A Statistical Associating Fluid Theory (SAFT) Framework for Aqueous Nonionic Surfactant Systems

An established molecular thermodynamic model of micellization and phase behavior of aqueous nonionic systems is incorporated into a statistical associating fluid theory (SAFT) framework to obtain a combined model capable of describing both microscopic aspects of solution behavior (i.e., micelle shape and size distribution) and bulk phase behavior including phase separation and osmotic pressure. The original form of the model advanced by Puvvada and Blankschtein [1] considers three distinct contributions to the Gibbs energy of the surfactant solution: 1) formation of the micellar distribution, 2) ideal mixing of solution species including monomers, micelles and water molecules, and 3) interaction between micellar aggregates. The strength of this model is in the detailed accounting for factors that account for micellization including transfer of hydrocarbon tails from water to the micellar core, formation of a curved hydrocarbon-water interface, hydrocarbon chain packing within the core, and steric head group interactions. However, the Blankschtein phenomenological model of micellization uses a mean field expression with an empirical parameter for the Gibbs free energy of interaction.

The SAFT approach [2] has been successfully applied to correlate and/or predict thermodynamic properties and phase behavior of a wide range of pure fluids and mixtures. This work draws on the SAFT methodology and develops a theoretically-based expression for the free energy of species interaction that replaces the mean field expression in Puvvada and Blankschtein’s original formulation [2]. A main feature of the new model is a recently proposed equation of state [3] for the repulsive free energy contribution. We show in this talk several examples of the combined model’s predictive ability for micellar and bulk aqueous solution properties of surfactants from the polyoxyethylene glycol monoether and glucoside families.

References

[1]. Puvvada, S. and D. Blankschtein, Molecular-Thermodynamic Approach to Predict Micellization, Phase Behavior and Phase Separation of Micellar Solutions. I. Application to Nonionic Surfactants, J. Chem. Phys. 92, 3710-3724 (1990).

[2]. Muller, E. A. and K. E. Gubbins, Molecular-Based Equations of State for Associating Fluids: A Review of SAFT and Related Approaches, Ind. Eng. Chem. Res. 40, 2193-2211 (2001).

[3]. Gow, A. S., S. Alkhaldi and S. Demir, Cubic and Quartic Hard-Sphere and Lennard-Jones Chain Equations of State as Foundations for Complex Fluid Modeling, Fluid Phase Equilibria 399, 1-15 (2015).