Adsorption Properties of Novel Gemini Surfactants with Nonidentical Head Groups

Abstract

Novel environmentally friendly gemini surfactants, each with two hydrophilic and two hydrophobic groups, have been synthesized and their physicochemical properties investigated. One of the hydrophilic groups is a methyl-capped polyoxyethylene chain with mol wt 350, 550, and 750 g/mol, respectively, and the other is a sulfate group; the hydrophobic part of the surfactant is made from oleylnitrile. This nitrile derivative of the fatty acid is used to achieve good hydrolytic stability. Du Nouy ring and maximum bubble pressure tensiometry were used for equilibrium and dynamic surface tensions, γe and γt, respectively. The aqueous-phase critical micelle concentrations of the heterogeminis (HGs) have been investigated. The results have been compared with those for mixtures of standard surfactants sodium decylsulfate and octaoxyethyleneglycol mono n-decyl ether under equivalent conditions. The HGs are shown to exhibit improved performance over the mixed system both in terms of micellization and surface tension lowering. Dynamic surface tension (DST) studies were performed to investigate air–water adsorption mechanisms. A diffusion-limited mechanism was confirmed in the initial stages of adsorption. However, closer to the equilibrium the DST data are inconsistent with a diffusion-only mechanism. In particular, the HGs show a larger deviation from diffusion control as compared to the model mixture, which is a signature of slower adsorption kinetics. In addition to air–water interfaces, properties of these HGs have also been investigated at solid silica–solution surfaces by optical reflectometry. These surfaces were either naturally hydrophilic or rendered hydrophobic by chemical modification. On either surface the maximum amount of adsorbed surfactant was found to increase when the polyoxyethylene chain length decreases.