Chemical transfer energetics of the –CH 2– group in aqueous glycerol: Solvent effect on hydrophobic hydration and its three-dimensional structure

Abstract

Standard transfer Gibbs energies, Δ G t 0 and entropies Δ S t 0 of the –CH 2– group have been evaluated in aqueous glycerol (GL) by determining the solubilities of a series of homologous alkylcarboxylates (RCOO −), mono-alkylammonium ions (RNH 3 +) and α-amino acids at five equidistant temperatures ranging from 15 °C to 35 °C by spectrophotometry and ‘formol’ titrimetry. The chemical contributions Δ G t,ch 0(i) as obtained by subtracting the cavity effect, Born type and ion–dipole type electrostatic effects in case of RCOO − and RNH 3 + ions and cavity and dipole–dipole interaction effects for α-amino acids, are indicative of the superimposed effect of increased acidity, increased dispersion and decreased hydrophobic hydration (H bH) effect of GL–water mixtures as compared to that in water, while TΔ S t,ch 0(i) are guided by structural effects as well. These values also helped to compute chemical transfer energetics of the –CH 2– group, Δ P t,ch 0(–CH 2–) [ P = G or S] by subtracting the values of lower homologues from those of the immediately higher homologues and also those of the H bH effect Δ P t,H bH 0(–CH 2–) by subtracting the dispersion interaction effect of the (–CH 2–) group from the corresponding Δ P t,ch 0(–CH 2–). These values are found to be more or less equal for all probes indicating their additivity. While Δ G t,H bH 0(–CH 2–) are found to be guided by decreased H bH of GL–water mixtures, Δ S t,H bH 0(–CH 2–) are indicative of three-dimensional (3D) structuredness of the solvents.