A simple approximation for fluids with narrow attractive potentials.pdf

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A simple approximation for fluids with narrow attractive potentials.pdf

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A simple approximation for fluids with narrow attractive potentials

a r X i v : c o n d - m a t / 0 1 0 9 3 1 1 v 2 [ c o n d - m a t .s o f t ] 1 3 N o v 2 0 0 1 A simple approximation for fluids with narrow attractive potentials D. Pini Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Universita? di Milano, Via Celoria 16, 20133 Milano, Italy A. Parola Istituto Nazionale per la Fisica della Materia and Dipartimento di Scienze Fisiche, Universita? dell’Insubria, Via Valleggio 11, 22100 Como, Italy L. Reatto Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Universita? di Milano, Via Celoria 16, 20133 Milano, Italy We study a simple modification of the optimized random phase approximation (ORPA) aimed at improving the performance of the theory for interactions with a narrow attractive well by taking into account contributions to the direct correlation function non-linear in the interaction. The theory is applied to a hard-core Yukawa and a square-well potential. Results for the equation of state, the correlations, and the critical point have been obtained for attractions of several ranges, and compared with Monte Carlo simulations. When the attractive interaction is narrow, the modified ORPA significantly improves over the plain one, especially with regard to the consistency between different routes to the thermodynamics, the two-body correlation function, and the critical temperature. However, while the spinodal curve of the modified theory is accessible, the liquid-vapor coexistence curve is not. A possible strategy to overcome this drawback is suggested. I. THE CLOSURE In the last years there has been a considerable interest in model fluids in which a hard-core or very steep repulsion in the interparticle potential is followed by a narrow and deep attractive well. This interest mainly stems from the fact that such potentials provide a modelization of the interaction in many colloidal suspensions and protein solutions which, albeit crude, succeeds nevertheless in capturing