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

• Published in: Journal of colloid and interface science Vol. 233; no. 2; pp. 295 - 305
• Main Authors: Walsh, Christopher B., Wen, Xinyun, Franses, Elias I.
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 15.01.2001; Elsevier
• Subjects: cationic surfactants; Chemistry; ellipsometry; Exact sciences and technology; Gas-liquid interface and liquid-liquid interface; gemini surfactants; General and physical chemistry; reflection absorption spectroscopy; Surface physical chemistry; Surface-active agents: properties; cationic surfactants; ellipsometry; reflection absorption spectroscopy; gemini surfactants; Gas liquid interface; Monolayer; Infrared reflection; Absorption spectrometry; Cationic surfactant; Dimer; Air water interface; Aqueous solution; Surface area; Experimental study; Arginine polymer; Ellipsometry
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1006/jcis.2000.7275

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 determining the surface densities of adsorbed layers of cationic surfactants in situ at the air–water interface. The surfactants were Nα-lauroyl-arginine methyl ester (LAM) and Nα, Nω-bis(Nα-lauroyl-arginine)-α,ω-alkylidenediamide (C6(LA)2). In ellipsometry, the ellipsometric phase angle Δ was obtained at various surfactant concentrations and was referenced to that of the solvent. Three algorithms were used for analyzing the data. The surface densities are 3.3±0.3×10−6 mol/m2 at 1 mM for LAM and 1.5±0.3×10−6 mol/m2 at 0.1 mM for C6(LA)2 by using an algorithm for which the monolayer thickness was estimated from molecular modeling. The corresponding surface densities from literature surface tension data and the Gibbs adsorption isotherm procedure are 2.2±0.4×10−6 mol/m2 and 1.2±0.2×10−6 mol/m2, respectively. In addition, IRRAS spectra were obtained from monolayers of LAM and C6(LA)2 at the air–water interface. The frequencies of the methylene stretching vibration bands indicate that the monolayers are liquid-like. 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±0.7×10−6 mol/m2 at 1 mM for LAM and 1.5±0.6×10−6 mol/m2 at 0.1 mM for C6(LA)2, which are in fair agreement with the ellipsometry- and the surface-tension-based surface densities.