Page 249 - MODERN ELECTROCHEMISTRY

ION–SOLVENT INTERACTIONS 185

46
water, but forming the head of the streamers and traveling across to the counter
electrode. The great outburst of electrons arising from reaching the conduction band
in water and flowing through the dielectric (opening of said flood gates, etc.) would
precipitate dielectric breakdown (Fig. 2.74).
This section briefly describes an intriguing and practical phenomenon found in
water and ionic solutions. A detailed comparison of this new model with experiment
would take a disproportionate amount of space. One matter only is mentioned. Does
the model stated explain the apparent avalanchelike effect shown in Fig. 2.71? Perhaps.
For there are always particles in practical solution, solid particles and some metallic.
The phenomena of breakdown are probably determined by many factors. A stream of
electrons from the cathodes could cause collisional phenomena in the solution and thus
secondary emissions from the particles struck by the electrons, which would then cause
many more electron–particle collisions and eventually an avalanche of electrons.

2.22. ELECTROSTRICTION
Ions exert electrical forces on solvent molecules in their vicinity. Because pressure
is defined as force per unit area, this means that ions exert a pressure on the solvent
and/or other nearby ions. As shown below, this pressure is very high (it may exceed
Pa) compared with pressures normally encountered in the laboratory. 47
Phenomena connected with this large pressure are referred to under the title of
electrostriction. Molecules and ions are squeezed and decrease in size. Electrostriction
is the reason that the partial molar volume of ions may become negative, for the
volume-decreasing effect of adding them to a system can be greater than the volume
increase caused by the addition of the ions themselves.
Effects of this kind are shown in Fig. 2.75. However, electrostriction has its limits.
As seen in Fig. 2.75, the value of the compressibility itself is reduced as the electric
field (and hence the local pressure) increases. Some details of this are worked out in
the next section.

2.22.1. Electrostrictive Pressure near an Ion in Solution
The molar volume of water in the natural state is but if water
molecules are close-packed in the liquid state, this volume would become only
a reduction of 33%. Thus, the volume available for the effects of electrical
constriction by ions on water molecules is as much as
It is easy to calculate a typical pressure exerted by an ion on a water molecule in
the first hydration shell. Thus, the energy of interaction of an ion of radius r i on a water
molecule of radius is

47
When a gas is compressed, the particles come into contact at pressures of a few thousand atmospheres. It
is impractical to deal with gases at substantially higher pressures.