Vapor-liquid equilibrium of a multipolar square-well fluid II. Effect of a variable square-well range

The multipolar square-well (MSW) fluid treated in part I [Physica A 202 (1994) 420] is studied in order to analyze the effect of SW range λ, which models the range of dispersion forces, on the vapor-liquid (VL) equilibrium. The model includes a simpler and more accurate treatment of the SW interactions of variable width. The MSW model covers systems ranging from a system similar to a multipolar Lennard-Jones fluid (λ = 1.5) to the multipolar van der Waals fluid (λ ⪢ 1). The relative influence of the dispersion and electrostatic forces on the critical properties and the VL phase diagram is analyzed. The critical temperature in most sensible to variations in λ, whereas the shape of the VL coexistence curve is affected more by the multipolar forces. The latter have a larger effect on the critical temperature than on the critical pressure for all ranges. A change in λ (at constant dipolar and quadrupolar moments strengths, μ∗and Q∗) and changes in μ∗and Q∗ (at λ constant) have similar effects. The electrostatic forces become systematically less important as λ increases. For μ∗and Q∗ ⩽ 1 their effect on the VL properties is almost negligible relative to that of the dispersion forces when λ ⩾ 2.5.