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11) Scatchard (ref 1) and Parsegian (ref 3) have calculated the partition coefficient of ions between water and the middle of a membrane. They do not treat the integrated membrane-water partition coefficient, which governs ionic permeability. (12) R. P. Bell, Trans. Faraday Soc., 27, 797 (1931), (13) The method of image charges is widely used in classical electrostatic calculations. It provides a handy way of satisfying Laplace's equation under a variety of conditions. See, for example,

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Xenon-1 29 NMR spectra have been measured for solutions containing Xe dissolved in a variety of n-alkanes and their mixtures at elevated pressures. The resonance frequencies measured for 129Xe dissolved in pure n-alkanes are all shifted to higher frequency relative to low-pressure 129Xe gas and vary with carbon number in a continuous but nonlinear fashion. Theoretical treatments of solvent-induced chemical shift based on a continuum model of the solvent are found to be inadequate when low molecular weight n-alkanes are involved. However, a simple model, which treats the dominant van der Waals contribution to the chemical shift of the 129Xe resonance as the average of individual pair interactions between X e and the individual methyl and methylene groups present in solution, is shown to accurately account for the chemical shifts observed in both pure n-alkanes and their mixtures. A modification of this simple model, which allows for multiple interactions between 129Xe and the carbon centers of each alkyl chain, is invoked to qualitatively explain the large difference observed between the shielding effects produced by the methyl and methylene groups of the various solvents.

The difference between the values of 0V and 0V°I s sufficiently small for this purpose.(50) In water, the slope has been found to be steeper for organic ions22,48•51 and for nonelectrolytes.52 When the data for R4NI obtained by Gopal et al.23 are plotted against molal volumes, the slope is found to be equal to that obtained from eq 8 In /V-methylacetamlde, to be smaller than that in dlmethylformamide, and to be larger than that In A/-methylpropionamlde. (51) K.

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The chemical shifts of Xe-129 dissolved in liquid C02, NzO, CZH6, and CsHs have been measured at 23 OC using a lightweight sapphire N M R tube-valve assembly designed for this purpose. The Iz9Xe chemical shifts, determined relative to gaseous Iz9Xe in the low-pressure limit, are found to be proportional to the molecular polarizability of the individual gases. The chemical shift of Iz9Xe in liquid COz agrees closely with the value predicted by extrapolating gas-phase 1z9XeCOz chemical shifts measured by Jamesen et. a1.I to a density corresponding to that of liquid C02. These facts suggest that the simple cage model, originally proposed by Rummens, Raynes, and Bernstein? satisfactorily describes the effect of electron fluctuations in neighboring solvent molecules on the nuclear shielding of 129Xe dissolved in solutions composed of small molecules. The sapphire N M R tube-valve assembly used in these studies is described in some detail as well.

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