Mitigating Complexity: Cohesion Parameters and Related Topics. I: The Hildebrand Solubility Parameter

The objective of this review in two parts is to present a compact overview of the development of the solubility parameter ( δ_i) concept: from the seminal work of van Laar in 1910, to the contributions of Scatchard , Hildebrand , Scott and Prausnitz , leading finally to the generalized multi - component ( multi - dimensional ) cohesion parameters, with the Hansen solubility parameter being the most prominent representative. In this first part, physico-chemical aspects concerning δ_i -related models in solution chemistry and chemical engineering will be presented, and recent theoretical efforts in this field, that is, equation-of-state approaches and computer simulation methods for the estimation of solubility parameters, will be indicated. Indeed, prediction of thermodynamic properties of liquid nonelectrolyte solutions from properties of the corresponding pure constituents has come a long way since the classic studies by Hildebrand and by Scatchard leading to regular solution theory (RST), in which the solubility parameter is the property of central importance. Selected aspects of RST will be discussed, including the influence of T and of P on δ_i and their reliable estimation, thereby clearing up misconceptions and pointing out pitfalls not generally recognized. Extending the dicussion to supercritical conditions, the use of solubility parameters in supercritical fluid (SCF) technologies will be indicated, focusing on practical implications of some of the unique phenomena happening in the near supercritical region, which provide the basis of SCF extraction in industries devoted to food-processing, nutraceuticals, pharmaceuticals and biotechnology.