J Solution Chem (2009) 38: 403–415 DOI 10.1007/s10953-009-9384-5 Spinodal Composition of the System Water + Chloroform + Acetic Acid at 25 °C Florin D. Buzatu · Radu P. Lungu · Daniela Buzatu · Roberto Sartorio · Luigi Paduano Received: 1 October 2008 / Accepted: 23 November 2008 / Published online: 12 February 2009 © Springer Science+Business Media, LLC 2009 Abstract A theoretical spinodal curve for the system water + chloroform + acetic acid at 25 °C is derived using a lattice model for ternary amphiphilic solutions: rod-like mole- cules covering the bonds of the honeycomb lattice with three-body interactions between the molecular ends associated to the same lattice site. The molecular model is equivalent to the standard Ising model on the same lattice; its mean-field solution is the most appropriate for reproducing, by local fitting, the experimental data for the binodal composition. The derived spinodal curve is in very good agreement with the spinodal composition determined also in the present work from the measured diffusion coefficients recently reported for the same system. Keywords Ternary amphiphilic solution · Spinodal curve · Molecular lattice model · Ising model 1 Introduction In a previous paper [1] we reported the values of the diffusion coefficients determined at five compositions of the system water + chloroform + acetic acid at 25 ◦ C. The system presents F.D. Buzatu ( ) Institute of Atomic Physics, M˘ agurele-Bucharest, 077125, Romania e-mail: f.buzatu@ifa-mg.ro F.D. Buzatu Dept. of Theoretical Physics, Horia Hulubei National Institute for Physics and Nuclear Engineering, M˘ agurele-Bucharest, 077125, Romania R.P. Lungu Dept. of Physics, University of Bucharest, M˘ agurele-Bucharest, 050107, Romania D. Buzatu Dept. of Physics, Politehnica University, Bucharest, 060040, Romania R. Sartorio · L. Paduano Dept. of Chemistry, Federico II University of Studies from Naples, Naples, 80126, Italy