A crossover of the solid substances solubility in supercritical fluids: what is it in fact?

TL;DR

This paper explores the shifting nature of crossover points in solubility isotherms of solids in supercritical fluids, revealing that these points are not fixed but depend on temperature and converge at a threshold, challenging previous assumptions.

Contribution

It demonstrates that crossover points in solubility curves are not fixed but shift with temperature, converging at a threshold, based on theoretical analysis and experimental data.

Findings

Crossover points shift with temperature changes.

At a certain threshold, crossover points converge into a single point.

Experimental data supports the shifting and convergence of crossover points.

Abstract

We investigate a well-known phenomenon of the appearance of the crossover points, corresponding to the intersections of the solubility isotherms of the solid compound in supercritical fluid. Opposed to the accepted understanding of the existence of two fixed crossover points, which confine the region of the inverse isobaric temperature dependence of the solubility, we have found that these points tend to shift with the change of the temperature and in the limit of the certain threshold value they converge to a single point. We demonstrate this analyzing the solubility data of a set of poorly soluble drug compounds, which have been computed in a wide area of the phase diagram via the approach, based on the classical density functional theory. Thorough analysis of the available in the literature experimental solubility data is found to be in an agreement with our conclusions, as one can find that the wider temperature region of the experimental study is, the more pronounced effect of the crossover points drift can be observed.