Handbook of Solvents - George Wypych - ChemTech - Ventech!

298 Kenneth A. Connors
5.5.3.6 Liquid chromatography
In reverse phase high-pressure liquid chromatography (RP HPLC), the mobile phase is usually
an aqueous-organic mixture, permitting the phenomenological theory to be applied.
LePree and Cancino 23 carried out this analysis. The composition-dependent variable is the
capacity factor k ′ , defined by eq. [5.5.51],
V −V
k′ =
V
R M
M
t − t
=
t
R M
M
[5.5.51]
where VR is the retention volume of a solute, tR is the retention time, VM is the column dead
volume (void volume), and tM is the dead time. It is seldom possible in these systems to use
pure water as the mobile phase, so LePree reversed the usual calculational procedure, in
which water is the reference solvent, by making the pure organic cosolvent the reference.
This has the effect of converting the solvation constants K1 and K2 to their reciprocals, but
the form of the equations is unchanged. For some solvent systems a 1-step model was adequate,
but others required the 2-step model. Solvation constant values (remember that these
are the reciprocals of the earlier parameters with these labels) were mostly in the range 0.1
to 0.9, and the gA values were found to be directly proportional to the nonpolar surface areas
of the solutes. This approach appears to offer advantages over earlier theories in this application
because of its physical significance and its potential for predicting retention
behavior.
5.5.4 INTERPRETATIONS
The very general success of the phenomenological theory in quantitatively describing the
composition dependence of many chemical and physical processes arises from the treatment
of solvation effects by a stoichiometric equilibrium model. It is this model that provides
the functional form of the theory, which also includes a general medium effect
(interpreted as the solvophobic effect) that is functionally coupled to the solvation effect.
The parameters of the theory appear to have physical significance, and on the basis of much
experimental work they can be successfully generated or predicted by means of empirical
correlations. The theory does not include molecular parameters (such as dipole moments or
polarizabilities), and this circumstance deprives it of any fundamental status, yet at the same
time enhances its applicability to the solution of practical laboratory problems. Notwithstanding
the widespread quantitative success of the theory, however, some of the observed
parameter values have elicited concern about their physical meaning, and it is to address
these issues, one of which is mentioned in 5.5.3.1, that the present section is included.
5.5.4.1 Ambiguities and anomalies
Consider a study in which the solubility of a given solute (naphthalene is the example to be
given later) is measured in numerous binary aqueous-organic mixed solvent systems, and
*
eq. [5.5.23] is applied to each of the mixed solvent systems, the solvent effect δMΔGsoln being
calculated relative to water (component 1) in each case. According to hypothesis, the parameter
gA is independent of solvent composition. This presumably means that it has the
same value in pure solvent component 1 and in pure solvent component 2, since it is supposed
not to change as x2 goes from 0 to 1. And in fact the nonlinear regression analyses
support the conclusion that gA is a parameter of the system, independent of composition.