Handbook of Size Exclusion Chromatography and Related ...

macromolecules may strongly differ from that of small molecules. Therefore,
some conclusions on the role of adsorption of low molecular analytes cannot be
directly applied in HPLC of macromolecules.
The schematics of polymer adsorption on the solid surface are shown in
Fig. 5. Asimilar picture is valid also for adsorption of macromolecules on liquid
(mobile phase)–(quasi) stationary phase interfaces. The extent of polymer
adsorption depends both on the affinity of macromolecules to the surface and the
eluent strength (see also Secs 1and 4.2). Weakly adsorbed macromolecules are
attachedtosolidsurfacesandtointerfaceswithrelativelyshortpartsoftheirchains
(trains are short, free ends are long, and loops are large). As 1increases, trains
become longer while the loops and free ends become less frequent and their sizes
decrease. To adsorb, macromolecules usually change their conformation (decoiling).
Consequently,adsorption of macromolecules is accompanied by large
changes in their conformational entropy.This is one of the reasons why extent of
polymer adsorption may increase with rising temperature of the system. Decoiling
of macromolecules also allows us to understand adsorption of large
macromolecules in narrow pores. At a certain, large 1 value, de-coiled
macromoleculesareabletoreptateintoporesfromwhichtheywouldbeexcluded
in the weak interaction regime (0 1 1cr) (Fig. 6) (15).
It is anticipated that in some systems the summing effect of otherwise not
very large segmental interactions may start playing a role at certain molar
masses. As aresult, the calibration dependence would exhibit an unusual backturn
curvature (Fig. 4, curve 10). Wehave also revealed that macromolecules
can penetrate along rather bulky groups bonded on asolid surface to adsorb on
active surface groups (for example, silanols in case of silica gel C18 bonded
phase) (16). In this case, polymer adsorption may be limited to rather short
trains or even to single active groups situated on macromolecules. To attain a
measurable change in retention volume, macromolecules must bend and attach
on the surface simultaneously with several moieties. This is possible only if
polymer molar mass is high enough and therefore the back-turn kink
on calibration dependences affected by adsorption is observed only at a
certain “limiting” size of macromolecules. Wespeak about “U-turn adsorption”
(Fig. 7) (15–17). Evidently, conformation of this hypothesis will need
Figure 5 Schematic representation of macromolecules adsorbed on a solid surface.
© 2004 by Marcel Dekker, Inc.