Occlusion or inclusion complexes results more from the architecture of molecules than from their chemical affinity. One of the constituents of the complex is trapped in the open lattice or cage-like crystal structure of the other to yield a stable arrangement.
Channel Lattice Type
In this complex, the host component crystallizes to form a channel-like structure into which the guest molecule can fit. Specifically, the guest molecule must possess a geometry that allows it to easily fit into the channel-like structure. Furthermore, channel lattice complexes provide a means of separating optical isomers.
Cholic acids (bile salts) are an example of this complex type. For instance, the crystals of deoxycholic acid are arranged to form a channel into which the complexing molecule can fit. Additionally, the well-known starch–iodine complex is a channel-type complex, consisting of iodine molecules entrapped within the spirals of glucose residues.
Layer Type
Layer type complex (or intercalation compound) is a type of inclusion complex in which the guest molecule is diffused between the layers of carbon atom, to form alternate layers of guest and host molecules. Montmorillonite, the principal constituent of bentonite, can trap hydrocarbons, alcohols, and glycols between the layers of their lattices. Graphite can also intercalate compounds between its layers.
Clathrates
The Clathrates are compounds that crystallize in the form of a cage-like lattice, within which the coordinating compound is entrapped. Specifically, one official drug, warfarin sodium, exists as a crystalline clathrate containing water and isopropyl alcohol. Moreover, clathrates have applications in separating optical isomers, showcasing their versatility in various chemical processes.
Hydroquinone crystallizes in a cage-like hydrogen-bonded structure, in which small molecules such as methyl alcohol, CO2, and HCl may be trapped in these cages. Size of the guest molecule is important for formation. complex If the size is too small, the guest molecule will escape from the cage of the host and if the size is too big, it will not be fit inside the cage.
Monomolecular Inclusion Complexes
A monomolecular inclusion complex involves the entrapment of guest molecules within the cage-like structure formed by a single host molecule. Typically, monomolecular host structures are represented by cyclodextrins (CD). These compounds are cyclic oligosaccharides that can form inclusion compounds in aqueous solutions due to the characteristic arrangement of their glucose units.
Reference:
- Felton. L. (2013). Remington Essentials of Pharmaceutics. London. UK: Pharmaceutical Press.
