Guest molecule hydrophobic interactions

Pillar[5] arenes bearing ten neutral amino groups or cationic ammonium, pyridinium and imidazolium moieties have also been prepared an found to be soluble in aqueous media, where they were used to capture linear mono-and diacid guests [24a, b]. In both of these cases, the driving force for the formation of the complexes was the formation of electrostatic interactions between the host and the guest molecules. Hydrophobic-hydrophilic interactions also occur in these systems to stabilize the complexation process because they are conducted in aqueous media. 

Molecular imprinting can be accomplished in two ways (a), the self assembly approach and (b), the preorganisation approach3. The first involves host guest complexes produced from weak intermolecular interactions (such as ionic or hydrophobic interaction, hydrogen bonding) between the analyte molecule and the functional monomers. The self assembled complexes are spontaneously formed in the liquid phase and are sterically fixed by polymerisation. After extraction of the analyte, vacant recognition sites specific for the imprint are established. Monomers used for self assembly are methacrylic acid, vinylpyridine and dimethylamino methacrylate. 

J.D. Badjic and N.M. Kostic, Unexpected interactions between sol-gel silica glass and guest molecules. Extraction of aromatic hydrocarbons into polar silica from hydrophobic solvents. J. Phy. Chem. B 4, 11081-11087 (2000)... 

Thus, it seems that both the hydrophobic interactions and the van der Waals interactions undoubtedly play a part in inclusion-complex formation, although the relative contribution of each type of interaction may vary with the chemical properties of the guest this would account for the ability of the cyclodextrins to form complexes with a wide variety of guest molecules. The existence of a close spatial fit between the guest and the cyclodextrin cavity is, however, a necessary requirement for the formation of a stable inclusion-complex. 

Very recently a new method was developed that opens the possibility to polymerize even hydrophobic monomers in aqueous solution. This method is based on the finding that hydrophobic monomers can be made water-soluble by incorporation in the cavities of cyclodextrins. It has to be mentioned that no covalent bonds are formed by the interaction of the cyclodextrin host and the water-insoluble guest molecule. Obviously only hydrogen bonds or hydrophobic interactions are responsible for the spontaneous formation and the stability of these host-guest complexes. X-ray diffraction pattern support this hypothesis. Radical polymerization then occurs via these host-guest complexes using water-soluble initiators. Only after a few percent conversion the homogeneous solution becomes turbid and the polymer precipitates. 

In addition to the hydrophobic interaction mentioned above to encapsulate guest molecules, other types of nonspecific interactions have also been explored to enhance binding. For example, block copolymer micelles based on PEO as hydrophilic segments and poly(/3-benzyl L-aspartate) as hydrophobic blocks have used to encapsulate doxombicin. The encapsulation efficiency of doxombicin, an aromatic anticancer drug molecule, has been found to be significantly higher. This observation has been attributed to the tt-tt interaction between the anthracycUne moiety of doxorubicin and the benzyl group of poly(/3-benzyl L-aspartate) (Cammas-Marion et al. 1999). 

If one polymer has much higher molecular weight than the other, a model host-guest is commonly applied (Figure 9.8). Smaller guest molecules are absorbed on the host molecule. Because hydrophobic interactions take place between created blocks, the molecule of the complex becomes more compact. Similar intermolecular interactions can lead to precipitation. It seems probable that similar process takes place at the very be-... 

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