4964
Langmuir 1998, 14, 4964-4966
Infrared External Reflection Spectroscopy of Sodium Dodecyl Sulfate Monolayers at the Air-Solution Interface: Removal of Bulk-Phase Water Concentration Effects Takeshi Kawai,* Hiroaki Kamio, and Kijiro Kon-No Department of Industrial Chemistry, Faculty of Engineering, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received March 26, 1998. In Final Form: July 8, 1998 Infrared external reflection (IER) spectra of a Gibbs monolayer of sodium dodecyl sulfate (SDS) at the air-solution interface were obtained at various concentrations of aqueous SDS. A new method is used to remove the effect of SDS dissolved in the bulk water phase on measured IER spectra. Corrected IER spectra reveal that (i) conformational order of the alkyl chain of SDS at the air-solution interface improves with increasing the SDS concentration, (ii) conformational order at concentrations >3 mM is nearly constant, and (iii) the alkyl chain of SDS is in a liquid crystalline state.
Introduction The adsorption of atoms and molecules at interfaces is an important aspect in research directed at elucidating the science of surfaces. With regard to characterizing the phenomena of surfactant adsorption at an air-solution interface, the Gibbs adsorption equation and surface tension measurements are primarily used.1 Precisely estimating the degree of adsorption is possible using various techniques such as the radiotracer method,2 microtome method,3 or moving bubble method.4 On the other hand, available information clarifying the adsorption state of surfactants is relatively limited.5-7 As a result of the pioneering efforts of Dluhy,8-13 infrared external reflection (IER) spectroscopy has proven to be valuable for studying the state of insoluble monolayers on water. Fina et al.5,6 applied this method to a solution of sodium dodecylsulfonate (C12S) and concluded that C12S adsorbed at the air-solution interface is in a solid state above 3.6 mM, while Ren et al.7 obtained IER spectra of water-soluble polymers of poly(ethylene oxide) samples end-capped with fluorinated carbon chains at the airsolution interface. The sample concentrations in both cases, however, were relatively low (3 mM, SDS at the air-solution interface is in a liquid crystalline state, being similar to the soap film prepared using aqueous solutions of SDS19 but completely different from solidstate C12S at the air-solution interface as determined from the observed wavenumber of the νa(CH2) band of 2917 cm-1.5,6 (17) The adsorption saturation of SDS at 3 mM is somewhat lower than the critical micellar concentration (cmc) at 8 mM. The occurrence of the saturation below the cmc in the SDS system has been already pointed out by Tajima et al. (ref 2). They also demonstrated that the amounts of SDS adsorption directly measured by the radiotracer method are in good agreement with those values calculated by the Gibbs adsorption equation applying the surface tension data. (18) Kawai, T.; Umemura, J.; Takenaka, T. Bull. Inst. Chem. Res., Kyoto Univ. 1983, 61, 314. (19) Umemura, J.; Matsumoto, M.; Kawai, T.; Takenaka, T. Can. J. Chem. 1985, 63, 1713.
Letters
Conclusions Our presented method for removing the effect of SDS dissolved in the bulk water phase on measured IER spectra was shown to be suitable for obtaining IER spectra of adsorbed molecules at the air-solution interface in a SDS concentrated system of 10 mM or greater, while the effect on the IER spectra of SDS dissolved in bulk solution was negligible at concentrations less than 10 mM. Measured IER spectra were found to be highly effective in clarifying the states of SDS monolayers at the air-solution interface. Acknowledgment. T.K. is pleased to acknowledge financial assistance from Science University of Tokyo Foundation. LA980341E
