Page 197 - Chiral Separation Techniques

6.5 Adsorption–Desorption Kinetics and Chromatographic Band Broadening 175

latter case, the site integrity will be compromised. The above factors have been stud-
ied for theL-PA model system. In aprotic media of low polarity, MAA and templates
containing polar functional groups are only weakly solvated, and the interactions
holding the monomer template assemblies together are mainly electrostatic in nature
[77]. In such cases the association of the monomer and template is associated with a
loss of one set of rotational and translational degrees of freedom which leads to a net
decrease in entropy [83]. From this follows that the interaction is weakened at
increasing temperature. On the other hand, when the monomer and the template is
more strongly solvated, the association may lead to release of part of the solvent
shell, leading in turn to a net increase in rotational and translational entropy. In this
case the interaction will be favored by increasing the temperature.

6.6.4 Factors Related to Polymer Structure and Morphology

For the formation of defined recognition sites, the structural integrity of the
monomer–template assemblies must be preserved during polymerization to allow the
functional groups to be confined in space in a stable arrangement complementary to the
template. This is achieved by the use of a high level of crosslinking, usually >80 % [18].
The role of the polymer matrix, however, is to contain the binding sites not only in a
stable form but also in an accessible form (Fig. 6-12). Porosity is achieved by carrying
out the polymerization in presence of a porogen. Most of the crosslinked network poly-
mers used for molecular imprinting have a wide distribution of pore sizes associated
with various degrees of diffusional mass transfer limitations and a different degree of
swelling. Based on the above criteria, i.e. site accessibility, integrity, and stability, the
sites can be classified according to different types. The sites associated with meso- and
macro-pores (>20 Å) (sites A and B in Fig. 6-12) are expected to be easily accessible
compared to sites located in the smaller micropores (<20 Å) (sites C) where the diffu-
sion is slow. The number of the latter may be higher since the surface area, for a given
pore volume, of micropores are higher than that of macropores. One undesirable effect
of adding an excess of template is the loss of site integrity due to coalescence of the
binding sites, which is related to the extent of template selfassociation. The optimum
amount of template is usually about 5 % of the total amount of monomer, but can be
higher when trivinyl monomers such as TRIM (24) are used as crosslinkers, where a
larger fraction of functional monomer is used [84]. In this case higher sample load
capacities have been observed. The amount of template is of course also limited by the
solubility and availability of the template, although recycling is possible.