Supramolecular Chemistry of Host-Guest Inclusion Complexes

SUPRAMOLECULAR CHEMISTRY OF HOST-GUEST INCLUSION COMPLEXES

Supra-molecular chemistry is one of the most popular areas of experimental chemistry and it seems set to remain that way for the coming future. Not only the chemists, but biochemists, environmental scientists, engineers, physicists, theoreticians, mathematicians, and a lot of other researchers are also attracted towards this field of chemistry. Supra-molecular science thus had crossed traditional boundaries and more and more scientists are attracted into this field. The basic reason behind this interest is because of the presence of a number of supra-molecular assemblies in nature. (1-3)

The basic difference in the classical and supra-molecular chemistry is the difference in the bonds responsible for the connectivity of the molecules. In classical chemistry, molecular building blocks are connected by permanent covalent bonds. However, in supra-molecular chemistry, these bonds are replaced by molecularly matching physical bonds. These bonds can be hydrogen bonds to form liquid crystals, polymer-like chains, side chain-liquid crystalline polymers or comb copolymers. The consequences of these physical bonds include ionic complexation, steric and charge match in the case of crown ethers/metal cation complexes, π−π stackings, and coordination complexation. Due to these interactions, highly specific complexes called supra-molecules are formed, which are able to form a hierarchy of structures. These specific interactions are typical examples of molecular recognition. Molecular recognition is characterized by the simultaneous stability of the supra-molecule and selectivity in its formation. An important example of such case in the biological systems is the formation of the double-stranded structure of DNA and the binding of enzymes. (4-7)

Colloidal systems are the significant part of supra-molecular chemistry. On the one hand colloidal systems are extensively used in the application of detergents, emulsifiers/demulsifiers, foaming agents, wetting agents, etc., in solving the day-to-day problems that exist in many fields of industrial and domestic processing. On the other hand, these phenomena are also critical to the very fundamental processes of biological membrane formation and function in living cells. Colloidal science has generated a number of fields such as polymers, semiconductors, liquid crystals, membranes, vesicles and micellar aggregates. Micellar aggregates have served as an important bridge between microscopic and macroscopic chemical species in the development of new technologies. (8-9)

Materials exhibiting the characteristic of modifying the interfacial interactions by way of enhanced adsorptions are referred to as SURFace ACTive AgeNTS or SURFACTANTS. The term Surfactant was coined by Antara Products in 1950. Surfactants are characterized by the presence of two moieties in the same molecule viz. one polar head group (soluble in water) and other non-polar aliphatic tail (insoluble in water). Surfactants, therefore, can simultaneously dissolve in both hydrophobic and hydrophilic medium. (10-12)

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Structure of a surfactant monomer

Surfactants can be classified depending on the charge present in the hydrophilic group i.e. its headgroup (after dissociation in aqueous solution):

1. Anionic surfactants - The surfactants are called anionic, if the head groups are negatively charged such as sulphate, sulphonate or carboxylate anions. The most common example is Sodium dodecyl sulphate (SDS).

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Sodium dodecyl sulfate (SDS)

2. Cationic surfactants - The surfactants are called cationic if the head groups are positively charged. The cationic surfactants are usually quaternary ammonium, imidazolinium or alkyl pyridinium compounds. For example- Cetyltrimethylammonium bromide (CTAB).

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Cetyltrimethylammoniumbromide (CTAB)

3. Zwitterionic surfactants - The surfactants are called zwitterionic surfactants if the head groups contain both positive and negative charges. The particular example is Lecithin.

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4. Non-ionic surfactant - The surfactants are called nonionic surfactants if the head group has no charge groups such as alkyl poly (ethylene oxide). For example- Polyoxyethylene (5) lauryl ether (C12E5).

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Polyoxyethylene (5) lauryl ether

The existence of groups with opposing characteristics is responsible for all the special properties of surfactants. The properties of surfactants fall into two broad categories:

1) Adsorption: Adsorption is the property of surfactant molecules to collect at an interface (water / oil or water / air).
2) Self-assembly: Self-assembly is the tendency of a surfactant molecule to organize themselves into extended structures in water.

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Upon increasing the concentration of the amphiphilic compound in water, at a certain point the solubility limit will be reached and phase separation will set in. Due to the efficient interactions between the polar headgroups and the surrounding water molecules, a complete phase separation is usually unfavourable. Instead, the process will be arrested in an intermediate stage with concomitant formation of aggregates of amphiphilic material, wherein the nonpolar parts stick together and are shielded from water, whereas the headgroups are located in the outer regions of the aggregate. A multitude of different aggregates can be formed in this way. The morphology of these assemblies is mainly determined by the shape of the individual surfactant molecules. These aggregates are called micelles. In non-polar medium, the micelle is inverted as compared to micelle in polar medium.

Structure of a micelle

The formation of micelles sets in after a certain critical concentration of surfactant (the critical micelle concentration, cmc) has been reached. Beyond this concentration the addition of more surfactant molecules will result in an increase in the number of micelles, while the concentration of monomeric surfactant remains almost constant. Micellisation is usually driven by an increase in entropy, resulting from the liberation of the water molecules from the hydrophobic hydration shells of the monomeric amphiphile molecules, whereas the enthalpy change is generally close to zero. (8-12)

Surfactants place a special role in modern day to day life and technological applications. Among the various types of surfactants, cationic surfactants are among the most extensively used surfactants in commercial products. Here the surface-active moiety has a positive charge, thus adsorbs strongly onto most solid surfaces and can impart special characteristics to the substrate, especially surfactants belonging to the class of long chain alkyl quaternary ammonium salts are very useful. They are unaffected by pH changes, positive charge remains in acidic, neutral and alkaline media. Generally they are used in textile softeners industrially and for home use in the rinse cycle of washing machines. They impart fluffy, soft “hand” to fabrics by absorbing on to them with hydrophobic group oriented away from fibers.

N-alkyltrimethyl ammonium chlorides are used as emulsifying agents for acidic emulsions or where adsorption of emulsifying agents on to substrate is desirable. Benzalkonium chloride is used as an antiseptic and spermicide. It is deemed safe for human use, and is widely used in eyewashes, hand and face washes, mouthwashes, spermicidal creams, and in various other cleaners, sanitizers, and disinfectants. Cetyltrimethylammonium bromide (CTAB) is one of the components of the antiseptic Cetrimide. Its uses include providing a buffer solution for the extraction of DNA. Lecithin is extracted from plant (soy) or eggs. There are very biocompatible. It’s a natural component of cell membrane.

In anionic surfactants, Sodium dodecyl sulfate (SDS) is used in household products such as toothpastes, shampoos, shaving foams and bubble baths for its thickening effect and its ability to create lather. Sodium laureth sulfate, or sodium lauryl ether sulfate (SLES), is found in many personal care products (soaps, shampoos, toothpaste etc.). It is an inexpensive and very effective foamer. All of the soaps (sodium oleate, etc) are fatty acid salts. Docusate sodium [USP] is used as a stool softener and is administered orally or rectally as a tablet disintegrant or as an emulsifier and dispersant in topical preparations.

Recent reports suggest that various biological surfactants are also playing an important part in enhancing solubility of sparingly soluble substances. They can increase the solubility of a molecule in desired case via micellisation. Hydrophobic groups of surfactant form a micelle core which is liquid hydrocarbon-like in character, while their hydrated polar groups constitute a micelle outer shell in contact with water. They are capable of reducing the interfacial tension and contact angle between solid particles and aqueous media thus improving the drug wettability and increasing surface availability of the drug dissolution.

The solubilizing power of micelles is associated with the hydrocarbon core. Thus the overall influence on the drug solubility relates to the inclusion of poorly soluble drugs in the non-polar cavity of micelles. The enhancement in solubility is due to adsorption of surfactant at biological membranes and their subsequent penetration into the membrane, altering fluidity and increasing permeability. Hence, micellar solubilization is a powerful alternative not only for dissolving hydrophobic drugs in aqueous environments but also in various drug delivery and drug targeting systems where they minimize drug degradation and loss, prevent harmful side effects and increase drug bioavailability. Micellar systems can stay in blood long enough to provide gradual accumulation in the required area and their sizes permit them to accumulate in areas with leaky vasculature. (12-23)

Aqueous solution of surfactants often exhibit unusual physiochemical properties associated with the ordering of water molecules around the solute, as they distort water structure and therefore increase the free energy of the system. This process can be decreased among others by the aggregation of surfactants into micelles, which is also related to an increase of the apparent molar and partial molar properties. Above the cmc, sudden changes in many physicochemical properties have been observed in aqueous solution of surfactants.

The physical properties like surface tension, interfacial tension and detergency changes below the cmc with concentration but there is no change in these properties above cmc. Some other physical properties like density, equivalent conductivity show a change in slope below and above the cmc. Thus cmc represents a fundamental micellar quantity to study self-aggregation of amphiphilic molecules in solution and depends on a variety of factors including the number of carbon atoms, the structural arrangement of the hydrophile, presence of added electrolyte, pH and temperature. Factors that lower the cmc usually increase the lifetimes of the micelles as well as the residence times of the surfactant molecules in the micelle. Thus to obtain the precise values of cmc is of scientific interest. (24-29)

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