Essay: Surfactants (surface active agents)

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Surfactants (surface active agents) are amphiphilic compounds which have the ability to reduce the surface tension of two phases; liquid and liquid, liquid and gas or liquid and solid.1 It is important to note that not all amphiphiles behave as surfactants, only those with somewhat equilibrated hydrophobic and hydrophilic regions.2 Principally, surfactants are made up of a polar hydrophilic head group and a non-polar hydrocarbon tail. In solution, they gather at the interface and arrange themselves so that the polar head group is in contact with the aqueous phase, and the non-polar hydrophobic chain forms weak bonds with the insoluble phase. At and above the critical micelle concentration (CMC), the interface becomes saturated with surfactant and as a result, micelles form encapsulating the insoluble drug. This in turn enhances stability, and shelf life of liquid formulations hence its widespread use. Pharmaceutical companies currently design and synthesise surfactants, one of the most recent types of novel synthetic surfactants are gemini surfactants.

Since the initial scientific literature on gemini surfactants was published in 1971, there has been growing interest on the design and synthesis of these gemini surfactants; a family of synthetic molecules made up of two identical or different amphiphilic moieties connected by a spacer group near or on the head group.3-7 These surfactants often have lower CMC’s and superior surface-active properties compared to the corresponding conventional surfactants of equal chain length.8-13 Surfactant solutions with lower CMC’s exhibit greater solubilisation and stability at low surfactant concentrations, and are therefore more desirable in the pharmaceutical industry.10 This has further given rise to the extensive study of dimeric and oligomeric surfactants which incorporate two (dimeric) or more (oligomeric) amphiphilic moieties covalently linked by a spacer group.8-10,14 Some dimeric surfactants with short spacer groups exhibit high viscosity at relatively low concentrations compared to their monomeric counterparts which is further amplified by shear-induced viscoelasticity.8,9,11 In essence, the chemical structure, length, hydrophobic or hydrophilic nature, and rigidity or flexibility of the spacer group, along with the associated amphiphiles, play a vital role in determining the properties of solutions, which include the aggregation behaviour, and interfacial characteristics.5,8,11

Oligomeric surfactants have not been studied in detail despite their promising benefits in the formulation of stable colloidal dispersions.10 This is mostly due to their more complex synthesis and purification.15 Since dimeric surfactants possess properties superior to the corresponding monomers, it is theoretically predicted that these properties would be further amplified with an increasing degree of oligomerization, with small spherical micelles forming at low concentrations, and wormlike or threadlike micelles forming at higher concentrations.11,16,17 Oligomeric surfactants can be classed into 4 different types according to their chemical compositions; anionic, cationic, non-ionic and zwitterionic. Polyoxyethylene type surfactants have a wide variety of commercial and industrial applications such as in agro-chemical, cosmetic, household, and therapeutic products.15,18-20 In this investigation, the polyoxyethylene type non-ionic oligomeric surfactant Tyloxapol (4-isooctylpolyoxyethylene phenol formaldehyde polymer) and its monomeric counterpart Triton X-100 were studied. Tyloxapol consists of a maximum of 7 units of Triton X-100 (octoxynol 9), covalently …

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