Ellipsometry and Infrared Reflection Absorption Spectroscopy of Adsorbed Layers of Soluble Surfactants at the Air-Water Interface

摘要

Optical techniques play an increasingly important role in the characterization of microstructure and surface densities of thin films at various interfaces. In this study, ellipsometry and infrared reflection absorption spectroscopy (IRRAS) were used for deter- mining the surface densities of adsorbed layers of cationic sur- factants in situ at the air-water interface. l'he surfactants were JV" -lauroyl-arginine methyl ester (LAM) and 1V" ,Nw-bis(N" - lauroyl-arginine )-a ,w-alkylidenediamide (C6(LA)z). In ellipsome- try, the ellipsometric phase angJe t,. was obtained at various sur- factant concentrations and was referenced to that of the solvent. Three algorithms }V~re used for analyzing the data. The sur- face densities are 3.3+-0.3 x 10-6 mol/mz at 1 mM for LAM and 1.5+-0.3 x 10-6 mol/mz at 0.1 mM lor C6(LA)z by using an algo- rithm for which the monolayer thickness was estimated from molec- ular modeling. The corresponding surface densities from literature surface tension data and the Gibbs adsorption isotherm procedure are 2.2+-0.4 x 10-6 mol/mz and 1.2 :1:0.2 x 10~(-6) mol/m2, respec- tively. .In addition, IRRAS spectra wereobtained from monolayers of LAM and C6(LAh at the air-water interface. The frequencies of the methylene stretching vibration bands indicate that the mono- layers are liquid-Iike. The surface densities were determined from the reflectance-absorbance data by using the model of either an isotropic film or an anisotropic film on the aqueous subphase. The IRRAS-based surface densities from either model, by using DPPC monolayers for calibration, are 2..4+-0. 7x 10~(-6) mol/m2 at 1 mM for LAM and 1.5 :1:0.6 x 10-6 mol/mz atO.1mMforC6(LA)z, which are in fair agreement with the eIlipsometry- and the surface-tension- based surface densities.