Enthalpies of transfer of formamide, N-methylformamide, N,N-dimethylformamide and urea from water to some aqueous alcohol solvent systems at 298 K

Abstract

The enthalpies of transfer, Δ_tH^⊖ of formamide, N-methylformamide and N,N-dimethylformamide from water to aqueous methanol, ethanol, propan-1-ol and 2-methylpropan-2-ol, and those of urea from water to aqueous propan-1-ol systems are reported. These data, and literature values for Δ_tH^⊖ of N-methylpyrrolidinone and urea, have been analysed in terms of solvation theory.

Analysis of the Δ_tH^⊖ data points to there being a discontinuity in the solvating properties of the solvent systems at intermediate solvent compositions. In the water-rich domain the solutes are substantially randomly solvated, while they are preferentially hydrated in the alcohol-rich regions. It is also found that, in the water-rich solvents, the extent to which the solutes disrupt the structure of the solvent is a simple function of the size of the non-polar alkyl groups on the solute, increasing linearly with the size of the alkyl surface. The rate of this increase depends on the nature of the alcohol, and increases from aqueous methanol to aqueous propan-1-ol, and decreases again for the aqueous TBA system. In contrast, the extent of disruption of solvent structure shows more complex variations, dependent on the solvent system, in the alcohol-rich region.