Thermodynamics of proton dissociations from aqueous threonine and isoleucine at temperatures from (278.15 to 393.15) K, molalities from (0.01 to 1.0) mol . kg(-1), and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volume

摘要

We have measured the densities of aqueous solutions of isoleucine, threonine, and equimolal solutions of these two amino acids with HCl and with NaOH at temperatures 278.15 <= T/K <= 368.15, at molalities 0.01 <= m/mol (.) kg(-1) <= 1.0, and at the pressure 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 in 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, in) for ionization of water to calculate parameters for Delta rCp,m (T, m) for the 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 threoninium and isoleucinium chloride and for sodium threoninate and isoleucinate 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 threoninonine, and isoleucine as functions of T and in. (c) 2006 Elsevier Ltd. All rights reserved.