Curie–Weiss Cell Fluid Model: Fitting to Experimental Critical-Point Data

We examine how the Curie–Weiss cell fluid model can be quantitatively related to real fluids by determining its microscopic parameters from experimental critical-point data for water, carbon dioxide, and three polyol ester lubricants (POE5, POE7, and POE9). The microscopic parameters of the model are determined from the experimental values of the critical temperature and critical density: the global attraction coupling constant is fixed by the critical temperature, and the cell volume by the critical density. The critical pressure then follows as an independent model prediction, deviating from experimental values by 10–41% across the studied substances. Varying the ratio of local repulsion to global attraction f = J₂/J₁ over a wide range has only a weak effect on this deviation. For f ≥ 1, the model’s critical compressibility factor lies within a narrow range Zc ≈ 0.382–0.386, remaining close to the mean-field van der Waals value Zc = 3/8 = 0.375 and consistently overestimating experimental values. The best agreement with experiment is obtained for the polyol ester lubricants and improves along the POE5–POE7–POE9 series with increasing molar mass, suggesting that multiple-occupancy cell fluid models are well suited to systems of large molecules, particularly soft-matter systems with bounded interaction potentials. We further show that generalizing the model by introducing a temperature- and phase-dependent effective attraction allows it to describe a triple point and to reproduce the broad range of critical-to-triple temperature ratios Tc/Ttr characteristic of real fluids.