Ion–Solvent Interaction of Amino Acids. The Role of the “Zwitterionic” and the “Ionic” Forms in the Modification of Water Structure over the Temperature Range 25–45 °C

Viscosities and densities of aqueous glycine, glycinium chloride, sodium glycinate, betaine, betainium chloride [(CH3)3+NCH2COOH·Cl−], and tetramethylammonium bromide solutions have been measured at 25, 30, and 40 °C over the concentration range 0.01 to 0.1 M. The sequence of the viscosity B-coefficient values obtained is in the order: Banion>Bcation>Bzwitterion for the glycine and betaine systems. The comparatively higher structure promoting behavior of the anion has been discussed in terms of the charge delocalization effect. Apparent molal volumes at infinite dilution have also been calculated, and the ratio B⁄φv° furnishes important information about the nature of solvation around the ion. The present study indicates that in the betaine molecule, the opposing nature of ion–water interaction at the two charge centers renders the system neither a “structure maker” nor a “breaker” in aqueous solution.