Thermodynamics of the first and second proton dissociations from aqueous L-aspartic acid and L-glutamic acid at temperatures from (278.15 to 393.15) K and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volumes of zwitteri

摘要

We have measured the densities of aqueous solutions of L-aspartic acid, L-glutamic acid, and equimolal solutions of these two amino acids with HCl and with NaOH at temperatures 278.15 <= T/K <= 368.15, at molalities 0.002 <= m/mol . kg(-1) <= 1.0 as solubity of the solutes allowed, and at p = 0.35 MPa using a vibrating tube densimeter. We have also measured the heat capacities of these solutions at 278.15 <= T/K <= 393.15 and at the same m and p using a twin fixed-cell differential temperature-scanning calorimeter. We used the densities to calculate apparent molar volumes V-phi and the heat capacities to calculate apparent molar heat capacities C-p,C-phi for these solutions. We used our results and values from the literature for V-phi(T, m) and C-p,C-phi(T, m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change Delta C-r(p,m)(T, m) for ionization of water to calculate parameters for Delta C-r(p,m)(T, m) for the first two proton dissociations from each of the protonated aqueous cationic amino acids. We used Young's Rule and integrated these results iteratively to account for the effects of equilibrium speciation and chemical relaxation on V-phi(T,m) and C-p,C-phi(T,m). This procedure gave parameters for V-phi(T,m) and C-p,C-phi(T, m) for L-aspartinium and L-glutaminium chlorides and for monosodium L-aspartate and L-glutamate which modeled our observed results within experimental uncertainties. We report values for Delta C-r(p,m), Delta H-r(m), pQ(a), Delta S-r(m), and Delta V-r(m) for the first and second proton dissociations from protonated aqueous L-aspartic acid and L-glutamic acid as functions of T and m. (c) 2006 Elsevier Ltd. All rights reserved.