Water solubilization by surfactant aggregates in organic solvents: Limitations of the multi-state water model

Abstract

Models for water solubilization by surfactant aggregates in organic solvents are discussed. In the multi-state model, water is present as interfacial-, bound-, intermediate-, and bulk-like water, respectively. The following evidence shows that this picture is an oversimplification: (i) interpretation of the dependence of a property of solubilized water, e.g., its chemical shift, on the ratio [water]/[surfactant], W/S, in terms of bound-, and bulk-like water is not unequivocal because the interpretation is model-dependent; (ii) values of the so-called deuterium/protium “fractionation factor, ϕ” for water solubilized by anionic-, cationic-, and non-ionic reverse aggregates indicate that it is not present in layers of different structures, as implied by the multi-state model; (iii) the same conclusion is deduced from IR studies of solubilized HOD. Therefore, available data indicate that water appears to be present as one layer whose properties vary continuously as a function of increasing W/S. The following forces preclude any long range order within the aqueous nanodroplet: (i) the surface potential of the micellar interface; (ii) water structure perturbation due to hydration of species present in the “water pool”; and (iii) dependence of surfactant0water interactions on W/S. Calculation of the decay of the electric potential within the water pool for two opposite surfactant molecules as a function of W/S shows that these potentials either cancel out, or their values reach zero mV within a short distance of each other, e.g., ca. 8 A at W/S of 50.