You are hereSURFACTANTS AND ITS APPLICATION IN PHARMACEUTICALS: AN OVERVIEW

SURFACTANTS AND ITS APPLICATION IN PHARMACEUTICALS: AN OVERVIEW


Modification of the wetting process by the use of surfactants, the effect of surfactants on the wetting process is a result of their adsorption at various interfaces with a resulting alteration of interfacial tensions. As has been noted from Young’s equation, the wetting process is promoted if either γLAor γSL or both are reduced with γSA remaining unchanged. Surfactants almost always cause a reduction in γLA, however, the same cannot be said for γSL and the effect on the interfacial tension depends on the nature of the adsorption. Thus the addition of a surface-active agent to the system does not always promote wetting, and spreading may in fact be made more difficult. If adsorption of the surfactant molecules at the solid–liquid interface occurs in such a manner that they are oriented with their polar ends toward the substrate and hydrophobic ends toward the liquid, the wettability of an aqueous solution is reduced. This orientation of surfactants molecules at the surface occurs if they are adsorbing to ionic or polar substrates (ion-exchange or ion-pairing mechanism). However, at higher concentrations of surfactant, the surfactant ions adsorb by hydrophobic interaction with the already adsorbed layer, thus exposing their hydrophilic ends to the solution in such a way that the surface becomes more readily wetted. Thus, the contact angle may first increase and subsequently decrease following the addition of more surfactant to a solution. In contrast, where adsorption occurs onto non-polar surfaces by, for example, van der Waals attraction, the surfactant molecules are oriented with their hydrophilic groups toward the liquid, the hydrophilicity of the substrate is increased, and it becomes more wettable.
The adsorption of surfactants onto solid surfaces is important with respect to their detergent properties, their use as wetting agents in solid pharmaceutical dosage forms, and as stabilizers for suspension formulations. The mode of action of surfactants in each of these systems is discussed further below.[2, 7, 19]

* Solubilization
Solubilization can be defined as ‘‘the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components.’’ The amphiphilic components (surfactants) must be introduced at a concentration at or above their critical micelle concentrations. Simple micellar systems (and reverse micellar) as well as liquid crystalline phases and vesicles referred to above are all capable of solubilization. In liquid crystalline phases and vesicles, a ternary system is formed on incorporation of the solubilizate and thus these anisotropic systems are not strictly in accordance with the definition given above.

Solubilization by micelles
The location of a solubilized molecule in a micelle is determined primarily by the chemical structure of the solubilizate. Solubilization can occur at a number of different sites in a micelle:


Fig.No.10.

In Aqueous SystemsSolubilization Of Drugs At Diff. Positions Of Micelle.[19]

1.      On the surface, at the micelle–solvent interface,
2.      At the surface and between the hydrophilic head groups,
3.      In the palisades layer, i.e., between the hydrophilic groups and the first few carbon atoms of the hydrophobic groups that comprises the outer regions of the micelle core.
4.      More deeply in the palisades layer, and in the micelle inner core.

Examples

1. Polar alcohols are soluble in aqueous solution, so it located in solution / on surface of micelle.
2. Phenol are having polar –OH group and non polar benzene ring. In which –oh gr. Located in hydrophilic environment and benzene ring in hydrophobic environment, so it located at the surface and between the hydrophilic head groups.
3. Semipolar materials, such as fatty acids are usually located in the palisades layer, the depth of penetration depending on the ratio of polar to non-polar structures in the solubilisate molecule.
4. Non-polar additives such as hydrocarbons tend to be intimately associated with the hydrocarbon core of the micelle.[17,18]

In non aqueous system
Reverse micelles
formed in non-polar solvent systems containing surfactant, polar additives may be solubilized in the core where a polar interaction of head groups occurs. A preferred location of the solubilisate molecule within the micelle is largely dictated by chemical structure.
However, solubilized systems are dynamic and the location of molecules within the micelle changes rapidly with time. Solubilization in surfactant aqueous systems above the critical micelle concentration offers one pathway for the formulation of poorly soluble drugs. From a quantitative point of view, the solubilization process above the CMC may be considered to involve a simple partition phenomenon between an aqueous and a micellar phase. Thus the relationship between surfactant concentration Csr and drug solubility Cdss is given by Eq. (1).

Cdss = Cdsa + P Cdsa Csr        ________________(1)

Where Cdss is the drug solubility in the absence of surface active agent and P is the distribution coefficient of drug between the micelle and bulk phases. A plot of Cdss versus Cs is linear with a slope of P Cdsa, which is the solubilizing capacity of the micelle. The effect of altering the pH of the vehicle, in the case of a partly ionized drug will be to alter the apparent partition coefficient. Thus the effect of increasing the pH of a vehicle containing an acidic drug is to reduce the proportion of drug in the micellar phase. If the surfactant is a weak electrolyte, it may induce a concentration-dependent change in pH thus altering drug partitioning and solubility. In general the solubilizing capacity for surfactants with the same hydrocarbon chain length increases in the order anionic < cationic < non-ionic, the effect being attributed to a corresponding increase in the area per head group, leading to looser micelles with less dense hydrocarbon cores which can accommodate more solute.

The solubilizing capacity for a given surfactant system is a complex function of the physicochemical properties of the two components which, in turn, influence the location or sites where the drug is bound to the micelle. The molar volume of the solubilisate together with its lipophilicity is important factors, the former reducing and the latter increasing solubilization.

Many pharmaceutical products contain a number of solutes potentially capable of being solubilized within the micellar phase. Thus competition can occur between solutes resulting in an altered solubilizing capacity. Furthermore, the addition of a second highly solubilized component to form a mixed micellar system may greatly alter the structure, size and solubilizing capacity of the system, thereby greatly enhancing drug solubility.[20]

Pharmaceutical Examples of solubilisation

  • The solubilization of phenolic compounds such as cresol, chlorocresol, chloroxylenol and thymol with soap to form clear solutions for use in disinfection.
  • Solubilised solutions of iodine in non-ionic surfactant micelles (iodophors) for use in instrument sterilization.
  • Solubilisation of drugs (for example, steroids and waterinsoluble vitamins), and essential oils by non-ionic surfactants (usually polysorbates or polyoxyethylene sorbitan esters of fatty acids).[17]

* Detergency
It is most important property of surface active agents. Surface active agents are referred as detergents.  The term Detergency is mostly used in the cleaning / removing of grease, oil and dirt from the solid surface. The principle of detergency is based on the formation of micelle.

The process needs many of the actions specific to surfactant molecules.

1. The surfactant requires good wetting properties to ensure good contact with the solid surface.
2. It also has the ability to remove dirt into the bulk liquid.

This property is achieved by lower the surface tension of the medium in which surfactants is dissolved. By lowering this interfacial tension between two media or interfaces (e.g. air/water, water/stain, stain/fabric) the surfactant plays a key role in the removal and suspension of dirt. The lower surface tension of the water makes it easier to lift dirt and grease off of dirty dishes, clothes and other surfaces, and help to keep them suspended in the dirty water. The water-loving or hydrophilic head remains in the water and it pulls the stains towards the water, away from the fabric. The surfactant molecules surround the stain particles, break them up and force them away from the surface of the fabric. They then suspend the stain particles in the wash water to remove them.If the dirt is oily it may be emulsified or solubilized by the surfactant.[2,7,21,22,25]

Fig.No.11. Process of Action Of Detergents.

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