Interdependence of enthalpic and entropic contributions to the second osmotic virial coefficient

The interdependence of the enthalpic contribution A_{2, H} and the entropic contribution A_{2, s} to the second osmotic virial coefficient for a given polymer-solvent system has been investigated from the experimental and the theoretical point of view. Experimentally, the following common facts were observed for various systems at temperatures and pressures below the critical values for the solvent. Both the isobaric and isothermal dependences can be approximated over relatively wide ranges of A_{2, H} by linear relations with a slope deviating only slightly, but in a characteristic manner from a value of ?1. When the temperature is increased at constant pressure one moves along an isobar towards higher A_{2, H}; when the pressure is increased at constant temperature, one moves along an isotherm in the opposite direction, i.e., towards lower A₂, _H. Theoretically this behavior can be described in a qualitative manner, starting from a relation derived by Patterson and Delmas on the basis of the Prigogine corresponding-states theory. The reasons for the lack of quantitative agreement are discussed.