Surfactants | classification of surfactants | Emulsifying agents | APPLICATIONS OF SURFACTANTS |

SURFACTANTS

Surfactants or surface-active agents are substances, which lower the surface tension of a liquid or the interfacial tension between two liquids. Such materials have a tendency to get preferentially adsorbed at the interface between the two phases. Surfactants are used for many reasons in almost every dosage form including liquids, semisolids, and solids.

 

Surfactants

Surfactants are amphiphilic meaning that the molecule or ion has a certain affinity for both polar and non-polar solvents. Thus surfactant molecules consist of a ‘polar’ as well as a ‘non-polar part and when they are placed in two phases of differing polarities, the non-polar part gets oriented towards the phase of low polarity while the polar part is oriented towards high polarity phase. Depending upon the number and nature of polar and non-polar groups present, the amphiphile may be predominantly hydrophilic (water-loving), lipophilic (oil-loving), or well-balanced between the two extremities.

 

Micelle formation: Surfactants are helpful in the solubilization of poorly water-soluble solute molecules through the formation of micelles. Solubilization is defined as the spontaneous passage of poorly water-soluble solute molecules into an aqueous solution of soap or a detergent in which a thermodynamically stable solution is formed. When a surfactant is dissolved in water in a very low concentration, a fraction of it will be adsorbed at the air-water interface whereas the remainder will reside in the bulk in the form of monomers.

 

When more of the Surfactant is added, the interface becomes fully saturated, and the surfactant is forced into the bulk of the liquid. At still higher concentrations, the molecules or ions of the surfactant aggregate to form particles of Colloidal size, which are referred to as micelles. The concentration at Which micelle formation (aggregation) occurs is called the Critical MiCelle Concentration (CMC). Micelles may be of various shapes such as Spheres, sausage-shaped rods, or lamellar plates.

 

HLB system: The functional utility of a surfactant depends upon the relative magnitude of hydrophilic and lipophilic groups constituting it. An arbitrary scale to serve as a measure of the hydrophilic-lipophilic balance (HLB) of surfactants was devised by Griffin. Using this scale it is possible to assign an HLB range of optimum efficiency for each class of surfactant.

 

In addition to the most commonly used HLB system, the relative proportion of hydrophilic and lipophilic portion in a surfactant can also be expressed in terms of H/L number and the water number.

CLASSIFICATION OF SURFACTANTS

The simplest classification of surfactants is based on their ionic behavior in solutions. Thus surfactants may be classified into the following four categories:

Surfactants

Anionic Cationic Non-ionic Ampholytic

(1) Anionic surfactants : 

Anionic surfactants or anionics are those surfactants, which ionize in aqueous media or whose surface activity is attributed to the anionic part. Anionic surfactants containing carboxylate, sulphonate, and sulfate ions are known as soaps and are generally prepared by the saponification of natural fatty acid glycerides in an alkaline solution.

The cations most commonly associated with soaps are sodium, potassium, ammonium, and triethanolamine. The chain length of fatty acids ranges from 12 to 18 without which the non-polar part becomes weak. Monovalent soaps are generally hydrophilic whereas divalent and trivalent soaps are hydrophobic in nature because of a higher proportion of hydrophobic moiety.

 

Several long-alkyl-chain sulphonates, as well as alkyl aryl Sulphonates, may be used to overcome the disadvantages of soaps. – The sulphonate ion is less prone to hydrolysis and precipitation in presence of multivalent ions. A group of commonly used sulphonates in pharmaceutical practice consists of dialkyl sodium sulphosuccinates, particularly di-(2-Ethylhexyl) sodium sulphosuccinate, also known as Aerosol OT, It reduces surface and interfacial tension to extremely low values and acts as an excellent wetting agent in many types of solid dosage forms.

 

Sodium lauryl sulfate is an alkyl sulfate, which is widely used as an emulsifier and solubilizer in pharmaceutical systems. Rosin soaps formed by combining abietic acid or its isomers with sodium, potassium, or amino groups are not popular in pharmaceutical practice.

 

(2) Cationic surfactants : 

These are chiefly quaternary ammonium Compounds. They are used in pharmaceutical practice for their antimiCrobial properties rather than as surfactants. Specifically, they have bacteriostatic activity probably because they combine with the carboxy) the group in the cell walls and membranes of microorganisms by’ cation exchange and cause the cell lysis. Examples of commonly used cationic. include cetylpyridinium chloride, benzalkonium chloride, and cetyltrimethylammonium bromide.

 

Another group of cationic surfactants comprises amine salts. They act as surfactants and possess good wetting, foaming, and detergent properties. Octodecylamine hydrochloride is an important example of this type of surfactant.

 

(3) Non-ionic surfactants : 

Non-ionic surfactants are most widely favored in pharmaceutical systems because of their significant advantages concerning compatibility, stability, and potential toxicity.

 

Non-ionic surfactants can be broadly divided into 

(i) relatively water-insoluble; and 

(ii) quite water-soluble. °

 

The major types of compounds under the first group are the long-chain fatty acids and their water-insoluble derivatives. 

These include (i) fatty alcohols, e.g. lauryl, cetyl, and stearyl alcohols; 

(ii) glyceryl esters such as the naturally occurring mono-, di-, and triglycerides; and 

(iii) fatty acid esters of fatty alcohols and other alcohols such as propylene

glycol, polyethylene glycol, sorbitan and cholesterol (free steroidal alcohol), etc.,

For the preparation of the second group of non-ionic agents, polyoxyethylene groups are added through an ether linkage with one of their alcohol groups. The most commonly used compounds of this group are the polyoxyethylene sorbitan fatty acid esters, which are widely employed in preparations for both internal as well as external applications. Polyoxyethylene glyceryl and steroidal esters as well as polypropylene esters are the other related compounds.

 

A direct linkage with the hydrophobic group is also possible as with polyoxyethylene-stearyl ether or a polyoxyethylene-alkyl phenol. Unlike esters, these ethers are quite resistant to acidic and alkaline hydrolysis and hence are advantageous,

 

(4) Ampholytic surfactants ; 

These are least common. The major group of compounds under this class is that containing carboxylate of phosphate groups as the anion, and the amino or quaternary amine groups the cation.

 

These are the ionic compounds and may be either positively or negatively charged in solution. Their anionic or cationic behavior! depends on the pH of the solution. N-dodecyl N: N-dimethyl betaine, thé alkyl amino acid, for example, has a maximal concentration of ions co-training both positively and negatively charged groups at its isoelectri¢

 

point. Such species are known as zwitterions. In general, long-chain ampholytic, which exist in solution in zwitterionic form, are more surface-active than ionic surfactants having the same group because, in effect, the oppositely charged ions are neutralized.

PHARMACEUTICAL APPLICATIONS OF SURFACTANTS

 

Surfactants constitute the most useful of the pharmaceutical additives. The major applications include wetting, emulsifying and solubilizing.

 

(a) Wetting agents : 

Wetting denotes the spreading of a liquid over a solid surface. As explained earlier, the availability of drugs from capsule dosage form depends upon the rate and extent of dissolution of the drug from the dosage form. The drug particles resulting from the disintegration of the capsule or tablet should be immersed in the dissolution medium. For this to occur, the liquid must displace air and spread over the surface of the solid.

 

If the wetting does not occur, the powder will float and tend to aggregate. The extent of wetting is measured in terms of contact angle, which is described as the angle between the tangent to the surface of the drop and the solid surface.

 

Marked increases have been shown in the dissolution rate of phenacetin and phenobarbital when Polysorbate 80 is placed in simulated gastric fluids in increasing amounts; the rate of dissolution is proportional to the reduction in surface tension of the solution.

 

Similarly, in the case of tablets, the presence of hydrophobic tablet lubricants such as magnesium and calcium stearates has been shown to decrease the dissolution rate of salicylic acid whereas the addition of sodium lauryl sulfate overcomes this difficulty by increasing the wettability of the tablet.

 

Surfactants having HLB values between 7 to 9 are good wetting agents.

 

(b) Emulsifying agents :

 An emulsion is a dispersed system containing at least two immiscible phases. The immiscible phases (water and oil) have considerable interfacial tension and dispersion can not be obtained unless the tension is reduced. Surfactants, which are used as emulsifiers, facilitate the formation of an emulsion by orienting themselves at the interface and reducing the interfacial tension. Surfactants with HLB values between 3 to 8 and 9 to 16 promote the formation of w/o and o/w types of emulsions, respectively.

 

(c) Solubilizing agents : 

Solutions of surfactants can dissolve substances, which are insoluble or at the best only sparingly soluble in the solvent alone. Drugs of limited aqueous solubility have been solubilized for both internal and external use by the process of solubilizing. Solubilization has proved to be a very useful technique.

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