Interpretation of the activity of a hydrotrope in lecithin/sodium xylenesulfonate/water microemulsions

Activity changes of sodium xylenesulfonate (SXS) in lecithin/SXS/water micr oemulsion phase were determined by equilibrating it with the SXS/water solu tion through a semipermeable membrane, which retained the lecithin while th e water and SXS were free to move. The activity of the SXS in the microemul sion phase was calculated from its equilibrium concentration measured in th e SXS/water phase. The activity changes were interpreted by both the surfac e excess approach and Hall's thermodynamic description. The calculation of the adsorbed amount of SXS took the electrostatic exclusion effect into con sideration. Combining the microemulsion with water leads to the formation o f vesicles. A relation between the size of these vesicles and the compositi on of the microemulsion droplets is suggested by considering a depletion ef fect.