Surfactant Modeling Using Classical Density Functional Theory and a Group Contribution PC-SAFT Approach

Models for surfactants need to incorporate the amphiphilic character of the molecules to describe key properties such as the adsorption at interfaces and the reduction of interfacial tensions. One possibility is to model the surfactant molecules as heteronuclear chains. Therefore, we revisit the heterosegmented density functional theory and present a theory consistent with the group contribution perturbed-chain statistical associating fluid theory equation of state. The model is used to study water/surfactant and water/surfactant/octane systems with surfactants from the group of polyethylene glycol alkyl ethers, a commonly used group of nonionic surfactants. The model parameters are obtained by fitting to pure component data of small surfactants. Binary interaction parameters are required to model the water/alkane subsystem and to account for the polarity of the head groups of the surfactant. The model is able to reproduce the significant enrichment of surfactant molecules at both vapor–liquid surfaces and liquid–liquid interfaces and the corresponding reduction of interfacial tensions. For liquid–liquid interfaces, the competing solubility of the surfactant in both phases has to be taken into account when searching for an optimal surfactant molecule.