Standard volumes of transfer for some alpha-amino acids from water to aqueous sodium acetate solutions at 298.15 K

Apparent molar volumes, V-2,V-phi, at 298.15 K have been obtained for the f ollowing alpha-amino acids in aqueous sodium acetate solutions from measure ments of density: glycine, alpha-alanine, DL-alpha-amino-n-butyric acid, DL -valine and DL-leucine. The infinite dilution apparent molar volumes, V-2,p hi(0),, were calculated by linear extrapolations using the least-squares me thod. These data were used to derive the standard volumes of transfer, Delt a(t)V(0), of the amino acids from water to aqueous sodium acetate solutions . It was shown that both V-2,phi(0), and Delta(t)V(0) vary linearly with in creasing number of carbon atoms in the alkyl chain of the amino acids. Thes e linear correlations have been used to estimate the contributions of the c harged end group (NH3+, COO-) and CH2 group of the amino acids to V-2,phi(0 ) and Delta(t)V(0). An increase in the transfer volume and a decrease in hy dration number of the amino acids with increasing electrolyte concentration s have been explained due to strong interactions of sodium acetate with the charged center of zwitterion for the amino acids compared to sodium acetat e-nonpolar group interactions. The volumetric interaction parameters of the amino acids with sodium acetate were calculated in water. The pair interac tion parameters are found to be positive and decrease with increasing alkyl chain of the amino acids, indicating that more hydrophobic amino acids und ergo more dehydration effect of sodium acetate. A comparison of the pair in teraction parameters of the amino acids with different electrolytes has sho wn that acetate ion has stronger interactions than chloride and thiocyanate ions. These interactions have been rationalized in terms of the cosphere o verlap model.