The influence of SDS (a hydrotrope) on the region of stability of a microemulsion of water, sodium dodecyl sulfate (SDS), pentanol (C3OH) and p-xylene was determined. The addition of the hydrotrope united the O/W and W/O regions of the phase diagram enlarging the bicontinuous region by reducing the lamellar liquid crystal one. Hence, the phase behavior and the low angle x-ray measurements show that the stratum corneum coupling action of a hydrotrope is in