THOMAS MURRAY FYLES. Can. J. Chem. 65, 884 (1987).A method is described for the estimation of two phase extraction constants from stability constants for complexation of alkali metal cations by crown ethers and cryptands. The total free energy change for extraction of alkali metal salts from water to a non-polar organic solvent is evaluated as a sum of free energy terms for complexation, transfers of cation, anion, and complex between solvents, and ion pairing in the non-polar solvent. The method gives satisfactory agreement with a variety of published extraction constants and can be used predictively. As an example, the influence of extraction constant on transport flux was examined using calculated extraction constants and the rate data of Lamb et al. (J. Am. Chem. Soc. 102,6820 (1980)); rates are adequately modelled by a simple analysis of diffusion.THOMAS MURRAY FYLES. Can. J. Chem. 65, 884 (1987). The transport of cations across artificial membranes using synthetic ionophores such as crown ethers or cryptands has received considerable attention over the past decade (1, 2). The focus of such studies varies from the assessment of the complexation selectivity of new ligands to the development of specific separation processes (1-3). The actual mechanism of the transport has received somewhat less attention but it is widely assumed that the transport is controlled by diffusion processes in the unstirred boundary layers adjacent to the organiclwater interfaces. A recent report summarized the predictions of a simple diffusion model and concluded that the narrow range of reported rates was consistent with a diffusion mechanism (4). Our interest in this area has centred on crown ether carboxylic acids which can exhibit cation-proton countertransport across artificial membranes (5, 6). This system has been examined in detail and it is clear that under the usual conditions of stirring the transport is not controlled by diffusion (5-7). We have identified the rate-limiting processes as adsorption and desorption of the carrier at the interface and have shown, as one.consequence, how the transport selectivity depends on the interfacial surface charge (8).In search of other systems which might be controlled by interfacial processes, we set out to compare the literature data with the predictions of the simple diffusion model (4). This search is immediately hampered by the paucity of data on two phase extraction constants. The use of extractions, particularly of metal picrates, to assess cation binding and selectivity has a long tradition in crown ether chemistry (9-1 1). However, such tests are often used qualitatively (% extraction) and, moreover,