Thermodynamics of proton dissociations from aqueous l-valine and l-2-amino- n-butanoic acid: apparent molar volumes and apparent molar heat capacities of the protonated cationic, neutral zwitterionic, and deprotonated anionic species at temperatures from 278.15⩽ T/K⩽393.15, at molalities 0.015⩽ m/mol · kg −1⩽0.67, and pressure p=0.35 MPa

Abstract

We have measured the apparent molar volumes V φ and apparent molar heat capacities C p,φ for aqueous solutions of l-valine and l-2-amino- n-butanoic acid in protonated cationic, neutral zwitterionic, and deprotonated anionic forms over the ranges 0.015⩽ m/mol · kg −1⩽0.67 and 278.15⩽ T/K⩽393.15 and at the pressure 0.35 MPa. We have applied Young’s rule and have accounted for chemical relaxation effects to resolve our observed V φ and C p,φ results for each solution into contributions from the various species that are present. We have also calculated estimates of Δ r V m, Δ r C p,m, Δ r H m, and pQ a for the two proton dissociation reactions for each aqueous amino acid over the same ranges of T and m using values of p K a and Δ r H m 0 from the literature as integration constants. Our results extend significantly the detailed description of the thermodynamic behavior of these two aqueous amino acid systems.