Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

We have measured the densities of aqueous solutions of L-methionine, L-methionine plus equimolal HCl, and L-methionine plus equimolal NaOH at temperatures 278.15 6 T/K 6 368.15, at molalities 0.0125 6 m/mol Ae kg 1 6 1.0 as solubilities 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 6 T/K 6 393.15 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 DrCp,m(T,m) for ionization of water to calculate parameters for DrCp,m(T,m) for the two proton dissociations from protonated aqueous cationic L-methionine. We integrated these results in an iterative algorithm using Young s Rule to account for the effects of speciation and chemical relaxation on V/(T,m) and Cp,/(T,m). This procedure yielded parameters for V/(T,m) and Cp,/(T,m) for methioninium chloride {H2Met+Cl (aq)} and for sodium methioninate {Na+Met (aq)} which successfully modeled our observed results. Values are given for DrCp,m, DrHm, pQa, DrSm, and DrVm for the first and second proton dissociations from protonated aqueous L-methionine as functions of T and m.