Molar heat capacities and solution thermochemistry of n-undecylammonium bromide monohydrate C₁₁H₂₈BrNO(s)

Abstract

Crystal structure of n-undecylammonium bromide monohydrate was determined by X-ray crystallography. Molar enthalpies of dissolution of the compound at different concentrations (m/mol kg⁻¹) were measured by an isoperibol solution reaction calorimeter at T = 298.15 K. According to the Pitzer electrolyte solution theory, molar enthalpy of dissolution of the compound at infinite dilution was determined as $\Delta _{\text{s}} H_{\text{m}}^{\infty } ({\text{C}}_{11} {\text{H}}_{28} {\text{BrNO}})$ = 32.40 kJ mol⁻¹ and Pitzer’s parameters ($\beta_{\text {MX}}^{(0)\text {L}}$ and $\beta_{\text {MX}}^{(1)\text {L}}$) were obtained. The values of apparent relative molar enthalpies (${}^{\varPhi}L$) of the title compound and relative partial molar enthalpies ($\overline{{L_{2} }}$ and $\overline{{L_{1} }}$) of the solute and the solvent at different concentrations were derived from experimental values of enthalpies of dissolution of the compound. Low-temperature heat capacities of the compound were measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 390 K. Two solid–solid phase transitions were observed for the title compound. The temperatures, molar enthalpies and entropies of the phase transitions were determined as T _trs,1 = (314.42 ± 0.229) K, T _trs,2 = (338.66 ± 0.228) K, Δ_trs H _m,1 = (15.542 ± 0.059) kJ mol⁻¹, Δ_trs H _m,2 = (6.431 ± 0.209) kJ mol⁻¹, Δ_trs S _m,1 = (49.803 ± 0.079) J K⁻¹ mol⁻¹ and Δ_trs S _m,2 = (17.658 ± 0.072) J K⁻¹ mol⁻¹, respectively, based on the analysis of heat capacity curve. Two polynomial equations of heat capacities as a function of temperature were fitted by least squares method. Based on the two polynomial equations, smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated and tabulated at 5-K intervals.