Surface active agents, also known as tensides, amphiphiles or surfactants (for short) is a general name for substances that tend to preferentially accumulate at the boundary (i.e., interface) between two phases. This adsorption at the various interfaces existing between solids, liquids, and gases causes a change in the nature of interfaces which are of considerable importance in pharmacy. Thus, the lowering of the interfacial tension between oil and water phases facilitates emulsion formation, the adsorption of surfactants on insoluble particles enables these particles to be dispersed in the form of a suspension, their adsorption on solid surfaces enables these surfaces to be more readily wetted, and the incorporation of insoluble compounds within micelles of the surfactant can lead to the production of clear solutions.
Surface-active compounds are characterized by having two distinct regions in their chemical structure, one hydrophilic or water-loving region (usually depicted with a circle) and the other hydrophobic or water-hating region (usually depicted with a wiggly chain or a rectangular box). The existence of such two regions in a molecule is known as amphipathy, and the molecules are consequently often referred to as amphiphiles or amphipathic molecules.
The hydrophobic portions are usually saturated or unsaturated hydrocarbon chains or, less commonly, heterocyclic or aromatic ring systems and are also known as the lipophilic group. The hydrophilic regions are typically the functional groups that bear electronegative atoms that can form hydrogen bonds with water and can participate in dipole-dipole interactions. Depending on the number and nature of the polar and nonpolar functional groups present, the amphiphile may be predominantly hydrophilic, predominantly lipophilic, or almost equal in hydrophilic and lipophilic characters.
The ability to reduce surface/interfacial tension (surface activity) of a surfactant depends on its ability to preferentially partition into the interface, which, in turn, depends on the balance between its hydrophilic and lipophilic properties. The surfactant molecules localize at the surface, with the hydrophobic regions pointing toward and bonding the hydrophobic liquid (or air), while the hydrophilic regions pointing toward and bonding the aqueous or hydrophilic liquid. Thus, the surfactant molecules replace the bulk liquid molecules on the surface with molecules that show mutual attraction for both sides of the surface, which reduces surface tension.
For air-water surfaces, an increase in the length of the hydrocarbon chain of a surfactant results in an increase in surface activity. Conversely, an increase in the hydrophilicity results in a decrease in surface activity.
Types of surfactants
There are several general classes of surfactants. For pharmaceutical purposes, surfactants are most satisfactory classified by means of the type of polar group into;
- Anionic surfactants
- Cationic surfactants
- Amphoteric Surfactants
- Non-ionic Surfactants
These are surfactants in which the hydrophilic portion of the molecule carries a negative charge such as R-COO−, RSO4−, or RSO3−, where R represents an organic group. Anionic surfactants are electrolytes, and a surface-active ion is an anion when the surfactants dissociate in water. The anionic surfactants adsorb on various kinds of substrates and give them an anionic charge. This action of the anionic surfactant contributes to the strong detergency and high foaming power of the agent. Consequently, anionic surfactants are used most widely and extensively in detergents, shampoos, and body cleansers.
Some anionic surfactants, such as sodium dodecyl sulfate (SDS), also known as sodium lauryl sulfate (SLS) are used to create sink conditions during in vitro drug release studies for new drug product development. It is very water-soluble and has bacteriostatic action against gram-positive bacteria. Therefore, sodium lauryl sulfate also finds use as a preoperative skin cleaner and in medicated shampoos.
Other examples of anionic surfactants include alkali metal and ammonium soaps such as sodium stearate (o/w), soaps of divalent and trivalent metals such as calcium oleate (w/o), amine soaps such as triethanolamine oleate (o/w), etc.
These are surfactants in which the cation is the surface-active component. They are also electrolytes, with a positive charge. Most materials have negative charges in an aqueous media, and the cationic surfactant molecules adsorb by orienting their hydrophilic head group toward the surface of the materials. This characteristic of the cationic surfactant is fully utilized in several products such as fabric softeners and hair conditioners.
In addition, cationic surfactants can destabilize biological membranes due to the interaction of their cationic groups with the negatively charged phospholipids on the cell membranes. This results in their germicidal activity. Thus, the quaternary ammonium and pyridinium cationic surfactants have bactericidal activity against a wide range of gram-positive and some gram-negative organisms and are commonly used as preservatives in pharmaceutical formulations at relatively low concentrations.
Cationic surfactants may also be used on the skin for cleansing of wounds. For example, solutions containing 0.1 % – 1 % cetrimide are used for cleaning the skin, wounds, and burns, as well as for cleaning contaminated vessels. Dilute solution of benzalkonium chloride (which is a mixture of alkyl benzyl dimethyl ammonium chlorides) may be used for the preoperative disinfection of the skin and mucous membranes, for application to burns and wounds, and for cleaning polyethylene tubing and catheters. Benzalkonium chloride is also used as a preservative in eye drops.
Amphoteric or ampholytic surfactants sometimes referred to as zwitterionic molecules are surface-active agents that possess both cationic and anionic groups in the same molecule. Their ionization state in solution depends on the pH of the medium and the pKa of ionizable groups. For example, the acidic functional groups, such as carboxylate, sulfate, and sulfonate, are negatively charged (ionized) at pH > pKa, while the basic functional groups, such as amines, are positively charged (ionized) at pH < pKa. The extent of ionization of functional groups, that is, the proportion of molecules in solution that bear the positive or the negative charge, at a given pH is governed by the Henderson–Hasselbalch equation [ pH = pKa + log([A–]/[HA]) ].
Amphoteric surfactants are rarely used as a main product component. Their primary use is as an important co-surfactant that boosts the detergency and the foaming power of anionic surfactants.
Examples of ampholytic surfactants include lecithin (used for parenteral emulsions), N-dodecyl 2-amino acetic acid, etc.
Non-ionic surfactants are surfactants that contain ether [–(CH2CH2O)nOH] and/or hydroxyl [–OH] hydrophilic groups. They make up the largest group of surfactants. Unlike anionic and cationic surfactants, non-ionic surfactants are nonelectrolytes; that is, their hydrophilic groups do not ionize at any pH value.
Non-ionic surfactants are commonly used for stabilizing oil-in-water (o/w) and water-in-oil (w/o) emulsions. Since the non-ionic surfactants do not contain an ionizable group, their properties are much less sensitive to changes in the pH of the medium and the presence of electrolytes. In addition, they have fewer interactions with cell membranes compared with the anionic and cationic surfactants. Thus, non-ionic surfactants are preferred for oral and parenteral formulations because of their low tissue irritation and toxicity.
Examples of non-ionic surfactants include sorbitan fatty acid esters, (Spans®), polysorbates (Tweens®), and poloxamer (Pluronics®). Sorbitan fatty acid esters such as sorbitan monopalmitate are oil-soluble emulsifiers that promote the formation of w/o emulsions. Polyethylene glycol sorbitan fatty acid esters (Tweens) are water-soluble emulsifiers that promote the formation of o/w emulsions. Pluronics are block copolymers of hydrophilic poly (oxyethylene) (POE) and hydrophobic poly(oxypropylene) (POP) represented by the general formula POEnPOPm-POEn, where n and m represent the number of OE and OP, respectively.
The Spans and the Tweens come in different molecular weight or size ranges, which differ in their physical properties.
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