Hydrophobic interactions described using hetero-segmented PC-SAFT: 2. Surfactants and their aqueous solutions

Despite their importance for industry and pharmaceuticals applications, description of aqueous solutions that contain surfactants is still a challenging task in thermodynamic modeling. As a first step towards a holistic modeling approach, which is also applicable for concentrated surfactant solutions, this work aimed to model the intrinsic behavior of surfactant molecules. For this purpose, we applied hetero-segmented PC-SAFT as a group contribution method to build surfactant molecules from different groups, which separately characterize the hydrophobic tail and the hydrophilic head of the surfactant. While the hydrophobic tail is modeled by the parameterization developed in the first part of this paper series (M. Rother, G. Sadowski, Fluid Phase Equilibria 582 (2024)), this work focuses on extending the parameter matrix to model the hydrophilic head. We considered the surfactant classes CiG1, CiEj and MEGA-i. The parameters for the surfactant head groups were adjusted to sorption data of surfactant/alcohol systems and to partition coefficients of the surfactants in n-alkane/water systems and n-alcohol/water systems. As a benchmark of the new parameterization, we modeled the critical micelle concentration as a function of temperature for these three surfactant classes using a newly developed, explicit equation for calculating this quantity. The results are in even quantitative agreement with the experimental data.