The generalized van der Waals partition function. III. Local composition models for a mixture of equal size square-well molecules

An improved Helmholtz energy model for non-polar fluids and their mixtures. Part 2: Application to mixtures of non-polar fluids

“…ij and Á ij of Eqs. (5) and (6), remain very close to unity although ij increases moderately with the absolute difference in acentric factors of the mixture components while Á ij shows the opposite behaviour. On the other hand, the parameter associated with the correction term of Eq.…”

“…→ max , which is equivalent to S → √ 2, it follows that lim (x 21 /x 11 ) = (x 2 /x 1 ), which corresponds to random mixing. This is the behaviour supported by the Monte Carlo simulation results of Lee et al [5]. Finally, the interaction potential parameters are calculated with the ordinary combining rules ij = ji = 1 2 ( ii + jj ) and ε ij = ε ji = (ε ii × ε jj ) 1/2 .…”

“…In the low-density limit it is known [5] that lim →0 g ji (r) = exp(−u ji (r)/k B T ), so that carrying out the integration of the radial distribution function with the SW potential the result is:…”

Clustering in supercritical fluid mixtures

“…The local compositions decrease toward the bulk values as the pressure increases, since the isothermal compressibility decreases. The effect of the attractive forces on the structure of the fluid decreases as the isothermal compressibility decreases since the molecules be- come less mobile, a phenomenon that has been observed in computer simulation data (Lee et al, 1986) and may be explained using high-temperature expansion perturbation theory (Weeks et al, 1971). The decay in the local composition behavior with pressure is larger for n-octane than for the polar cosolvents.…”

“…The density-dependent local composition (DDLC) concept (Mollerup, 1981(Mollerup, , 1983Whiting and Prausnitz, 1982;Mathias and Copeman, 1983;Hu et al, 1983Hu et al, , 1984 shows promise for describing asymmetric mixtures over a wide density range, although several deficiencies have been identified using computer simulation data (Lee et al, 1986;Nakanishi et al, 1982;Nakanishi and Tanaka, 1983;Wong and Johnston, 1984). The density dependence is incorrect in many of these models, for local ordering should decrease as the density is increased, as explained below.…”