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J. Phys. Chem. C 2008, 112, 2040-2044
Adsorbed Monolayers of Mixed Surfactant Solutions of Sodium Dodecylsulfate and Cetylpyridinium Chloride Studied by Infrared External Reflection Spectroscopy Takeshi Kawai,* Yukinori Yamada, and Takeshi Kondo Department of Industrial Chemistry, Faculty of Engineering, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ReceiVed: August 22, 2007; In Final Form: October 29, 2007
Infrared external reflection (IER) spectra of monolayers adsorbed on mixed surfactant solutions of sodium dodecylsulfate (SDS) and cetylpyridinium chloride were measured as a function of mole fraction of SDS (XSDS). The alkyl chains of the surfactants in the adsorbed monolayer (Gibbs monolayer) at the air-solution interface were in all-trans conformations in the solid state, regardless of XSDS. The adsorbed monolayers at any XSDS had a single layer structure, except for XSDS ≈ 0.6-0.7, which had multilayer structures. Furthermore, the time dependency of the IER spectra revealed that the formation of the multilayer structures occurs in two processes: a faster single layer formation followed by subsequent slower multilayer formation.
Introduction
Experimental Section
The adsorption of surfactant molecules at interfaces is an important aspect in surface science research directed at elucidating surface structures. Monomolecular films at the air-water interface have been a considerable focus of recent model systems for biological membranes and two-dimensional pattern formation, photomechanical responses, and molecular recognition systems.1-9 Although a number of techniques have been employed to investigate the microscopic structure and morphological properties of insoluble monolayers on water, the application of these techniques to water soluble surfactant monolayers adsorbed at the air-solution interface (Gibbs monolayer) is still limited.10-18 Infrared external reflection (IER) spectroscopy is a nondestructive and sensitive method of monitoring molecular conformation. Since Dulhy et al. first applied IER to Langmuir films,19 extensive applications have been developed to characterize the structure of various Langmuir and Gibbs film systems. In a previous article,20 we measured IER spectra and IR transmission spectra of aqueous SDS solutions and revealed the relationships between molecular states at the air-solution interface and in the bulk, and the effect of temperature on the adsorbed sodium dodecylsulfate (SDS) state at the air-solution interface. Aqueous mixtures of cationic-anionic surfactants have recently received a great deal of attention because they showed spontaneous vesicle formation or much higher surface activity than their individual components.21-25 Patist et al.26 demonstrated the effects of molecular association in a mixed surfactant system of SDS and cetylpyridinium chloride (CPC) on interfacial properties, such as surface tension, surface viscosity, and foam stability; however, no molecular information on the surface monolayer at the air-solution interface was provided. In this article, IER spectra of adsorbed (Gibbs) monolayers of SDS and CPC at the air-solution interface were measured, and the effect of the mixing ratio of SDS and CPC on the state of the alkyl chain and the composition of the adsorbed monolayer was studied.
Cetylpyridinium chloride (Nacalai Tesque, Inc.) was recrystallized three times from acetone. Sodium dodecylsulfate (Nacalai Tesque, Inc.) was recrystallized two times from acetone. Deuterated SDS (SDS-d25, Cambridge Isotope Laboratories, Inc.) was used without further purification. Distilled deionized water was used for all experiments. Aqueous solutions of mixed surfactant were prepared from stock solutions of 1.0 mM SDS and 1.0 mM CPC. All experiments were performed using a total concentration (Mtot) of [SDS] + [CPC] ) 1 mM. A given volume of mixed solution was transferred into a trough, and a waiting time of 6-12 h was allowed for adsorption equilibrium to be reached. The sample was then subjected to FTIR single-beam reflectance (R) at 28 ( 1 °C. An exact volume of pure water was used to collect the single-beam background (R0), because the effect on the IER spectra of surfactant dissolved in bulk solution was negligible at concentrations less than 10 mM of the surfactant.20 The reflection spectrum was defined as -log(R/R0). The complex of SDS and CPC was made from an equivalent molar mixture of SDS and CPC in methanol, and then the mixture was evaporated to dryness. The dried mixture was dissolved in chloroform. The Langmuir monolayer of the SDS and CPC complex was obtained by spreading the chloroform solution of SDS + CPC (1 mM) on water. To prepare a Langmuir-Blodgett (LB) film of the SDS + CPC complex, the vertical dipping method was used for deposition onto a ZnSe attenuated total reflectance (ATR) plate at a surface pressure of 35 mN/m. The surface pressure was measured using the Wilhelmy technique and a glass plate attached to an HBM film balance (Kyowa Interface Science Co., Ltd.). IR spectra were recorded on a Nicolet 510M equipped with a narrow band MCT detector. IER spectroscopy was performed using a modified ATR attachment (Spectra-Tech Inc.). An unpolarized beam at an incident angle of 30° was used to obtained interferograms, which were accumulated 50-2000 times and had a resolution of 4 cm-1. For ATR measurements, an ATR attachment (Spectra-Tech Inc.) was used and the incident angle of the IR beam was 45°.
* Corresponding author. Phone: +81 3 5228 8312. Fax: +81 3 5261 4631. E-mail:
[email protected].
10.1021/jp0767260 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/19/2008
Mixed Adsorbed Monolayers of SDS and CPC
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TABLE 1: Peak Positions and Intensities of the νa(CH2) Band in the IER Spectra of Adsorbed Monolayers of Mixed Surfactants of SDS and CPC XSDS
wavenumber/cm-1
-log(R/R0)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
2924.0 2918.3 2918.4 2918.2 2918.3 2917.4 2917.5 2917.3 2918.6 2919.4 2925.2
2.07 × 10-3 5.54 × 10-3 5.30 × 10-3 5.70 × 10-3 5.54 × 10-3 6.19 × 10-3 8.34 × 10-3 7.74 × 10-3 5.68 × 10-3 4.45 × 10-3 0.87 × 10-3
Results and Discussion State of the Alkyl Chains of SDS and CPC in the Adsorbed Monolayer. It is generally known that wavenumbers of CH2 stretching modes are conformation sensitive and can be empirically correlated with the conformational order (trans/ gauche ratio) of the alkyl chains.27,28 To examine the conformational order of the alkyl chains of CPC and SDS in the adsorbed monolayer at the air-water interface, we collected IER spectra of the monolayers of mixed surfactant systems (CPC + SDS). The stronger bands for the antisymmetric and symmetric CH2 stretching modes [νa(CH2) and νs(CH2)] of the alkyl chains of CPC and SDS were observed at ∼2920 and ∼2850 cm-1, respectively. Table 1 shows the wavenumbers and intensities of the νa(CH2) band for SDS + CPC monolayers adsorbed at the air-solution interface as a function of SDS mole fraction (XSDS). The wavenumbers of the νa(CH2) for mixed surfactant systems were ∼2918 cm-1, regardless of XSDS, whereas those for the pure surfactant systems (XSDS ) 0 and 1) were ∼2924 cm-1. A wavenumber of ∼2918 cm-1 is characteristic of highly ordered conformations with preferential alltrans characteristics, while that of ∼2924 cm-1 is characteristic of melting of the methylene chains, in which a number of gauche conformers exist.27,28 Thus, these results indicate that the alkyl chains of the adsorbed monolayers for the mixed surfactant systems of SDS + CPC are in the all-trans conformation (i.e., the solid state), while those in a pure SDS or CPC system are in a fused state. Surface Density of Adsorbed Monolayer. The intensities (peak heights) of the mixed surfactant systems were larger than those of pure surfactant systems (XSDS ) 0 and 1), as shown in Table 1. This is due to the difference in the state of the alkyl chains, because, in general, the νa(CH2) bands in the solid state show a greater peak height and narrower bandwidth than those in fused states.29 In addition, the larger intensities of the mixed surfactant systems are caused by increases in the surface density of CPC and SDS. Interestingly, the intensities of the mixed surfactant systems were in the range of 0.0045-0.0062, except for XSDS ) 0.6 and 0.7, and the average value was 0.0055. To evaluate the surface density of CPC and SDS from the IER intensities in Table 1, we then collected the IER spectra of spread (Langmuir) monolayers of SDS and CPC. The spread monolayer of the complex was prepared by spreading a chloroform solution of the mixture on water. The complex monolayer showed a condensed phase above a surface pressure of 20 mN/m in the π-A curve (Figure 1), and a steady-state surface pressure of 35 mN/m was maintained for a long time. The peak position of the νa(CH2) band in the Langmuir monolayer of CPC + SDS was ca. 2918 cm-1, and the intensity was 0.0055 at a surface pressure of 35 mN/m. This intensity is almost equal to those of the adsorbed monolayers of mixed
Figure 1. Surface pressure and surface area isotherm for monolayer of CPC + SDS on water at 28 °C.
Figure 2. IER spectra of adsorbed monolayers of mixed surfactants of SDS and CPC in the SO3- stretching region as a function of XSDS.
Figure 3. FTIR-ATR spectra of (a) one-monolayer and (b) threemonolayer LB films of SDS and CPC ion complexes on ZnSe, prepared at a surface pressure of 35 mN/m.
surfactant systems, except in the cases of XSDS ) 0.6 and 0.7. We concluded that the adsorbed monolayer consists of a single layer surfactant structure, while at XSDS ) 0.6 and 0.7, the adsorbed film is a multilayer structure. This conclusion was also confirmed by the fact that the band intensity of the head group of SDS at XSDS ) 0.6 and 0.7 is larger than the others, as shown in Figure 2. In addition, the spectral features of the asymmetric SO3- band at XSDS ) 0.6 and 0.7 differ from the others. Namely, at XSDS ) 0.2 and 0.4 the spectral features are similar to that of pure SDS solution (XSDS ) 1.0), with a broad peak at around 1230 cm-1. In XSDS ) 0.6 and 0.7, strong bands were observed at 1258 and 1213 cm-1. This difference is probably due to the
2042 J. Phys. Chem. C, Vol. 112, No. 6, 2008
Kawai et al.
Figure 4. IER spectra of adsorbed monolayers of mixed surfactants of SDS-d25 and CPC in the CH and CD stretching region as a function of XSDS.
structure of the film. That is, for XSDS ) 0.6 and 0.7, the film has a multilayer structure where the SO3- groups can have interlayer and intralayer interactions with the SO3- and pyridinium groups, while for the other XSDS values, the SO3- groups only have intralayer interactions with the pyridinium groups. To confirm the above estimation, we observed ATR spectra of one-monolayer and three-monolayer LB films of SDS and CPC complexes on a ZnSe plate. Figure 3 shows the ATR spectra of one-monolayer and three-monolayer LB films. The spectral features of the three-monolayer LB film were very similar to those of adsorbed film of XSDS ) 0.6 and 0.7, while the spectrum of the one-monolayer LB film showed a broad band. Therefore, it is reasonable to conclude that the adsorbed film for XSDS ) 0.6 and 0.7 has a multilayer structure. Composition of Adsorbed Monolayer. Although we attempted to elucidate the composition of the adsorbed monolayers using IR bands of the head groups of CPC and SDS, we could not find a suitable band for monitoring the head group of CPC. We therefore used deuterated SDS (SDS-d25) to study the monolayer composition by using CD and CH stretching bands, which are easily observed by IER measurements. Figure 4 shows the IER spectra of the adsorbed monolayers of mixed surfactant CPC + SDS-d25. Four strong bands at ∼2920 and ∼2850 cm-1 in the CH stretching region and ∼2190 and 2090 cm-1 in the CD stretching region are assigned to the antisymmetric and symmetric CH2 stretching modes for CPC and the antisymmetric and symmetric CD2 stretching modes [νa(CD2) and νs(CD2)] for SDS, respectively. As reported by Mao et al.,30 the band intensity observed in the IER spectrum depends both on the surface density of the molecules and on the orientation of the transition moments of the IR band. Since the alkyl chains of SDS and CPC are in an all-trans conformation as mentioned above, it is reasonable to assume that the alkyl chains of SDS and CPC have an identical molecular orientation regardless of XSDS, except in the cases of XSDS ) 0 and 1.0. Thus, the band intensity observed reflects the surface density of the adsorbed monolayers at the airsolution interface. In other words, the ratio of the intensity of the νa(CH2) band to that of the νa(CD2) band can be used as a measure of the surface composition.
Figure 5. Intensity ratio of the antisymmetric CH2 stretching band for CPC to the antisymmetric CD2 stretching band for SDS in the IER spectra of adsorbed monolayers (O) and transmission spectra of the KBr pellets (9) as a function of XSDS.
Figure 5 shows the intensity ratio (O) of the νa(CH2) band to the νa(CD2) band as a function of XSDS. The intensity ratios showed a constant value of 3-4, indicating a constant surface composition independent of XSDS. We also observed the infrared transmission spectra of KBr pellets containing mixtures of CPC and SDS-d25 at various XSDS’s. The intensity ratios (9) of the νa(CH2) band to the νa(CD2) band in the KBr spectra are also plotted in Figure 5. The ratio of 3-4 is consistent with the ratio of XSDS ) 0.5 in the KBr spectra. Thus, the composition of adsorbed monolayers is estimated to be XSDS ) 0.5 (i.e., [CPC]/ [SDS] ) 1.0), regardless of the composition of the bulk solution.31 Patist et al.26 have proposed a two-dimensional hexagonal arrangement of surfactant molecules of SDS/CPC ) 3:1 and 1:3 in mixed surfactant systems. However, from the IER measurements in our present work, the surface composition was evaluated to be SDS/CPC ) 1:1. The bulk solution consists of an ion pair amphiphile (i.e., a catanionic surfactant of SDS and CPC and the excess surfactant).26 The catanionic surfactant is preferentially adsorbed at the air-solution interface because of its high surface activity. Thus, these results are consistent with an equimolecular quantity of SDS and CPC as the composition of the adsorbed monolayer. Time Dependence of Adsorbed Monolayer. To clarify the formation process of the adsorbed monolayer at the air-solution interface, the changes in the IER spectra of mixed solutions
Mixed Adsorbed Monolayers of SDS and CPC
J. Phys. Chem. C, Vol. 112, No. 6, 2008 2043 the total concentration, Mtot, on the multilayer formation at XSDS ) 0.6. The single monolayer formation was faster with increasing Mtot, and a multilayer structure did not form at a lower concentration of Mtot ) 0.4 mM. Thus, it was found that the multilayer formation depends on both Mtot and XSDS, although the question remains why the single monolayer develops into the multilayer at XSDS ) 0.6-0.7 and Mtot ) 1 mM. To determine the reason for this multilayer formation, future studies on the concentration of free surfactants and ion pair amphiphiles of SDS and CPC and the interaction between the free surfactants and the ion pair amphiphile in bulk solution must be conducted. Conclusion
Figure 6. Time dependence of intensity (9) and wavenumber (b) of the νa(CH2) band in the IER spectra of adsorbed monolayer at XSDS ) 0.2.
We demonstrated that IER spectroscopy is a very powerful tool in the elucidation of the molecular states and compositions of adsorbed monolayers at the air-solution interface. IER spectra of the mixed surfactant solutions of SDS and CPC showed that the alkyl chains of the surfactants in the adsorbed monolayers at the air-solution interface were in all-trans conformations in the solid state, and the compositions of the monolayers were [CPC]/[SDS] ) 1.0, regardless of XSDS. Furthermore, the time dependency of the IER spectra revealed that the formation of the multilayer structures at XSDS )0.6 occurs in two processes: a faster single layer formation followed by subsequent slower multilayer formation. Acknowledgment. This study was partially supported by a Grant-in-Aid for Scientific Research (No. 17510091) and by the “High-Tech Research Center” project for private Universities (2005-2008) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References and Notes
Figure 7. Time dependence of intensity of the νa(CH2) band in the IER spectra of adsorbed monolayer at XSDS ) 0.2, 0.6, and 0.8.
were measured as a function of time, after the surface of the mixed solution was swept out with a Teflon bar to make a fresh surface. The intensity (9) and wavenumber (b) of the νa(CH2) band in the IER spectra of XSDS ) 0.2 with time are shown in Figure 6. The intensity gradually increased with time until 3 h, reaching a constant value of 5.3 × 10-3, which is in agreement with the intensity of the single monolayer as mentioned above. In contrast, the corresponding wavenumber gradually decreased from 2926 to 2918 cm-1, indicating that the alkyl chain of the surfactant (SDS and CPC) changes from a fused state to the solid state. This result indicates that the crystallization and adsorption of CPC and SDS simultaneously occurred at the airsolution interface, instead of the crystallized surfactant molecules adsorbing with time. Figure 7 shows the intensity of the νa(CH2) band in the IER spectra at XSDS ) 0.2, 0.6, and 0.8 over time. This result reveals that formation of the multilayer structure at XSDS ) 0.6 consists of a two-step process at the air-solution interface (i.e., faster single monolayer formation within 2 h and subsequent slower multilayer formation). Furthermore, we examined the effect of
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