Page 158 - MODERN ELECTROCHEMISTRY

98 CHAPTER 2

However, as the shell builds up and breaks more and more H bonds in the surrounding
solvent, the effect of ions on liquid water begins to become more structure-breaking
(Fig. 2.31).
So far, only singly charged ions have been mentioned and there is a good reason
for this. A flow of univalent ions can be generated by electron pulses in an atmosphere
of water vapor. If the electron energy is sufficiently high to bring about the second
ionization of the metal atoms to form ionization of water also occurs and hence
experiments are rendered useless. The results of measurements of equilibria become
too complex to interpret. However, in 1984, Yamashita and Fenn introduced a
technique that sprays ions already in solution into a mass spectroscope. It is possible
to spray into the instrument any ions that exist in solution. Such electrospray techniques
have opened up an exciting new area of possibilities in gas phase solvation studies, but the
database in the 1990s is as yet too sparse to support broader conclusions.

2.14. INDIVIDUAL IONIC PROPERTIES

2.14.1. Introduction
It is usually relatively easy to find the solvation-related property of an electrolyte
(as, e.g., the heat of hydration, Section 2.5.2) or the partial molar volume (Section
2.6.2) of a salt in solution. However, experiments that reflect the properties of individual
ions are difficult to devise, the only simple, direct one being the transport number of an
ion (Section 2.10) and the associated individual ionic mobility (Section 2.10.1).
It is important to separate the two contributions to ionic solvation in a salt such
as NaCl. Thus, the degree of hydration depends first upon the ion–solvent distance.
The crystallographic radii of cations are less than those of the parent atom, but those
of anions are larger than those of the parent. Cations, therefore, tend to be more
hydrated than anions because the attraction rendered by the ion is inversely propor-
tional to its radius. However, when, as with and the crystallographic radii are
essentially identical, there is even then a nonequal heat of hydration for each ion. This
is because the dipole moment of the water molecule is not symmetrically distributed
with respect to its geography; its center, which determines the degree of interaction
with the ion, is nearer the positive than the negative ion, so that the former is again
favored in respect to solvation compared with the latter.
Obtaining the individual properties of ions with solvation numbers from meas-
urements of ionic vibration potentials and partial molar volumes is not necessary in
the study of gas phase solvation (Section 2.13), where the individual heats of certain
hydrated entities can be obtained from mass spectroscopy measurements. One injects
a spray of the solution under study into a mass spectrometer and investigates the time
of flight, thus leading to a determination of the total mass of individual ions and
adherent water molecules.