The effect of the molecular structure of alkyl ether carboxylate surfactants on the oil–water interfacial tension

Abstract

• OPLS-AA is not applicable to study surfactants at the alkane-water interface. • Trappe-UA and OPLS-AA underestimate the effect of surfactants on the IFT. • The adsorption limit decreases with the increase in the surfactant headgroup length. • Doubling the tailgroup length leads to a small decrease in the adsorption limit. • The effect of a surfactant molecule size on the adsorption limit is asymptotical. Alkyl ether carboxylate surfactants at the alkane–water interface are explored experimentally through interfacial tension measurements with the spinning drop method and molecular dynamics simulations. The experimental interfacial tension dependence on bulk surfactant concentration is interpreted with the Redlich-Peterson adsorption model, which fits the experiments well. The critical micelle concentrations obtained from the experimental isotherms agree reasonably with published data on other surfactants of the same class. The length of the ethylene oxide hydrophilic segment significantly influences the dependence of the CMC and the interfacial tension at CMC on temperature. The excess surface density of the surfactant at the decane-water interface is calculated with the Redlich-Peterson model and compared with the molecular dynamics simulations, which are performed with two different forcefields. The simulations showed only semiquantitative agreement with the experiments: surfactants interfacial concentrations corresponding to a particular value of tension are systematically higher than the values derived from the experiments, for both forcefields by a certain factor that appears constant. Simulations of surfactants with different lengths of the alkyl tail and ethylene oxide head segments revealed general tendencies related to the surfactants interactions in the layer. Initially, the ethylene oxide segment elongation decreases the monolayer density and increases the effective elasticity. However, as the number of ethylene oxide monomers exceeds approximately eight, the density and compressibility become invariant of the length, which is likely explained by the dominance of electrostatic forces between the charged carboxylate groups that decay with the distance slower than the steric repulsion between the ethylene oxide chains.