Enthalpy of solution of 1,4-naphthoquinone in CO2 plus n-pentane in the critical region of the binary mixture:: Mechanism of solubility enhancement

The enthalpy of solution (Delta(solv)H(m)) and solubility of 1,4-naphthoquinone in CO2 + n-pentane were measured at 308.15 K in the critical region of the binary fluid. In order to study the effect of phase behavior of the mixed solvent on Delta(solv)H(m), the experiments were carried out in the supercritical (SC) and subcritical region of the binary solvent. The density of the mixed solvent in different conditions was determined. The isothermal compressibility (K-T) of the mixed solvent, and the partial molar volume (Vn-pentane) of n-pentane in the solution were calculated. It was demonstrated that the Delta(solv)H(m) was negative in all conditions. Delta(solv)H(m) is nearly independent of pressure or density in all the solvents in a high-density region, in which compressibility of the solvent is very small; this indicates that the intermolecular interaction between the solvent and the solute is similar to that for liquid solutions. It is very interesting that Delta(solv)H(m) in the mixed SC fluid differs from the Delta(solv)H(m) in mixed subcritical fluids. The absolute value of (_)Delta(solv)H(m) in the mixed SC fluid is close to that in pure SC CO2 in the high-density region, and is much lower than that in pure SC CO2 in the low-density region. In the mixed subcritical fluids, the Delta(solv)H(m) is also close to that in the pure CO2 in the high-density region. However, at the same density, the absolute value of Delta(solv)H(m) in the binary subcritical fluid is larger than that in pure CO2 in the high-compressible region of the mixed solvent. The main reason for this is that the degree of clustering in the SC solutions is small at the density in which the degree of clustering is large in the subcritical solutions. It can be concluded that solubility enhancement by n-pentane in the mixed SC fluid is entropy driven. In contrast, the solubility enhancement by n-pentane in subcritical fluids is enthalpy driven. The intermolecular interaction in the SC solutions and subcritical solutions can be significantly different even if their densities are the same.