Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

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 m and p using a twin fixed-cell differential temperature-scanning calorimeter. We used the densities to calculate apparent molar volumes V and the heat capacities to calculate apparent molar heat capacities Cp, for these solutions. We used our results and values from the literature for V(T, m) and Cp,(T, m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change ?rCp,m(T, m) for ionization of water to calculate parameters for ?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(T, m) and Cp,(T, m). This procedure gave parameters for V(T, m) and Cp,(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 ?rCp,m, ?rHm, pQa, ?rSm, and ?rVm for the first and second proton dissociations from protonated aqueous threonine and isoleucine as functions of T and m.