THE DEMIXING AND GAS-LIQUID INSTABILITY OF A BINARY YUKAWA FLUID

For a binary mixture we relate the second order expansion of the grand canonical potential OMEGA to direct correlation functions and to thermodynamic expressions. The matrix of second functional derivatives of OMEGA with respect to the particle density fluctuations is diagonalized. The zero of the smallest eigenvalue indicates a phase instability vs particle density fluctuations. The related eigenvector characterizes the instability uniquely. Calculations of correlation functions via the reference hypernetted chain integral equations are performed for a binary mixture of hard core particles with attractive Yukawa interactions; the spinodals are established for different concentrations and different interaction potentials using the calculated correlation functions. Along the spinodal we find a continuous change from predominantly demixing to mainly condensation instabilities.