Surface Phase Behavior of a Mixed System of Anionic−Nonionic

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Articles Surface Phase Behavior of a Mixed System of Anionic-Nonionic Surfactants Studied by Brewster Angle Microscopy and Polarization Modulation Infrared Reflection-Absorption Spectroscopy Md. Nazrul Islam,† Tomomichi Okano,‡ and Teiji Kato*,† Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan Received April 11, 2002. In Final Form: October 15, 2002 A first-order phase transition in the mixed monolayer of ethylene glycol mono-n-dodecyl ether (EGDE) and sodium 3,6,9,12-tetraoxaoctacosanoate (TOOCNa) has been studied by Brewster angle microscopy (BAM) at an EGDE/TOOCNa molar ratio of 3:2. From the study of surface tension as a function of mole fraction, it has been found that at this molar ratio the surfactant mixture shows the maximum surface activity. A cusp point followed by a plateau region with subsequent formation of bright two-dimensional condensed domains in a dark background visualized by BAM indicates the first-order phase transition. The appearance of the cusp point shifts to early time upon mixing TOOCNa with EGDE. The patterns of the adsorption kinetics and surface morphology of the mixed monolayers differ from those of pure systems. For the pure EGDE monolayer, the condensed domains are of a fingering pattern whereas those for the mixed monolayer are circular, indicating that the condensed domains are formed from both of the surfactants. The composition and the interaction parameters between the molecules in the mixed monolayers have been calculated. The values of the interaction parameters are found to be negative, suggesting that there exists a weak interaction between the molecules in the mixed monolayer. Polarization modulation infrared reflection-absorption spectroscopy has been used for better understanding of the conformational order and packing of molecules in the mixed monolayer. The spectra were collected using relatively concentrated solutions of the amphiphiles in H2O/D2O under equilibrium conditions to ensure that the surface is fully covered by the condensed domains. The stretching modes of COO- are clearly visible in the spectra of the mixed monolayer collected from the D2O subphase, suggesting that TOOCNa molecules are also present in the condensed domains. The shift of the νas(CH2) band to a lower frequency indicates better conformational order of the hydrocarbon chain in the mixed monolayer.

Introduction Mixtures of surfactants are commonly used in many practical surfactant applications since they often show superior performance compared to single surfactant systems alone. The addition of a nonionic surfactant to an ionic surfactant can reduce the electrostatic repulsions between the charged surfactant heads and thereby brings about changes in the properties of mixed systems. Depending on the interaction between two surfactants in a binary mixture, the surface properties as well as the bulk properties of mixed surfactant systems lie either between or outside the solution properties of the two single surfactant solutions.1 Besides, this type of understanding can help one to choose suitable surfactant mixtures that will result in desired properties at an appropriate mixing ratio. Sugihara et al.2 reported the formation of azeotrope in the mixed adsorbed films of an anionic and a nonionic * To whom correspondence should be addressed. Phone: +81-028-689-6170. Fax: +81-028-689-6179. E-mail: teiji@ cc.utsunomiya-u.ac.jp. † Utsunomiya University. ‡ Lion Corp. (1) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley-Interscience: New York, 1989; Chapter 11. (2) Okano, T.; Tamura, T.; Abe, Y.; Tsuchida, T.; Lee, S.; Sugihara, G. Langmuir 2000, 16, 1508.

surfactant showing a remarkable difference in the composition of the monolayers from that in the micelles. An important step toward understanding the behavior of the surfactants in a mixture is to study the adsorption kinetics and morphological features as well as the nature of the interaction among the surfactant molecules in the mixed system. A first-order phase transition can also occur in the adsorbed layers of water-soluble surfactants.3-7 A cusp point followed by a pronounced plateau region in the π-t adsorption isotherm with subsequent formation of condensed domains at the solution surface is indicative of this type of transition. The development of sensitive microscopic techniques such as Brewster angle microscopy8,9 allows in situ visualization of the condensed domains just after the phase transition at the air/water interface without using any probe impurity. Recently, experimental evidence has been provided that at a definite temperature a minimum bulk concentration is necessary (3) He´non, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148. (4) Revie`re, S.; He´non, S.; Meunier, J. Phys. Rev. E 1994, 49, 1375. (5) Melzer, V.; Vollhardt, D. Phys. Rev. Lett. 1996, 76, 3770. (6) Vollhardt, D.; Melzer, V. J. Phys. Chem. B 1997, 101, 1997. (7) Melzer, V.; Vollhardt, D.; Brezesinki, G.; Mo¨wald, H. J. Phys. Chem. B 1998, 102, 591. (8) He´non, S.; Meuner, J. Rev. Sci. Instrum. 1991, 62, 936. (9) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.

10.1021/la020345q CCC: $22.00 © 2002 American Chemical Society Published on Web 12/02/2002

Phase Behavior of a Mixed Surfactant System

for the phase transition in Gibbs monolayers.10-12 The change of temperature causes a change of the properties of the molecules in the bulk such as the critical micelle concentration (cmc) and their tendency to aggregate as micelles which in turn affects the surface concentration and the morphological features of the adsorbed monolayers.13 In recent years, there has been a growing interest in the two-dimensional (2-D) phase behavior of mixed surfactant systems at the air-water interface, since the stoichiometric association of the molecules can strikingly alter the properties of the system.14,15 Furthermore, contamination in surfactants, whether occurring in the original preparation or later, can be properly addressed from the understanding of the phase behavior of surfactants in mixed systems. Vollhardt et al.16,17 studied the mixed monolayers of sodium dodecyl sulfate (SDS) with dodecanol or tetradecanol. Their study demonstrates that the presence of SDS in the subphase lowers the surface tension of the subphase and changes the surface morphology of the monolayers. Sum frequency vibrational spectroscopic study18-21 of adsorbed monolayers of cationic or anionic surfactants with dodecanol or tetradecane reveals that the molecules adsorbed from the solutions of pure surfactants are conformationally disordered and become more disordered as the solution concentration is decreased or the temperature is increased. In contrast, highly ordered and densely packed robust films are found to form in the mixed monolayers. In a previous paper,22 we have provided evidence that the morphological features and texture of condensed domains in the mixed monolayers are quite different from those of pure systems. In this work, we discuss the effect of interaction between the surfactants on the adsorption kinetics and the surface morphology as well as the subcell packing of the molecules in the mixed monolayer of the ethylene glycol mono-ndodecyl ether (EGDE)/sodium 3,6,9,12-tetraoxaoctacosanoate (TOOCNa) system at the air-water interface. We chose this surfactant pair considering the fact that both surfactants bear ethylene oxide groups; therefore, steric hindrance between the molecules to form adsorbed film would be minimum. Of the two surfactants, the anionic surfactant, TOOCNa, cannot form condensed domains in the adsorbed layers under the experimental conditions, whereas EGDE can form condensed domains under the same conditions. The adsorption kinetics and the surface morphology of the mixed system have been studied at the EGDE/TOOCNa molar ratio of 3:2. At this molar ratio, the surfactant mixture shows the maximum surface activity. The interaction parameters and the surface compositions of the surfactants in the mixed monolayers at different molar ratios were calculated to know the nature of interaction between the molecules. (10) Hossain, M. M.; Yoshida, M.; Kato, T. Langmuir 2000, 16, 3345. (11) Hossain, M. M.; Kato, T. Langmuir 2000, 16, 10175. (12) Vollhardt, D.; Fainerman, V. B.; Emrich, G. J. Phys. Chem. B 2000, 104, 8536. (13) Islam, M. N.; Kato, T. J. Colloid Interface Sci. 2002, 252, 365. (14) Shah, D. O. J. Colloid Interface Sci. 1971, 37, 744. (15) Patist, A.; Devi, S.; Shah, D. O. Langmuir 1999, 15, 7403. (16) Seigel, S.; Vollhardt, D. Thin Solid Films 1994, 284/285, 424. (17) Fainerman, V. B.; Vollhardt, D.; Emrich, G. J. Phys. Chem. B 2001, 105, 4324. (18) Johal, M. S.; Usadi, E. W.; Davies, P. B. Faraday Discuss. 1996, 104, 231. (19) Casson, B. D.; Braun, R.; Bain, C. D. Faraday Discuss. 1996, 104, 209. (20) Casson, B. D.; Bain, C. D. J. Phys. Chem. B 1999, 103, 4678. (21) McKenna, C. E.; Knock, M. M.; Bain, C. D. Langmuir 2000, 16, 5853. (22) Hossain, M. M.; Islam, M. N.; Okano, T.; Kato, T. Colloids Surf., A 2002, 205, 249.

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The surface phase behavior of the mixed adsorbed film was investigated using Brewster angle microscopy (BAM) to know the effect of interaction between the surfactants on the morphological features of the mixed film at the air-water interface. The BAM results were combined with the infrared spectra to assess the conformational order and the subcell packing of the molecules in the monolayer. We used the polarization modulation infrared reflectionabsorption spectroscopy (PM-IRRAS) technique23-27 to know the structure of the monolayer on a molecular scale. This technique involves an external reflection of an IR beam from the film-covered air-water interface under a controlled condition of surface pressure. The main feature of the PM-IRRAS technique is that absorption due to the air moisture and the water subphase is isotropic in nature and can be effectively eliminated by the polarization modulation technique.26,27 As a result, the signal-to-noise ratio is much improved. Experimental Section Materials. The amphiphiles EGDE (Nikko Chemical Co., Tokyo, Japan) and TOOCNa (Lion Corp., Japan) were supplied with a purity of >99%. Deuterium oxide (D2O; purity, 99.9%) was purchased from Cambridge Isotope Laboratories, Inc. All the materials were used as received. For adsorption kinetics and surface morphology study, solutions of the amphiphiles of 1.3 × 10-5 M were prepared separately in ultrapure water of resistivity 18 MΩ cm (Elgastat UHQ-PS). For PM-IRRAS study, 3 × 10-5 M solutions of the amphiphiles were prepared separately either in ultrapure water or in D2O so that the system attains equilibrium surface pressure rapidly and the surface becomes fully covered by the condensed-phase domains. In both cases, to study the behavior of the mixed solution, 3:2 mixtures of the amphiphiles were prepared by mixing the pure solutions volumetrically. Adsorption Kinetics and Surface Morphology. The experimental setup consisted of a film balance above which a Brewster angle microscope was mounted. The Brewster angle microscope is composed of a 20 mW He-Na laser, a GlanThomson polarizer, an analyzer, a zooming microscope with a CCD camera of high sensitivity, a TV monitor, and a video recording system. Details of the instrumentation have been described elsewhere.28 Surface pressure was measured by the Wilhelmy method using a small rectangular glass plate. To attain equilibrium with the desire experimental temperature, the solution was allowed to stand for about 25 min before the start of the experiment. The molecules already adsorbed at the surface during this time were removed by sweeping the surface with the movable Teflon barriers at a constant sweep rate. The increase of surface pressure was then measured with time, and the change in surface morphology was monitored by BAM. Surface Tension Measurement. Surface tension measurements of the solutions of both pure and mixed systems at different concentrations were carried out by a surface tensiometer (Kru¨ss K 10) equipped with a platinum plate. The solutions were transferred into a vessel that was thermostated by circulating water of the desired temperature. Establishment of equilibrium was checked by taking a series of readings after 15-min intervals until no significant change occurred. PM-IRRAS Study. A small Langmuir trough of dimensions 10 cm × 8.5 cm × 0.2 cm was used for PM-IRRAS experiments. The trough was placed in a sealed box. The PM-IRRAS spectra were recorded using a Nicolet 860 FTIR, equipped with a PM (23) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectrosc. 1991, 45, 380. (24) Blaudez, D.; Turlet, J. M.; Dofourecq, J.; Bard, D.; Buffeleteau, T.; Desbat, B. J. Chem. Soc., Faraday Trans. 1996, 92, 525. (25) Blaudez, D.; Buffeleteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlet, J. M. Appl. Spectrosc. 1993, 47, 869. (26) Mao, L.; Ritcey, A. R.; Desbat, B. Langmuir 1996, 12, 4754. (27) Mendelsohn, R.; Brauner, J.; Gericke, A. Annu. Rev. Phys. Chem. 1995, 46, 305. (28) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34, L911.

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Figure 1. Dependence of surface tension as a function of mole fraction of EGDE for the mixture of 1.3 × 10-5 M solutions of each of EGDE and TOOCNa at 10 °C. Table 1. Values of Critical Micelle Concentration, Surface Composition, XEGDE, and Molecular Interaction Parameters, βσ, of the EGDE-TOOCNa Mixed System at 10 °C REGDE cmc (M) XEGDE 0.0 0.2 0.4

0.56 0.60 0.70

0.21 0.36

βσ -0.74 -0.74

REGDE cmc (M) XEGDE 0.6 0.8 1.0

0.85 1.10 1.75

0.50 0.66

βσ -0.75 -0.73

part. The subphase temperature was set to 10 °C by using a large number of Peltier elements attached to the back of the base plate of the trough and was detected by a platinum wire temperature sensor. For carbon dioxide and water vapor to be reduced, the assembly was continuously purged at a constant speed from a Whatmann 75-52 JA 100 purge gas generator. To keep the exchange of H2O with D2O as low as possible, the assembly was purged for about 2 h before pouring the amphiphile solutions in D2O. Details of the PM-IRRAS measurement have been described elsewhere.23,24 Briefly, the incidence angle of the IR beam onto both H2O and D2O subphases was 75° with respect to the surface normal. The PM-IRRAS spectra were recorded at 4 cm-1 and 1000 scans. The IR beam was passed through a ZnSe photoelastic modulator (Hinds PEM 90) which modulated polarization at a frequency of 2ω ) 100 kHz. The maximum phase retardation at 7100 nm (1408 cm-1) was set to π. The light intensity I arriving at the liquid-nitrogen-cooled MCT detector was demodulated to give Ip and Is, representing the intensities of the p and s polarized parts, respectively. The PM-IRRAS spectra were recorded as S ) (Ip - Is)/(Ip + Is). In this paper, the spectra were reported as -S(d)/S(o), where d and o denote the monolayercovered and uncovered subphase, respectively.

Results and Discussion Adsorption Kinetics and Surface Morphology. Figure 1 represents the equilibrium surface tension against the mole fraction of EGDE for EGDE-TOOCNa mixtures at a total concentration of 1.3 × 10-5 M at 10 °C. The cmc values of the mixtures over the mixing ratios of 4:1 to 1:4 were found to vary from 1.1 × 10-5 to 0.68 × 10-5 M (Table 1). The surface tension values plotted in Figure 1 for the mixed solution are the equilibrium values at this temperature. It is clearly visible in Figure 1 that at the molar ratio of 3:2, the surface activity of the mixture is the maximum. Therefore, we concentrate on the adsorption kinetics and the morphological features of the mixed system at this molar ratio. Figure 2 shows the adsorption kinetics of 1.3 × 10-5 M solutions of pure EGDE, TOOCNa, and their mixture at the molar ratio of 3:2 at 10 °C. The rate of adsorption of TOOCNa is faster than that of EGDE. However, the net rate of adsorption for the mixed system is between those of the pure systems, indicating the adsorption of both the surfactants simultaneously. Pure TOOCNa cannot show any indicative feature of phase

Figure 2. The π-t adsorption kinetics of 1.3 × 10-5 M solutions of each of EGDE, TOOCNa, and their 3:2 mixture at 10 °C.

transition in the adsorbed layers even with a solution of concentration 1.3 × 10-5 M, while the critical micelle concentration of the amphiphile at this temperature is around 0.56 × 10-5 M. Similar phase behavior has been observed previously at higher temperatures with higher concentrations of the amphiphile.22 The well-known reason behind this fact is the strong hydration, the dipole-dipole and the electrostatic repulsions among the bulky hydrophilic headgroups that do not allow the molecules to cohere in a condensed-phase domain. On the other hand, the π-t curves of pure EGDE and its mixture with TOOCNa show well-defined cusp points followed by pronounced plateau regions indicating the existence of a first-order phase transition in the adsorbed monolayers. The appearance of the cusp point moves to early time for the mixed monolayer (≈1000 s) in comparison with that for pure EGDE (≈1900 s). The critical surface pressure necessary for the phase transition in the mixed system is lower than that for the pure EGDE. With the same hydrophilic group, the critical surface pressure necessary for the phase transition decreases with increasing the length of the hydrocarbon moiety7,13 and it is expected to increase with increasing the size of the headgroup. The resultant decrease of the critical surface pressure necessary for the phase transition for the EGDE-TOOCNa mixed system compared to that for pure EGDE is due to the influence of the longer hydrocarbon chain of TOOCNa over its headgroup. All these changes clearly indicate that the existence of phase transition in the EGDE-TOOCNa system results from the interaction of both of the surfactants. Generally it is considered that the nonionic surfactant molecules being inserted into the monolayers of the ionic surfactant reduce the repulsive interaction between the ionic headgroups. Although TOOCNa is ionic, three ethylene oxide units in its headgroup distance the charge from the hydrocarbon tail and thereby reduce its ionic character. Furthermore, both the surfactants have ethylene oxide groups; therefore, their simultaneous adsorption at the solution surface to form a close-packed mixed monolayer should not be sterically hindered. The in situ BAM observation of 1.3 × 10-5 M EGDE and the mixed solution containing the components at a molar ratio of 3:2 at 10 °C is shown in Figure 3. After the cusp point, the formation of bright two-dimensional condensedphase domains in a homogeneous dark background is observed for the monolayers of both pure EGDE and its mixture with TOOCNa. The appearance of the condensed domains observed by BAM in both cases further confirms the existence of a first-order phase transition. The domains of pure EGDE are of a fingering pattern with optical isotropy all over the domains at this temperature (image A). With increasing time, both growing and broadening

Phase Behavior of a Mixed Surfactant System

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Figure 3. Formation of condensed domains in the adsorbed monolayers of pure EGDE (image A) and the mixed monolayer of EGDE with TOOCNa (images B-D) at 10 °C. The bar in image A indicates 100 µm. Chart 1. Chemical Structures of the Surfactants

of fingers in the domains occur simultaneously. When the domains touch each other, they fuse and the surface becomes fully covered by the condensed domains at equilibrium. On the other hand, in the mixed monolayers, the domains are circular and they grow in size in a shapepreserving way (images B and C). With an increase in the adsorption time, the domains become gradually larger and start to touch each other. At this stage, deformation of the domains takes place; nevertheless they do not fuse (image D). The change of the surface morphology reveals that the condensed phase and the surrounding less dense continuous phase are formed from the contribution of both the amphiphiles. At a definite temperature, the shape of a condensed domain is governed by the balance between the long-range dipolar repulsions of the molecules and the line tension of the interface between the two phases. The fractal domain in the case of the pure EGDE monolayer is the result of the predominance of repulsive interaction over line tension. The TOOCNa molecule has a longer hydrocarbon chain and a charged headgroup with three ethylene oxide units. As shown in Chart 1, these ethylene oxide units distance the negative charge from the hydrophobic part and thereby reduce its ionic character. As a result, the effect of the hydrocarbon tail outweighs the effect of the headgroup in raising the line tension of the condensed domains. Besides, the presence of three ethylene oxide units in the headgroup of TOOCNa molecules provides a considerable flexibility of the ethylene oxide moiety. Because of this flexible nature, the terminal part of the headgroups of TOOCNa molecules being situated below the headgroups of the adsorbed EGDE molecules can escape the electrostatic repulsions (Chart

Chart 2a

a Because of electrostatic repulsions between the headgroups of TOOCNa, the monolayer gives only the expanded phase (top). Insertion of EGDE molecules into the TOOCNa monolayer reduces the electrostatic repulsions and favors the formation of densely packed condensed-phase domains in the mixed system (bottom).

2). This facilitates gathering of several TOOCNa molecules at the interface which in turn favors strong van der Waals interactions between the long alkyl chains of TOOCNa molecules and thereby increases the line tension of the

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Figure 4. Surface tension versus concentration (M) curves for pure EGDE (I), TOOCNa (II), and their 3:2 mixture (III) at 10 °C.

condensed-phase domains. Thus incorporation of TOOCNa molecules imparts higher line tension which accounts for the circular condensed domains in the mixed monolayer at the same temperature. Interaction Parameters in the Adsorbed Film. Figure 4 shows the equilibrium surface tension versus bulk molar concentration plots for pure EGDE, TOOCNa, and their mixture at a molar ratio of 3:2 (data for other mixtures are not shown). These are used to calculate the interaction parameter, βσ, of mixed monolayer formation. The sign and magnitude of βσ are a good measure of interaction between two surfactants in a mixture. A negative value of βσ means that the attractive interaction between the two different surfactants in a mixture is stronger than the attractive interaction between the two surfactants of the same kind.1,30 Table 1 shows the interaction parameters, βσ, and the composition of the surfactants in the mixed monolayer at various bulk compositions. The values of βσ for the mixed monolayer formation at different molar ratios are calculated using the following equations.1

X12 ln(R1C/X1C1) (1 - X1) ln[(1 - R1)C/(1 - X1)C2]

)1

(1)

and

βσ )

ln(R1C/X1C1) (1 - X1)2

(2)

where X1 is the mole fraction of surfactant 1 in the total mixed monolayer; R1 is the mole fraction of surfactant 1 in the total surfactant concentration in the solution phase; and C1, C2, and C are the solution-phase concentrations of surfctant 1, surfactant 2, and their mixture at a definite molar ratio required to produce a definite surface tension value. Substitution of the values of C1, C2, and C from Figure 4 at the mole fraction R1 of surfactant 1 for a definite surface tension value in eq 1 gives the value of X1, which on substitution in eq 2 yields the interaction parameter β σ. The value of βσ for mixed monolayer formation at the air/water interface is governed by the electrostatic interaction between the hydrophilic headgroups of the two different surfactants.1,29 The calculated βσ values are found to be negative over the mixing ratios (Table 1), suggesting (29) Rosen, M. J.; Zhou, Q. Langmuir 2001, 17, 3532.

Figure 5. PM-IRRAS spectra of adsorbed monolayers of (a) EGDE and (b) mixed EGDE/TOOCNa at a molar ratio of 3:2 at the water surface at 10 °C in the C-H stretching vibration region.

that there exists an attractive interaction between the adsorbed molecules in the mixed monolayer. Such negative values of the interaction parameter for anionic-nonionic surfactant mixtures have been observed previously.29,30 According to Rosen et al.,1 the concentrations used in calculation should be as close to the cmc of the mixed system as possible to ensure that the molecules are closely packed in the mixed adsorbed film. The values of the interaction parameters are found to be almost constant over the mixing ratios. The composition of the surfactants in the monolayer varies with the change of the molar ratio of the surfactants in the bulk. At the molar ratio of 3:2, the composition of the surfactants in the monolayer is found to be equal (Table 1) and shows better surface activity than for other mixing ratios. This is probably because of equal distribution of EGDE and TOOCNa molecules in the monolayer. The negatively charged heads of TOOCNa being separated by the uncharged heads of EGDE reduces the electrostatic repulsions between the charged TOOCNa molecules and thus facilitates densely packed mixed monolayer formation. PM-IRRAS Study. PM-IRRAS spectra of the adsorbed monolayer of pure EGDE and its mixture with TOOCNa were recorded at equilibrium surface pressure. For this purpose, we used 3.0 × 10-5 M solutions for both pure and mixed monolayers to ensure the rapid establishment of equilibrium. It takes about 30 min to attain the equilibrium pressure at this concentration. Under this condition, the surface is fully covered by the condensed-phase domains. No conclusive result is obtained from the PMIRRAS spectra of the pure TOOCNa monolayer, as the peak intensity is too weak compared to the noise (data are not shown). This is due to the repulsive interaction between the large negatively charged heads of TOOCNa, which prevents close packing of the molecules in the monolayer. Figure 5 shows the PM-IRRAS spectra of the monolayers of EGDE and its 3:2 mixture with TOOCNa at 10 °C in the region 3000-2800 cm-1 at the water surfaces. The spectra recorded from the monolayers of pure EGDE and its mixture with TOOCNa show clear bands corresponding to the methylene asymmetric (νas(CH2)) and symmetric (νs(CH2)) stretching regions. It is important to note here that the CH2 stretching modes appear as positive bands in the PM-IRRAS spectra, indicating that the corresponding transition dipole moments (TDMs) are preferentially oriented parallel to the water surface. The position of the (30) Zhu, D.; Zhao, G. Colloids Surf. 1990, 49, 269.

Phase Behavior of a Mixed Surfactant System

Figure 6. Normal transmission spectra of EGDE and TOOCNa cast on a CaF2 substrate by manually drying a few drops of solutions of the pure samples in hexane (a) and PM-IRRAS spectra of adsorbed monolayers of pure EGDE and its 3:2 mixture with TOOCNa at the air/D2O interface in the range between 1700 and 1050 cm-1 (b).

νs(CH2) mode is unchanged for both pure and mixed monolayers. On the other hand, the νas(CH2) mode for the monolayer of pure and mixed systems appears at 2920 and 2916.5 cm-1, respectively. According to the available literature, asymmetric methylene stretching vibrations are sensitive to the conformational order of the hydrocarbon chain and it can be related empirically with the order with respect to the trans/gauche ratio. A lower vibrational frequency is characteristic of better conformational order with preferential increase of the trans/ gauche ratio.31-33 Therefore, the decrease of the asymmetric CH2 stretching frequency suggests the incorporation of TOOCNa molecules, which accounts for better conformational order of the hydrocarbon chain in the mixed monolayer compared to that of the pure EGDE monolayer. To detect what molecules are present in the condensed phase, we need to probe the polar headgroup region (17001050 cm-1). It is also possible to gather information about the subcell packing in the monolayer from the methylene bending δ(CH2) mode that also lies within this region. In PM-IRRAS spectra, a broad negative band appears in the region between 1700 and 1600 cm-1 23,24 due to the δ(H2O) mode of liquid water. Figure 6a shows the transmission spectra of TOOCNa in the region of 1600-1050 cm-1. The bands at 1608 and 1428 cm-1 are assigned to asymmetric νas(COO-) and symmetric νs(COO-) stretching modes of the carboxylate group, respectively. Therefore, it is not straightforward to explore the signal of the COO- stretch(31) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1988, 88, 334. (32) Weers, J. G.; Scheuing, D. R. Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; American Chemical Society: Washington, DC, 1991; p 91. (33) Gericke, A.; Hu¨nerfuss, H. J. Phys. Chem. 1993, 97, 12899.

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ing modes in the strong absorption region of water to corroborate the presence of TOOCNa molecules in the adsorbed monolayer along with EGDE. To facilitate the interpretation of the spectra of the mixed monolayer, we carried out the experiments using D2O as a subphase under the same experimental conditions. In this measurement, the δ(H2O) mode completely disappears and is replaced by the δ(D2O) mode which appears around 1200 cm-1. Figure 6b shows the PM-IRRAS spectra of monolayers of pure EGDE and the EGDE/TOOCNa mixed system in the region between 1700 and 1050 cm-1 on D2O subphases. The bands at 1595 and 1430 cm-1 are assigned to the νas(COO-) and νs(COO-) stretching modes, respectively. This value is in good agreement with the value observed for the isotropic spectrum collected by drying the sample from hexane solution on a CaF2 substrate (Figure 6a). The bands of the νas(COO-) and νs(COO-) modes are oriented upward and downward, respectively. According to the PM-IRRAS selection rule,23,24 the direction of a band depends on the orientation of the TDM of the absorbing molecular group with respect to the electric field of the polarized radiation. An upward-oriented band implies that the corresponding transition moment is preferentially aligned in the plane of the monolayer, whereas a downward-oriented band implies that the corresponding transition moment is perpendicular to the plane of the monolayer. Information about the packing of molecules in the monolayer can be obtained from the analysis of the δ(CH2) mode, since it is sensitive to interchain interaction and often used as a tool to check the state of molecular packing. The band around 1472 cm-1 in Figure 6b is assigned to the δ(CH2) mode of the hydrocarbon chains of the mixed monolayer. The δ(CH2) bands of genuine triclinic and hexagonal unit cells should be sharp with an fwhm (full width at half-maximum) of