Surfactants are widely used due to their amphiphilic properties and their solubility in water and many organic solvents. The phase behaviour of surfactant mixtures is therefore of great interest, both from the scientific and industrial perspective. Transferable molecular models of the alykylpolyoxyethylene (C_iE_j) non-ionic surfactants are developed within the Wertheim TPT1 formalism for use with the statistical association fluid theory for potentials of variable range (SAFT-VR) [1]. Short-range attractive interaction sites allow the explicit treatment of water and surfactant hydrogen bonding. The fluid phase equilibria of binary aqueous surfactant solutions are examined with a particular emphasis on the regions of liquid immiscibility known as closed-loop regions. Additional association sites for the surfactant are used to better describe the experimental closed-loop; the physical significance of these extra sites can be explained in terms of the micellar aggregation of the surfactants where only the hydrophilic head groups are exposed to water so that the solubility of the surfactant is effectively one of a fully hydrophilic chain with sites all along its length. The fluid phase equilibria of ternary water + hydrocarbon (oil) + C_iE_j solutions are examined to represent the liquid two and three phase regions. Transferable molecular interaction parameters are determined which give an excellent description of the phase behaviour of these systems. The interaction parameters provide a predictive platform for surfactant mixtures of various molecular weights and permit to examine global behaviour. The pressure dependence of the liquid phase behaviour in these systems is also studied. The closed-loop is observed to diminish with increasing pressures. At low pressures and with increasing temperature the three phase region in water + oil + C_iE_j solutions opens at the water rich side and closes at the oil rich side in correspondence to experimental data. The phase behaviour of aqueous non-ionic surfactant solutions under the influence of salt is studied by extending the SAFT-VR equation of state to incorporate electrolytes, and preliminary results of the effect of salinity on the phase behaviour is described.

References:

[1] A. Gil-Villegas, A. Galindo, P.J. Whitehead, S.J. Mills, G. Jackson, A.N. Burgess, J. Chem. Phys. 106 (1997) 4168–4186.