Adsorption Equilibria and Desorption Rates of Charged Ethoxylated Surfactants on Octadecyl Silica: Role of Electrostatics

David E. Keller, Ruben G. Carbonell and Peter K. Kilpatrick

Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905

Available online 26 April 2002.

Abstract

The effects of ethoxy chain length and ionic strength on surfactant adsorption to porous octadecyl-bonded silica (ODS) are reported for four surfactants of the general formula CH₃(CH₂)₁₅(OCH₂CH₂)_n pyridine, where n = 0, 3, 5, and 8. The critical micelle concentrations (CMCs), equilibrium surface densities on ODS, and the subsequent desorption rates from ODS were determined for each surfactant at varying ionic strengths. The equilibrium surface density of the surfactants above the CMC decreased with increasing ethoxy chain length, which is consistent with a larger head group occupying a greater area on the surface. Surface density also increased with higher ionic strength for all the surfactants, indicative of reduced repulsion between the charged head groups at higher ionic strength. The desorption rates from the surfactant-coated octadecyl silica decreased with both increasing ionic strength and increasing ethoxy chain length, suggesting that electrostatic repulsion between the head groups heavily influences desorption of the surfactant. The effect of ionic strength on CMC was most pronounced for the surfactant with no ethoxy groups. These experimental findings were shown to be consistent with a thermodynamic model which couples repulsive electrostatic interactions predicted by the Poisson-Boltzmann equation with the hydrophobic interactions of the surfactant with the octadecyl bonded silica.