Temperature Independent Adsorbate Structure of 4-Octyl-, 4-Decyl-, and 4-Dodecylbenzenesulfonates at the Al2O3/Water Interface

Abstract

In situ Fourier transform infrared (FTIR)-attenuated total reflection (ATR) spectroscopy and linear dichroism (LD) analysis were used to characterize the Gibbs' surface excess (Γ) and molecular orientation (ϑ, mean tilt from the interface normal) of sodium 4-n-octylbenzenesulfonate (SOBS), sodium 4-n-decylbenzenesulfonate (SDeBS), and purified sodium 4-n-dodecylbenzenesulfonate (SDoBS) adsorbed at the Al2O3/water interface. The Γ values were confirmed by the solution depletion method. The ϑ values were determined separately for the phenyl rings, alkyl chains, and SO3- head groups. Thin films (150−220 nm) of Al2O3 on the surface of ZnSe IR internal reflection elements allowed in situ IR ATR adsorption measurements on a model hydrophilic solid from the near IR to 950 cm-1. Both Γ and ϑ for SOBS and SDeBS were insensitive to ionic strength, but were slightly sensitive to pH and temperature. The crystal structure of SOBS·1/2H2O was determined by single-crystal X-ray diffraction and compared with polarized sin...