Effect of branching on the interfacial properties of nonionic hydrocarbon surfactants at the air-water and carbon dioxide-water interfaces

The interfacial tensions, surface pressures, and adsorption of nonionic hydrocarbon surfactants at the air-water (A-W) and carbon dioxide-water (C-W) interfaces were investigated systematically as a function of the ethylene oxide (EO) unit length and tail structure. Major differences in the properties are explained in terms of the driving force for surfactant adsorption, tail solvation, area per surfactant molecule, and surfactant packing. As the surfactant architecture is varied, the changes in tail-tail interactions, steric effects, areas occupied by the surfactant at the interface, and tail hydrophobicity are shown to strongly influence the interfacial properties, including the surfactant efficiency (the concentration to produce 20 mN/m interfacial tension reduction). For linear surfactants at the A-W interface, high efficiencies result from dense monolayers produced by the high interfacial tension driving force for adsorption and strong tail-tail interactions. At the C-W interface, where a low interfacial tension leads to a much lower surfactant adsorption, the contact between the phases is much greater. Branching or increasing the number of tail chains increase the hydrophobicity, tail solvation, and adsorption of the surfactant. Furthermore, the area occupied by the surfactant increases with branching, number of tails, and number of EO monomers in the head group, to reduce contact of the phases. These factors produce greater efficiencies for branched and double tail surfactants at the C-W interface, as well as surfactants with longer EO head groups.