20) Rao, B. G.; Singh, U. C. J. Am. Chem. Soc. 1990, I l l , 3803. (21) Weiner, S. J.; Kollman, P. A,; Case, D. A.; Singh, U. C.; Ghio, C.; Alagona, G.; Profeta, S. Jr.; Weiner, P. J. Am. G e m . Soc. 1984,106,765. (22) Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D. J.; Sw-aminathan, S.; Karplus, M. derivatives of these partial molar quantities, the nature of the soluttsolute interactions was discussed in relation to the global information about the mixture: existence of phase separation, the way in which the phase boundary slants, and existence of azeotropy and addition compound. = 0.7. Indeed, in the concentration range 0.15 < xBUT < 0.7, our 0022-3654/92/2096-10025$03.00/0 the signs of the slopes of H, P (W) and Sm(W) remain the same AppcadixI fhht&3oh1te Intmctiona nenr Phase & d r y .If a binary mixture exhibits phase separation with an upper critical solution temperature (UCST), it is qualitatively understood that the interaction energy between the like molecules is more negative, or the like molecules attract each other in terms of enthalpy. At lower temperatures where the enthalpy effect becomes more predominant, the like molecules attract so much that two phases,
