Discrete water molecules in the vicinity of two oppositely charged ions
An attempt is made to describe the water molecules in contact with an ion pair in an aqueous solution. One possible arrangement consisting of 10 nonpolarizable water dipoles is considered and a comparison of various energy terms is made in order to decide what factors are most important. The two ions and their attached water dipoles are enclosed in a spheroidal cavity embedded in a dielectric continuum to represent the bulk of the water. It is shown that the energy associated with the water molecules lying in the central region between the two ions is practically unaffected by the cavity size. On the other hand, although the behavior of the "outer" water molecules is extremely sensitive to the cavity size it is a good approximation to the more precise statistical problem to place them in the position of minimum potential energy.