Hildebrand [15,16] introduced a regular solution model with a
solubility parameter DHvap/V for van der Waals molecules. In these
molecules, bonding is the result of temporary dipoles which are
formed by dispersion forces. Therefore, the cohesive energy stems
from these temporary dipoles. Hildebrand [15,16] assumed that
‘‘the interaction energy between pairs of unlike atoms is the geometric
mean of the interaction energy between pairs of like atoms
or molecules’’ [43]. The mixing takes place under constant volume
conditions and the excess entropy of mixing is negligible (i.e., entropy
of mixing is ideal). His solubility parameter adds a positive
contribution to the enthalpy of mixing