Electrolyte solutions are important in all branches of chemistry, but especially in analytical chemistry, and biochemistry. These systems by their nature are always non-ideal, and represented an early challenge to theoreticians interested in describing their thermodynamic properties. The solute components are ions, cations, and anions, which carry opposite charges and thus interact very differently with one another. The existence of electrolyte solutions depends on the polar properties of the solvent through which the individual ions are stabilized. When one recognizes the molecular nature of the solvent, one must also consider the interactions between solvent dipoles and the ion. This results in changes in solvent structure in the immediate vicinity of the ions. It follows that a complete description of an electrolyte solution at the molecular level requires the consideration of ion–dipole, ion–ion, and dipole–dipole interactions. In addition to these simple electrostatic interactions, one must also consider the role of hydrogen bonding in protic solvents like water. In very dilute electrolyte solutions, the most important consideration is ion– dipole interactions. One expects these interactions to be different for cations and anions. This follows from the fact that the solvent molecule is not a simple dipole in the electrostatic sense but instead it has a chemical structure which is different at each end of the molecular dipole. Each ion interacts locally with four to six solvent molecules in its immediate surroundings. In the case of water, the concentration of water molecules in the pure liquid is 55.5 M; it follows that the number of water molecules experiencing direct interaction with ions in dilute solutions represents a small fraction of the total number. As the electrolyte concentration increases, ion–ion interactions become more important in determining the thermodynamic properties of the solution. The electrostatic field of an ion is long ranged, decreasing with the reciprocal of the distance from the charge center of the ion. As a result a given ion has an ionic atmosphere in which the concentration of oppositely charged ions in its vicinity is slightly greater on the average than that of ions of the same charge.
Effect of truncating electrostatic interactions on predicting thermodynamic properties of water–methanol systems