(502a) Microemulsions: Modeling of Temperature and Concentration Limits

Microemulsions have been successfully applied as reaction media (e.g., hydrogenation reaction) and for separation of reaction products in reaction engineering as well as in the fields of surfactant-enhanced oil recovery, cosmetics, pharmaceuticals etc.

Microemulsions are mixtures of oil, water, and, surfactant. They exist in a certain range of temperatures and compositions and are thermodynamically stable (one-phase) [1]. On the one hand, oil-water-surfactant systems enable the mixing of poor water soluble organic compounds and water by adding non-ionic surfactants. On the other hand, such systems offer the advantage to build one, two, or three phase systems changing only the temperature or the amount of surfactant that can be used for effective separation processes.

The temperatures T_u, T_l, T̃ and the parameters γ₀ and γ̃ characterize the phase limits in the microemulsion system. A microemulsion middle phase with excess phases of water and octane occurs in the temperature range between T_u and T_l with the middle temperature T̃ and in the fraction range of surfactant in the ternary system between γ₀ and γ̃ at fixed ratio of water and oil.

This paper focuses on thermodynamic modeling of the characteristic temperatures and compositions in the microemulsion sytems. Two predictive models are used here: the group-contribution method UNIFAC and the a priori Conductor like Screening Model for Real Solvents (COSMO-RS) based on quantum mechanics. Both models have been successfully used to model micellar systems [2]. The main goal of the present study is to show the applicability of these models to predict the phase boundaries in the oil-water-surfactant systems with microemulsions.

As model systems mixtures of octane, water and non-ionic surfactants (ethoxylated alcohols such as Tetra-/Penta-/Hexaethylenglycolmonododecylether and Tetra-/Pentaethylenglycolmonodecylether) are considered. For the prediction of the phase boundaries, an efficient method is developed. The method is based on the assumption of constant distribution of one of the three system components between the two coexisting phases built up from the two other components. Using this method, the temperatures and system compositions that define the limits of three-phase region are determined. As results, the temperatures T_u, T_l, T̃ and the parameters γ₀ and γ̃ are predicted and compared with experimental values found in literature.

Both the UNIFAC and COSMO-RS models have been shown to be able to predict the temperature dependence of the three-phase region characteristics as well as their dependence on the amount of surfactant in the ternary system. Those predictions minimize the experimental effort significantly.

[1] K. Holmberg, B. Jönsson, B. Kronberg, B. Lindman; Surfactants and polymers in aqueous solution, 2nd edition, John Wiley & Sons Ltd., Chichester, 2003 [2] L. Mokrushina, M. Buggert, I. Smirnova, W. Arlt, R. Schomäcker, Ind. Eng. Chem. Res. 2007, 46 (20), 6501