J. Aerosol Sci., Vol. 26. Suppl I, pp. $627-$628, 1995 x ~Der"anlon Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00 Effective one-component theory of binary nucleation v.I. Kalikmanov and M.E.H. van Dongen Eindhoven University, Physics Department, WfJS, P. O.Box 513, 5600 MB, Eindhoven, The Netherlands Classical theory of binary nucleation (CBN) formulated by Reiss in 1950 and developed by Stauffer and Wilemski [1] states that the Gibbs free formation energy AG(nA, riB) of a liquid droplet, containing na molecules of component A and nB molecules of component B, in a supersaturated vapour mixture forms a saddle-shaped surface such that AG* represents the height at the saddle. The steady-state nucleation rate, J, is given by J = Joexp(-AG*/ksT) (1) The crucial point of any nucleation theory is the choice of a model for AG which manifests itself in the determination of the size and composition of the critical nucleus. Recent experiments [2] showed, in agreement with earlier studies, that CBN fails because it inaccurately predicts the composition of the critical nucleus and does not take into account the curvature effects. Experimental investigations until now focused on nearly ideal mixtures. For nonideal mixtures, exhibiting a retrograde condensation behaviour, composition, size and curvature effects should be even more pronounced. We propose an effective one-component model of binary nucleation at conditions of retrograde condensation which means that the nucleation point characterized by the total pressure p, temperature T and vapor molar fraction YA is situated inside the vapor-liquid coexistence region of the phase diagram. An effective model should predict the energy barrier close to that of the original system. Comparison of the expression for AG* given by CBN with its one-component classical analogue shows that the quantity S* <,,®Tt ~,'~ = °A '-'B , represents a supersaturation in the effective system; here S7 is partial critical supersat- uration of component i and asterisk denotes the dependence on the composition x~ of the critical cluster. We rewrite S* as: ( 1 ~h+l S* = ~(1 + ~*) with 3' = eza'°c'~B'° and ~* = a~ °) \-~1 , "a,o "B,o , (2) Here S~,o = YjY~,o; Y~,o(P, T) and x~,o(p, T) are equilibrium vapour and liquid molar fractions, respectively; r* is the radius of the criticalcluster. Function ~* accounts for composition, surface enrichment and curvature effects incorporated in S*. We show that nucleation in the original binary system is similar to nucleation in the effective system with supersaturation ~ and characterized by the renormalized surface tension $627