372
The second result indicated that for polymers containing only a small proportion
of cyclodextrin, the quantity of pollutant bound by the material was often much
greater than the quantity of cyclodextrin present in the material, contradicting the
notion that one molecule of cyclodextrin traps one pollutant molecule. For contaminants containing aromatic groups, we also introduced the occurrence of hydrophobic interactions leading to pollutant stacking (π–π interactions) and/or the formation
of multilayers of contaminants at the surface of the polymers, in agreement with
Freundlich’s model. In the presence of phenolic derivatives with high dipole
moments, electrostatic interactions of the dipole-dipole type between pollutant molecules were also possible, in particular at high concentrations (Crini and Peindy
2006; Crini et al. 2007). Another surprising result was the type of cyclodextrin
incorporated into the gel. We prepared materials based on α-, β-, and γ-cyclodextrin
using the same experimental conditions during the synthesis. The results showed
that contaminants could be removed regardless of the type of cyclodextrin polymer
used. For example, the cross-linked α-cyclodextrin polymer can adsorb Acid Blue
25 dye, which is too large to be a guest. For the three types of polymers (with a close
cyclodextrin content but with different swelling properties), the performance could
be comparable (Crini 2005a, b). A response was found in the structure of each macromolecular network. Similar conclusions have been published by Professor Yilmaz
(Yilmaz Ozmen and Yilmaz 2007, 2008). The last result was related to the shape of
the materials. As expected, the more regular the structure and spherical distribution
of the beads, the higher their performance. However, the results were independent
of the amount of cyclodextrin but dependent on the degree of cross-linking. With the
beads, kinetic studies have indicated short contact times necessary to reach equilibrium, suggesting rapid adsorption surface. This led us to highlight the importance of
physisorption in the process of pollutant removal by ECP polymers. This physisorption mechanism acts as a complement to chemisorption by complexation (Crini and
Peindy 2006; Crini 2008).
We explained these four results mainly by the network structure of the materials
and their shape and swelling properties, closely related to the degree of crosslinking, and also by the presence of cyclodextrin units (Morin-Crini and Crini 2012,
2013; Morin-Crini et al. 2015). For ECP materials, the question arises as to the
predominance of inclusion complexes due to the cyclodextrin molecules or association complexes due to the polymer network. Currently, the consensus is rather for
the latter, with the results being mainly due to the structure of the macromolecular
network independent of the quantity of cyclodextrin actually present (Morin-Crini
et al. 2018a).
The concept of association complexes is less simple since there can be a cooperative effect, not only between the cyclodextrin cavities themselves (particularly for
large guest molecules) but also between the cyclodextrin cavities and those of the
polymer network. To demonstrate this conclusion, we synthetized materials composed of non-cyclic oligosaccharides (linear dextrins, sugars such as sucrose which
has similar dimensions and chemical composition to cyclodextrin moieties) and
polysaccharides (starch fractions rich in amylose or amylopectin components, chitosan) under the same experimental conditions as the ECP polymers (Badot et al.
G. Crini
The second result indicated that for polymers containing only a small proportion
of cyclodextrin, the quantity of pollutant bound by the material was often much
greater than the quantity of cyclodextrin present in the material, contradicting the
notion that one molecule of cyclodextrin traps one pollutant molecule. For contaminants containing aromatic groups, we also introduced the occurrence of hydrophobic interactions leading to pollutant stacking (π–π interactions) and/or the formation
of multilayers of contaminants at the surface of the polymers, in agreement with
Freundlich’s model. In the presence of phenolic derivatives with high dipole
moments, electrostatic interactions of the dipole-dipole type between pollutant molecules were also possible, in particular at high concentrations (Crini and Peindy
2006; Crini et al. 2007). Another surprising result was the type of cyclodextrin
incorporated into the gel. We prepared materials based on α-, β-, and γ-cyclodextrin
using the same experimental conditions during the synthesis. The results showed
that contaminants could be removed regardless of the type of cyclodextrin polymer
used. For example, the cross-linked α-cyclodextrin polymer can adsorb Acid Blue
25 dye, which is too large to be a guest. For the three types of polymers (with a close
cyclodextrin content but with different swelling properties), the performance could
be comparable (Crini 2005a, b). A response was found in the structure of each macromolecular network. Similar conclusions have been published by Professor Yilmaz
(Yilmaz Ozmen and Yilmaz 2007, 2008). The last result was related to the shape of
the materials. As expected, the more regular the structure and spherical distribution
of the beads, the higher their performance. However, the results were independent
of the amount of cyclodextrin but dependent on the degree of cross-linking. With the
beads, kinetic studies have indicated short contact times necessary to reach equilibrium, suggesting rapid adsorption surface. This led us to highlight the importance of
physisorption in the process of pollutant removal by ECP polymers. This physisorption mechanism acts as a complement to chemisorption by complexation (Crini and
Peindy 2006; Crini 2008).
We explained these four results mainly by the network structure of the materials
and their shape and swelling properties, closely related to the degree of crosslinking, and also by the presence of cyclodextrin units (Morin-Crini and Crini 2012,
2013; Morin-Crini et al. 2015). For ECP materials, the question arises as to the
predominance of inclusion complexes due to the cyclodextrin molecules or association complexes due to the polymer network. Currently, the consensus is rather for
the latter, with the results being mainly due to the structure of the macromolecular
network independent of the quantity of cyclodextrin actually present (Morin-Crini
et al. 2018a).
The concept of association complexes is less simple since there can be a cooperative effect, not only between the cyclodextrin cavities themselves (particularly for
large guest molecules) but also between the cyclodextrin cavities and those of the
polymer network. To demonstrate this conclusion, we synthetized materials composed of non-cyclic oligosaccharides (linear dextrins, sugars such as sucrose which
has similar dimensions and chemical composition to cyclodextrin moieties) and
polysaccharides (starch fractions rich in amylose or amylopectin components, chitosan) under the same experimental conditions as the ECP polymers (Badot et al.
G. Crini