Volumetric properties of aqueous solutions of amino acids from the singular to critical temperature

The partial volume -V20 of amino acids in aqueous solution is assumed to be zero for T = 227 K (singular temperature) and T= Tc (critical temperature). The literature data for -V20(T) of ten amino acids at 278–328 K are reproduced by the two-parameter equation with a standard deviation of 0.06–0.16 cm3/mol. Only for asparagine and tryptophan the standard deviation exceeds 0.3 cm3/mol. In the case of glycine and alanine, the relation -V20(T) in the high temperature range is obtained. -V20 is divided into contributions (∂-H20/∂p)T and —T·(∂-S20/∂p)T. Their dependence on the temperature and amino acid nature is discussed. Positive values of ∂-C20/∂p)T characterize amino acids as water structure breakers; however, the differentiation of compounds by this feature is not successful. The behavior of amino acids in aqueous solutions is compared with the behavior of urea.