Self-association of purine base, 6-methylpurine, in water - organic component mixtures

Thermodynamic quantities of the self-association of 6-methylpurine in water (1)-dimethylsulfoxide (DMSO) (2) mole fraction, x2 less than 0.1) and water (1) - N,N-dimethylformamide (DMF) (2) (x2 less than or equal to 1.0) mixed solvents have been obtained through heat of dilution measurements, at 25 degrees C. In the water-DMSO solvent system, the standard enthalpy and entropy changes, delta Ho and delta So, of the association exhibited an abrupt behavior. They decreased remarkbly with the the mole fraction of DMSO until about x2 = 0.012 and after that, they increased steeply. In the case of water-DMF solvent system, the values of delta Ho and delta So didn't show the abrupt behavior. They decreased steeply until about x2 = 0.1 and, at higher mole fractions, became relatively constant. These behaviors are rationalized on the basis of solvent structural effects and solvation in these association systems.