The use of polar hydrotropes in extractive distillation and in liquid-
liquid extraction has been extensively reported in the literature. In
this article, the effect of hydrotropes on increasing the effectivenes
s of separation is explained in terms of the effect of the dipole-indu
ced dipole and the ion-induced dipole interactions on the activity coe
fficients of the neutral solutes in the aqueous phase. The self-consis
tent local composition theory is used to correlate quantitatively the
observed behavior in systems involving liquid-liquid equilibria. The t
heory yields composition-dependent expressions for the effect of each
independent pair interaction on the activity coefficient of a neutral
solute in the aqueous phase. The ratio of the activity coefficients of
the two solutes is of interest in this context. The resulting express
ion for the separation factor is elegant and admits simple physical in
terpretation. The essential features of hydrotrope-assisted extraction
are correctly described by the theory. The ability of the theory to c
orrelate data is demonstrated for two systems.