Organization of Amphiphiles VI.A Comparative Study of the Orientation of Polyoxyethylated Alkyl Phenols at the Air-Water and the Silica-Water Interface

摘要

The adsorption behavior on silica of a series of polyoxyethylated alkyl phenols of varying hydrophobicities and hydrophilicities was studied by determining the adsorption isotherm along with the zeta potential of silica.An increase in the concentration of each of the surfactants caused the zeta potential to become less negative and ultimately to reach a plateau;similarly,the adsorption isotherm also plateaued.These results were attributed to the flat orientation of the surfactants caused by hydrogen bonding of the oxygen atom of the oxyethylene group with the silanol group of the silica,which led to the formation of a thin oxyethylene layer at the silica-water interface.This layer masked a constant fraction of the silica surface.A model for the orientation of the surfactants at the silica-water interface was proposed,based on a "molecular parking area" (i.e.,space occupied by the molecule during adsorption at the silica-water interface) of the surfactants on the silica surface.