Energies of transport of individual ions out of water into aprotic solvents: Specific features of the acetate ion transport

An analysis of techniques used for the determination of the free energy of the transport of an individual ion out of water into an aprotic solvent shows that the most reliable technique at present seems to be a method based on the determination of potentials relative to a cobalticenium electrode in combination with the energy required for the transport of the cobalticenium ion calculated after Born. This method is combined with relevant original and literature data to determine the transport energies for more than thirty ions out of water into propylene carbonate, dimethyl sulfoxide, dimethylformamide, acetonitrile, and acetone. The solubility of anhydrous sodium acetate in the above solvents are measured and the obtained data are used to determine the transport energies for the acetate ion and a neutral molecule of acetic acid. It is shown that, because of a large contribution of short-range interactions, especially the hydrogen bonds, the energies in question cannot be calculated with the aid of continuum electrostatics. This proves that the purely electrostatic method, which has been extensively used in biophysics for calculating pK of acid groups of proteins, is fallacious.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 325–332.Original Russian Text Copyright © 2005 by Bunakova, Khanova, Krishtalik.