Characteristics of AOT Microemulsion Structure Depending on Apolar Solvents

By using synchrotron radiation small-angle X-ray scattering (SR-SAXS), we have studied the structural properties of reversed micellar systems depending on hydrocarbon chains of apolar solvents, where we have treated water/bis(2-ethylhexyl)sulfosuccinate (AOT)/n-hexane, n-heptane, n-octane, and isooctane systems. With increasing water/surfactant molar ratio w0 ()[H2O]/[AOT]), we have found that three different phases (oligomeric phase, transient phase, monomeric phase)appear successively in the above systems the same as in the water/AOT/isooctane system. In the present experiments we have found other features of the reversed micellar structure depending on apolar solvents. Thus, the w0 region of the transient phase broadens with increasing the hydrocarbon chain length. The slope of the linear relation between micellar radius and w0 changes systematically depending on the length of linear hydrocarbon chain of apolar solvent, which clearly suggests that the penetration of the solvent molecules into the surfactant layer of the micelle greatly depends on the length of the linear hydrocarbon chain. According to the systematic difference of the above slope from 1.5, where 1.5 is a well-known value obtained from geometrical constraint of micellar packing, we can estimate the penetration limit of apolar solvent with linear hydrocarbon chain to be C9.5; therefore, over C9.5 the penetration of apolar solvent molecules would hardly occur.