Nuclear magnetic resonance studies of preferential solvation. Part 1.—Hydrogen peroxide + water

Abstract

Studies of the alkali metal (⁷Li, ²³Na, ⁸⁷Rb, ¹³³Cs) and halide (¹⁹F, ³⁵Cl) resonance chemical shifts have been made in H₂O₂+ H₂O solvent mixtures up to 85 % w/w peroxide with the addition of 10^–3 mol kg^–1 ethylenediaminetetra-acetic acid (EDTA) to complex transition metal ion impurities. When the chemical shifts were dependent on salt concentration, results were extrapolated to zero salt concentration. The variation of these infinite dilution shifts with peroxide mole fraction shows Rb⁺, Cs⁺ and F^– to be preferentially solvated by peroxide and Li⁺ preferentially solvated by water. These conclusions can be put on a quantitative basis by assuming that the solvation changes can be represented by a series of n competitive equilibria, where n is the solvation number, assumed to be the same for both water and peroxide. The free energy of preferential solvation has been determined where ΔG^⊖_ps=–RT ln K and K is the equilibrium constant for the overall process X(H₂O)_n+ H₂O₂⇌ X(H₂O₂)_n+nH₂O. K is related to the equilibrium constant for the ith step by K=[iK_i/(n+ 1 –i)]ⁿ. The relation of ΔG^⊖_ps to the free energy of transfer of a neutral combination of ions from one solvent to another, ΔG^⊖_t, is discussed.