Correlations of surface free energy and solubility parameters for solid substances.

HYPOTHESIS: Both the surface free energy (γ) and solubility (δ) parameters of substances are related to their cohesive energies which are decided by intermolecular interactions, and there should be some intrinsic relationships between the two parameters. Understanding of the γ-δ correlations is of great fundamental and practical importance. Several empirical γ-δ equations have been proposed so far, but their application to solids is limited. This is because the molar volume (V~) as a parameter exists in these equations while the V~ of solids is commonly hard to be obtained. Hence, the development of γ-δ equations without the parameter V~ is essential for solids. METHOD: The γ and δ data of 21 solids including polymers and layered solid materials were chosen, and possible γ-δ relationships were systematically explored using the parameter data of solids by a trial and error fitting method. FINDING: Six γ-δ equations without the parameter V~ are proposed. The γ parameters include total (γt), dispersive (γd), and polar (γp) ones, and the δ parameters include the Hildebrand parameter (δt) and the Hansen dispersive (δd), polar (δp), and hydrogen-bonding (δh) ones. Interestingly, the so-obtained V~-free γ-δ equations are also valid for most liquids including nonpolar and polar ones. These γ-δ equations can provide a way to estimate non-measurable parameters from measurable parameters for solid materials, which is beneficial to the application of the characteristic parameters (γ and δ) for solid material engineering.