration are displayed in Figure 5. Drastic changes centered at 34 OC correspond to the gel to liquid crystal phase transition. The band features of the CH2 stretching vibrations in the liquid crystal and gel spectra are in agreement with the corresponding ones obtained in the surface-charge-induced phase transition experiment. The temperature dependences of the wavenumber and band area of the OH stretching band are shown in Figure 6 . The band area decreases with increasing temperature in the gel phase and nearly reaches the lowest value at the phase transition temperature T,. Above T,, the intensity of the water band keeps its value constant. The band area of the OH stretching band is principally affected by the hydrogen bond strength, which depends on the kind of ionic species?3 its c~ncentration,~~ temperature, and so forth. Generally speaking, the stronger the hydrogen bond, the larger the stretching band area.j3 The weakening of the hydrogen bond with temperature is realized by the shift in the OH stretching frequency (33) Motojima, T.; Ikawa, S.; Kimura, M. J . Quanf.Specfrosc. Radiaf Transfer 1981. 26. 177.
Tian from 3383 to 3424 cm-I in Figure 6 . The intensity decrease expected from the above frequency shift (and hence the weakening of the hydrogen bond) amounts to ca. 4%.33 Thus, the much larger change of the band area observed in Figure 6 can be ascribed to the decrease in the thickness of the aqueous core. The thickness above T, is roughly evaluated to be smaller than 1 nme9 This equilibrium thickness corresponds to the Newton black film.S*28 In fact, the infrared spectra measured above T, exhibit features typical of those of Newton black film9 Therefore, these findings reveal that the change from the common to the Newton black film occurs accompanying the gel to liquid crystal phase transition. For the black soap film composed of electrically neutralized monolayers, the main repulsion force between the monolayers is the hydration one. In order to describe the hydration force, Kjellander suggested,34on the basis of the measurements of interlayer forces, that the major force is composed of an enthalpic part, which is repulsive, and an entropic part, which is attractive. At low temperatures, the enthalpic part is predominant and the net interaction is repulsive, while, at high temperatures, the interaction is attractive due to a dominance of the entropic part. Therefore, in a film system which does not undergo a phase transition, rising temperature shifts the interaction toward less repulsion or, for high temperatures, gives rise to an attraction. As a result, the equilibrium thickness decreases with the increase in temperature. This relation was experimentally assured by Claesson et al.3s for a nonionic surfactant double layer system by a direct measurement of temperature-dependent interactions. The fact shown in Figure 6 may be understood by the shift of the balance between the hydration repulsion and attraction with increasing temperature. This also provides an experimental support for Kjellander's suggestion. Once the Newton black film is formed, the equilibrium thickness falls into a sharp potential minimum, losing mobile water molecules, and the intensity of the OH stretching band becomes constant. In addition, the OH stretching band shifts to higher frequency on the phase transition, as shown in Figure 6. The gel spectra show an absorption maximum ofthe band at ca. 3383 cm-I, in good agreement with that obtained above. The liquid crystal spectra show the maximum at 3424 cm-I. This suggests that the water molecules of the liquid crystalline (Newton) film are in a highly ionic atmosphere due to the sulfonate groups of MO-.'9J2 Because the water structure alters in the process of the phase transition, the dehydration of the head group of the monolayers may contribute an additional effect to the change from the common to the Newton black film.
Concluding Remarks Surface-charge- and temperature-induced phase transitions are first observed in black soap film composed of CTAC and MO. The terms mobile black film and rigid black film refer to liquid crystalline and gel black films, respectively. This offers new insights into structural aspects of black film research, and may open up a wide range of potential new development in this field. First, black soap film is advantageous for spectroscopic study as a good model to understand membrane systems. Second, F'f-IR study on the black film provides very useful information about the property of short-range interaction between surface monolayers such as hydration force. Acknowledgment. The author expresses his gratitude to Professor Tohru Takenaka of Kyoto University, Japan, for the constant guidance and encouragement during this work. Thanks also go to Dr. Junzo Umemura of Kyoto University for helpful suggestions and discussion, and to Professor Yingqiu Liang of Jilin University for continuous support. Financial support from the National Education Commission of China is gratefully acknowledged, which made the author's stay at Kyoto University possible. (34) Kjellander, R. J . Chem. Soc., Faraday Trans. 2 1982, 78, 2025. (35) Claesson, P. M.; Kjellander, R.;Stenius, P.:Christenson, H. K. J . Chem. SOC.,Faraday Trans. I 1986, 82, 2735.
