The structure of water and electrolyte solutions.

Intermolecular forces tend to organize water molecules into a tetrahedral lattice, conferring "crystalline" properties on the liquid. The simple ions, such as Na+, K+, and Cl-, have dinmensions which are of the same order as the water nmolecule. Therefore, in aqueous solution, ions can substitute for water molecules in the water lattice. However, because the force between ion and water molecule differs from the force between 2 water molecules, the properties of the lattice are perturbed by an ion. Transport processes in electrolyte solutions are analyze,d in terms of the sp,ecific lattice perturbation produced by each ion. This approach is contrasted with the classical hydrodynamic model in which ions in solution are treated as spheres in a continuous fluid. The relevance of ionic transport in aqueous solution to the transport across biologic membranes is brieflv discussed. Theory of Dissociation ABOUT 70 years ago Arrhenius invented the theory of dissociation to account for the conduction of ani electric current through an electrolyte solution. In his day the textbooks taught that electrolytes were neutral molecules which decomposed into charged particles as a result of the applied voltage. However, while thinking about the measurements of conductance which he was making for his doctor's thesis, it ocecLrred to Arrheniiis that the charged particles that make a solution conduct might exist before current passed through the solution. This wouLld mean that an electrolyte dissociated into ions the moment it dissolved, and that these ions would carry an electric current wirhen driven by an applied voltage. The theory of dissociation worked well in explaining several important chemical problems of the day. For example, van't HToff was studying the osmotic pressure and the lowering of the freezing point of solutions. He discovered that a nonelectrolyte produced an osmotic pressure, or lowered the freezing point, according to the number of molecules in the solution. But electrolytes were anomalous; they had a much greater effect than expected on the basis of the number of mnolecules. By taking dissociation into account and calculating the effect according to the nui-nber of particles in the solution, the difference between 818 electrolytes and nonelectrolytes disappeared. The question of why electrolytes dissociated was sidestepped by Arrhenius. A few years later, though, Thomson, and then Nernst, correlated dissociation with the dielectric constant of the solvent. They pointed out that the forces between charged particles would be considerably reduced in a medium of high dielectric constant. …