Frans_M_Everaerts_Isotachophoresis_378342.pdf

CHOICE OF THE SOLVENT 85
In general, a solvent suitable for isotachophoresis in capillary tubes must meet the
following requirements:
(1) It must have as small a self-conductance as possible, as a large conductivity results
in undesirable elution phenomena.
(2) The sample must dissolve in the solvent chosen. With regard to the process of
dissolution, two aspects of solvent behaviour are important. The first aspect is the
tendency of the solvent molecules to interact with or to solvate the sample molecules.
For example, water molecules can be considered as dipoles that can arrange themselves
around either positive or negative ions. For this reason, the dipole moment of the solvent
has to be taken into account. The second important role of the solvent is to decrease the
electrostatic interactions between the oppositely charged particles of the ionic substances
as a result of its dielectric constant, according to Coulomb’s law:
The dielectric constant alone is not an adequate measure of the suitability of a solvent
and plays only a minor role in the solubility of ionic substances. Of particular importance
is the specific solvation of anionic and cationic species.
(3) The sample must form charged particles in the solvent.
The sample substances can form charged particles by accepting or losing a proton and by
dissociation, whereby the ions formed will be solvated. In a strongly acidic solvent, the
dissolved substance will accept a proton. For example, an amino acid in 98% formic acid
will give
RNH2 + HCOOH RNg, + HCOO- (5.2)
In a basic solvent, e.g., liquid ammonia, the dissolved amino acid will lose a proton:
RCOOH + NH3 RCOO- f NP4 (5.3)
The dissociation of salts can be expressed by an equilibrium:
M+X-zW+X- (5 -4)
In solvents with high dielectric constants and in very dilute solutions, only free ions are
present, while at lower dielectric constants, the equilibrium lies towards the left-hand side.
According to Brdnsted’s theory (see ref. 2), solvents can be divided into eight classes,
with respect to three properties of water, viz., the basicity, the acidity and the dielectric
constant. The dielectric constant is assigned as positive for a value higher than 30 and as
negative for a value lower than 30. The acidity and basicity can also be expressed
numerically.
Analogous to the expression of Peters (see ref. 2) for a redox system:
RT ox
E,, = E& +- In -
F red
*The terms ‘ox’ and ‘red’ indicate the activities of the oxidant and reductant.
(5.5)