Thermodynamic Solvation of a Series of Homologous α-Amino Acids in Aqueous Mixtures of 1,2-Dimethoxyethane

Abstract

The standard Gibbs energies $$ \left( {\Updelta {}_{\text{t}}G^\circ (i)} \right) $$ and entropies $$ \left( {\Updelta {}_{\text{t}}S^\circ } \right) $$ of transfer in aqueous mixtures of 1,2-dimethoxyethane (DME) containing 0, 20, 40, 60, 80, 100 wt-% DME have been determined from the solubility data of a series of homologous α-amino acids, evaluated by the formol titrimetric method. The observed result of Δt G°(i) and TΔt S°(i) against DME concentration profiles are complicated due to the various interaction effects. The chemical effects on the transfer Gibbs energies ( $$ \Updelta_{\text{t}} G_{\text {ch}}^{ \circ } (i) $$ ) and entropies of transfer $$ T\Updelta_{\text{t}} S_{\text {ch}}^{ \circ } (i) $$ have been obtained after elimination of the cavity effect, calculated by the scaled particle theory, and dipole–dipole interaction effects, estimated by the use of Keesom-orientation expression for total transfer Gibbs energies Δt G°(i) and entropies Δt S°, respectively. The chemical transfer energetics of the zwitterionic homologous α-amino acids are guided by the composite effects of increased dispersion interaction, basicity and decreased acidity, hydrogen bonding capacity and hydrophobic hydration of the DME mixed solvent as compared to that of reference solvent, water.