THERMODYNAMICS OF ION PAIR CONVERSION IN ALCOHOL SOLUTIONS OF HYDROGEN CHLORIDE

Abstract

The thermodynamic quantities of the dissociation stages of contact ion pairs (CIP) and solvent-separated ion pairs (SSIP) into ions, conversion of CIP into SSIP of the ionization process of HCl in n-alcohols from methyl to n-octyl at 278.15 – 328.15 K, i.e. in non-aqueous solvents that are most frequently used in chemical research and in chemical technology have been calculated. The proton and chloride ion forming HCl are modeled in a comparative study of the solvation process and the molecular mechanism of acid ionization in various solvents, which emphasizes the relevance of this study, in which the influence of the nature of the solvent, hydrogen chloride ions and temperature on the above thermodynamic quantities are considered. The calculation of the dissociation constants of CIP was carried out according to Ebeling’s equation, which takes into account the short-range repulsive forces between the ions in the ion pair and the dispersion interactions between the solvent molecules. The dissociation constants and thermodynamic quantities of SSIPs have been calculated by the Schwartz method, which evaluates the physical and chemical parameters of the solvent (besides the dielectric permittivity also the free volume of the solvent molecules, isothermal compressibility, etc.) that significantly affect the properties of ion pairs. The main conclusions drawn from our analysis of the array of thermodynamic data on the ionization stages of HCl in n-alcohols from methyl to n-octyl: (a) in methanol the predominant among the ion pairs are SSIPs, in ethanol the concentration of CIPs and SSIPs is approximately the same, and in other alcohols, the CIPs predominate; b) the concentration of CIPs increases with increasing alkyl radical of the alcohol when passing from methanol to n-octanol respectively from 30% to 95%; c) ratio of enthalpic (ΔconvH°) and entropic (–TΔconvS°) factors in methanol and ethanol ensures spontaneous conversion of CIP to SSIP; d) in alcohols from n-propyl to n-octyl values and sign ΔG° for the stages of dissociation of CIP and SSIP into ions and conversion of CIP to SSIP are determined by the entropic term (–ТΔS°), which leads to deconversion of ion pairs, i.e., the conversion of SSIPs into CIPs.