Thermodynamics of ion-solvation in nonaqueous solutions

Teck Siong Pang

This thesis is concerned with the study of the thermodynamics of ion-solvation in pure organic solvents and in aqueous organic solvent mixtures. The thermodynamic quantities studied are enthalpy, entropy, partial molal volume and partial molal heat capacity. As the partial molal volumes and heat capacities of electrolytes in organic solvents are relatively unexplored, a review is deemed to be useful and is treated as a major part of the thesis. The review includes a survey of reported data together with summaries of methods for determining partial molal quantities of electrolytes and ions. Heats of solution of electrolytes were measured calorimetrically in acetonitrilewater, ethanol-water and ethylene glycol-water mixtures and standard enthalpies and entropies of transfer calculated. In addition, densities and heat capacities of electrolytes in ethanol-water mixtures were also measured and apparent molal volumes and heat capacities of transfer calculated. A reference electrolyte method based on tetraphenylphosphonium tetraphenylborate (TPTB) was used to obtain the ionic values. The dependence of the thermodynamic transfer quantities on solvent composition is discussed in terms of sol vent-solvent interactions. Densities and heat capacities of electrolytes were also measured in pure ethanol, N,N-dimethylformamide, N-methylfomamide, dimethylsidphoxide and nitromethane and ionic partial molal volumes and heat capacities determined again using the TPTB assumption. Except for N, N-dimethylformamide and dimethylsulphoxide, the Hepler equation is (mostly) in agreement with the ionic volumes. The depart ure of the ionic volumes from the Hepler model for these two solvents is attributed to steric hindrance. A statistical analysis was carried out to relate ionic mo!a! volumes to ion and solvent properties. For large ions such as RX. Ph4P and BPhf, ionic radius (as r3) and solvent molar volume were found to be the dominating factors while ionic radius (also as r3), ion hydrogen bonding ability and the solvent solubility parameter are significant for the small ions. Ionic partial molal heat capacities are discussed in terms of the theory of Frank and Wen. Cp° (str) (solution structure contribution) were obtained by subtracting Cp° (int) (intrinsic heat capacity) from Cp° and were explained in terms of solvent properties.

Thermodynamics of ion-solvation in nonaqueous solutions

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