Standard Volumes of Transfer for Some α-Amino Acids from Water to Aqueous Sodium Acetate Solutions at 298.15 K

Apparent molar volumes, V_2,Φ, at 298.15 K have been obtained for the following α-amino acids in aqueous sodium acetate solutions from measurements of density: glycine, DL-α-alanine, DL-α-amino-n-butyric acid, DL-valine and DL-leucine. The infinite dilution apparent molar volumes, V⁰_2,Φ, were calculated by linear extrapolations using the least-squares method. These data were used to derive the standard volumes of transfer, Δ_tV⁰, of the amino acids from water to aqueous sodium acetate solutions. It was shown that both V⁰_2,Φ,and Δ_tV⁰vary linearly with increasing number of carbon atoms in the alkyl chain of the amino acids. These linear correlations have been used to estimate the contributions of the charged end group (NH⁺₃, COO^-) and CH₂ group of the amino acids to V⁰_2,Φ, and Δ_tV⁰. An increase in the transfer volume and a decrease in hydration number of the amino acids with increasing electrolyte concentrations have been explained due to strong interactions of sodium acetate with the charged center of zwitterion for the amino acids compared to sodium acetate-nonpolar group interactions. The volumetric interaction parameters of the amino acids with sodium acetate were calculated in water. The pair interaction parameters are found to be positive and decrease with increasing alkyl chain of the amino acids, indicating that more hydrophobic amino acids undergo more dehydration effect of sodium acetate. A comparison of the pair interaction parameters of the amino acids with different electrolytes has shown that acetate ion has stronger interactions than chloride and thiocyanate ions. These interactions have been rationalized in terms of the cosphere overlap model.