Thermodynamic studies of cryptand 222 and cryptates in water and methanol

Abstract

Enthalpies of solution in water and in methanol are reported for a number of cryptate electrolytes ([M⁺222]+X^–) and in conjunction with data on X^– yield values for the enthalpy of transfer, ΔH^° _t, of [M⁺222] ions from water to methanol. Enthalpies of complexing of cations with cryptand 222 have been determined in water and methanol; when combined with known values of ΔH^° _t(M⁺) and the presently determined value of ΔH^° _t(222), they yield, via a thermodynamic cycle, values of ΔH^° _t for the ions [M⁺222] where M⁺= Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Ag⁺. The two methods of obtaining the ΔH^° _t values are in good agreement with each other. Solubility measurements on the perchlorates of [Na⁺222] and [K⁺222] yield values of ΔG^° _t([M⁺222]) from water to methanol. These values are also obtained from a thermodynamic cycle involving known values of ΔG^° _t(M⁺), the free energies of complex formation in water and methanol and the presently determined value of ΔG^° _t(222). The direct values and the cycle values are again in good agreement. ΔH^° _t, ΔG^° _t and ΔS^° _t values for transfer of the complexed ions [M⁺222] vary considerably with the central cation M⁺(M⁺= Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Ag⁺) and it is clear that the surrounding cryptand does not isolate the central ion from the environment. Also, various single-ion assumptions that require the constancy of ΔG^° _t([M⁺222]) and ΔH^° _t([M⁺222]) with M⁺ are not valid for the water to methanol transfer. Partition coefficients for the hypothetical extraction process M⁺(aq)+222(aq)→[M⁺222](methanol) have been obtained and it is shown that by comparison with the simple partition M⁺(aq)→ M⁺(methanol), greatly enhanced cation selectivities are observed. The largest selectivity enhancement occurs with the ions Ag⁺ and Li⁺, where the complex extraction equilibrium favours the extraction of Ag⁺ by a factor of 4 × 10⁹ over the simple partition. Ion-pair partition coefficients have also been obtained for the species [Na⁺222]ClO^– ₄ and [K⁺222]ClO^– ₄; extraction of the latter is favoured by a factor of 1.6 × 10². This may be compared to a factor of 2.7 × 10² in favour of the potassium salt when extracted as the pair of ions ([M⁺222]+ ClO^– ₄) and to a factor of 0.61 for extraction as the uncomplexed K⁺ and Na⁺ cations.

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