Thermochemical Study of Solvation of Aliphatic Carboxamides in Aqueous Formamide Solutions

The entahlpies of solution of formamide, acetamide, and propionamide in aqueous formamide solutions (formamide concentration 8 mol kg^-1) at 298.15 K were measured. The entahlpies of transfer of amides from water to the mixed aqueous-organic solvent were calculated and compared with published data on the enthalpies of transfer of amides into aqueous urea solutions. The opposite trend in variation of the enthalpies of transfer of amides in these systems with increasing concentration of the nonaqueous component is due to different proton-donor power of formamide and urea relative to water. The enthalpy coefficients of pair interaction of amides with formamide in ternary aqueous solutions were calculated. Their positive values are due to endothermic interaction with formamide of the alkyl groups of amide molecules. The relative hydrophobicity of amides with different degrees of substitution of the amide group was estimated.