Solutions of alkali soaps and water in fatty acids VI. Studies of the refractive index and light scattering

Abstract

Experimental results of measurements of refractive index and light scattering yield interesting information about the structure of the L₂-phase. In accordance with previously presented studies they show that one, with respect to the role of water, has two main regions, one extending from the water-free phase up to a content of water of about 40% and the other at water contents above about 55%. An intermediate transition region lies between about 40% and 55% of water. In the first main region the incorporation of water into the acid octanoate molecules influences the magnitude of the refractive index decrement and light scattering intensity. In the latter region, the partial contributions of water and the amphiphilic substance for the magnitude of the refractive index remain almost unchanged, but here it seems to be the distribution into different domains, bordering each other, of the two types of water, that in the domains with hydrated polar groups and that in the domains with unbound bulk-water, which causes the changes in the intensity of the light scattering. The structure of the L₂-phase, among others, the diversity on the molecular level and the very high concentrations of the active amphiphilic components in large parts of the phase, render quantitative estimations of particle size impossible on the basis of light scattering measurements. Information about phase structure obtained from other experimental investigations, however, makes it possible to show the connection between phase changes on the molecular level and changes in the light scattering intensity. This has also created a basis for some, although partially hypothetical, explanations of which factors play a role in the changes mentioned, as with information about the structure. Among the latter it may be mentioned that the amphiphilic substance, up to very high water contents, retains some of the properties that the first 40% of water have caused. The differences in properties between unbound bulk-water and water bound to the polar groups are considerable; the influence of the bulk-water on the phase properties seems to vary with the manner of its distribution and the properties of the bound water are different, depending on whether it belongs to the solvation water shell of the sodium ion, or whether it is bound via hydrogen bridges.