Taking cyclohexane as an inert reference solvent

the expanded solubility parameters of l-methyl-2-pyrrolidone

nitrobenzene and benzonitrile have been obtained by measuring total vapor pressures. An improved regular associated model was used to treat total pressure-composition data of binary mixtures of 1-methyl-2-pyrrolidone

nitrobenzene or benzonitrile with styrene or o-xylene. A program of non-linear least-square fitting was written in ALGOL language to fit the curves and to estimate the parameters on a (121) computer. The contribution of chemical interaction to activity coefficient was calculated. For l-methyl-2-pyrrolidone as a solvent

the result showed that chemical interaction is an important factor in separating styrene from o-xylene.