Transport and Equilibrium Phenomena in Gas-Elastomer Systems. II. Equilibrium Phenomena

Abstract

The solubility of nitrogen, ethylene, and the n-paraffins from methane to pentane has been measured in a series of natural rubber vulcanizates in relation to chain length of paraffin, temperature, and degree of cross linking of vulcanizates which contained between 1.7 to 21.9% combined sulfur. For smaller, less soluble paraffins the degree of vulcanization had little influence upon the solubility but for higher molecular paraffins such as pentane this influence became significant. A qualitative interpretation of the effects observed was given. The solubility data are in all cases represented by a statistical theory of one of the authors for a mean frequency of vibration of the solute in the medium corresponding to the infrared (0.5 to 1.0 × 1012 sec.−1). The solubility constants, σ, and critical temperature, Tc, are also empirically related by the equation: Heats and standard free energies of solution for the earlier homologous paraffins show steady trends as the chain length increases. The heats are exothermal and are adequately interpreted as the sum of the heats of liquefaction together with a small heat-of-mixing term for liquid and rubber which was usually close to that given by Hildebrand's cohesive energy density equation: As anticipated from statistical theory, entropies were usually several entropy units more negative than those for solutions of gases in monomeric solvents.