Surface Tension and the Solubility of Polymers and Biopolymers: The Role of Polar and Apolar Interfacial Free Energies

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Abstract
Surface tension data can be used for estimating the solubility of polymers in liquids. By determining the apolar and the polar components of the surface tension of polymers and of solvents, the attractive free energy, δG 121, of a polymer (1) in a given solvent (2) can be established. By also taking into account the contactable surface area of two polymer molecules, immersed in a liquid, δG 121 can be expressed in units of kT. Solubility then is favored when -1.5 kT < δG 121 < 0 for apolar systems, and when -1.5 kT < δG 121 for polar systems. In polar solvents, hydrogen bonding can often increase δG 121 from <-1.5 kT to > + 1.5 kT. Positive values are frequently attained and this strongly shifts the behavior from insolubility to solubility. A number of proteins exemplify this behavior.