Enthalpies of Transfer of the −CH₂− Moiety into Aqueous Amide Solvent Systems; Values from n-Alcohol Solutes

Abstract

The enthalpies of solution of the n-alcohols, ethanol to hexanol, have been measured in aqueous mixtures of formamide, N,N-dimethylacetamide and N-methylpyrrolidinone. The average enthalpy of transfer of the −CH2 − moiety, Δt H θ(CH2), from the organic solvents to the mixtures and to pure water are determined from the differences between the enthalpies of transfer of the alcohols. The Δt H θ(CH2) values pass through maxima in the highly aqueous composition region, with the height of the maximum increasing linearly with the area of the hydrophobic surfaces of the cosolvent. These results are compared to similar data for the aqueous alcohol solvent systems where the maximum in Δt H θ(CH2) is also sensibly linear in the area of the cosolvent hydrophobic surfaces but increases more rapidly with increasing hydrophobic area.

Enthalpies of transfer of the CH2 moiety into aqueous acetonitrile mixtures; comparison of values from n-alcohols and tetraalkylammonium ions; effect of temperature variation

“…The sharp rise in D t H (CH 2 ) at high water concentrations is common to all of the aqueous organic solvent systems studied [2][3][4]. In contrast, minima at intermediate compositions havenÕt been observed in other mixed solvent systems.…”

“…It was found that both the height of the maxima and the composition at which they occur depended on the organic cosolvent, the height increasing linearly with the area of the non-polar CH n surfaces of the cosolvent molecule [4]. Importantly, this variation in the height of the maximum also depended on the core molecular structure, with CH n surfaces of n-alcohols cosolvents having a greater affect than those of amides.…”