Handbook of Size Exclusion Chromatography and Related ...

widely used also for the semiquantitative characterization of eluent strength
towardsilicagelandother polarcolumnpackings.Snyderproposedtoexpressthe
solvent strength for binary eluents AB, 1 0 AB as
1 0 AB ¼10 A
þ log NB
anB
where 1 0 A issolvent strength of the weaker eluent component, A, NB is mole
fraction of the stronger component B, and nB is the effective molecular area of an
adsorbed molecule B. The adsorbent surface activity function, a, is defined as
(4)
log K 0 ¼log Va þaf(X,S) (5)
whereK 0 issample adsorption distribution coefficient (milliliters pergram), Va is
the volume of an adsorbed solvent monolayer per unit weight of adsorbent, and
f(X,S) is afunction describing properties of sample X and solvent S. The
simplified Eq. (4) holds for A plus B mixed eluents with not very low
concentration of the stronger component B. The role of molecular interactions
between Aand Bsolvents is not considered.
Solubility of analytes plays an important role not only in phase separation
but also in partition-based HPLC of macromolecules. Thermodynamic quality
of the eluent allows controlling of both phase separation and enthalpic partition
polymer retention mechanisms and to some extent it may also influence polymer
adorption. The thermodynamic quality of asolvent for apolymer is expressed by
the Flory–Huggins interaction parameter x (6) or by the exponent in the
Staudinger–Mark–Houwink–Sakurada viscosity law
[h] ¼KvM a
where [h] is the limiting viscosity number of linear macromolecules with molar
mass M (in practice, molar mass of the species that is most abundant in the
sample), and aand Kv are constants for agiven polymer–solvent system. [h]
reflects the volume of polymer coils in the infinitively diluted solution. In
thermodynamically good solvents, [h] values little depend on temperature (Fig.
10) while theyrapidly change at thevicinityof the theta point. The product of [h]
and M is the hydrodynamic volume of polymer coils and represents a base for the
famous Benoit’s SEC “universal calibration dependence” (51), which is a plot of
log[h]M vs. VR for appropriate polymer standards (largely polystyrenes). In the
absence of enthalpic interactions (1 0) universal calibration plots for a particular
SEC column coincide for different coiled polymer species in different eluents.
For many linear macromolecules, exponent a in Eq. (6) assumes values from
0.5 (for theta solvents) up to 0.7–0.8 (for thermodynamically good solvents).
Both x and a values for numerous polymer–solvent systems are collected,
for example, in Polymer Handbook (52).
© 2004 by Marcel Dekker, Inc.
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