Thermodynamic Study of the Self-association of 6-Methylpurine in Water-1,4-Dioxane Solvent

Determination of the thermodynamic quantities of the self-association of 6-methylpurine in water(1)–1,4-dioxane(2) (x2=0–0.36) mixed solvent has been made from heat of dilution measurements in various organic contents, at 25 °C. It was confirmed experimentally that the dissociation process of associating purine bases in solution is very fast by dilution. It was characteristic of this solvent system that the standard enthalpy and entropy changes, ΔH° and ΔS°, of the association exhibited an abrupt behavior. These values decrease remarkably with 1,4-dioxane content up to a water-rich solvent composition below x2=0.012. After that, they approach their relatively high values for the pure organic component, at first with a steep slope, later more gradually. This behavior is related to the structural changes of the solvent in an extremely sensitive way. In the case of water (1)–methanol(2) (x2<0.19) mixed solvent system which has been investigated for comparison with the above system, minimum values of the same kind were found in the vicinity of x2=0.12, at 25 °C.