INTRODUCTION TO SURFACTANTS

SURFACTANTS: 

SURFACTANTS D.LOHITHASU M PHARM

surfactants: 

surfactants Molecules and ions that are absorbed at the interface is termed as surfactants Surfactants have two distinct regions in their chemical structure ,one of which is water-liking or hydrophilic and the other of which is water-hating or hydrophobic . These molecules are referred to as amphiphilic or amphipathic molecules or simply as surfactants or surface active agents.

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The functional groups such as- OH,-cooH,-Sulphate,quaternary ammonium contribute to hydrophillic portion Alkyl chains contribute to lipophillic nature of molecules The polar end oriented towards the water as well as the non polar end projected upwards to space.

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When the same agent added to the two immiscible liquids such as oil and wate r, the molecules of surface active agents get absorbed at the interface between the two immiscible liquids. At low concentration of surfactants, the surface of water is not completely filled with surfactant molecules.

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Surfactants concentration is increased with marked decrease in surface tension . HLB VALUES: The balance between the hydrophilic and the lipophilic nature may be given in terms of HLB values GRIFFIN ,the oil soluble surfactants have low HLB values and ,the water soluble surfactants have high HLB values .

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1.Griffin method: HLB values of polyhydric alcohols fatty acid esters may estimated by HLB=20(1-S/A) S= Saponification number of ester A=Acid number of fatty acid 2.Davies method: HLB values surfactant estimatedby addition of group numbers . HLB= ∑(Hydrophilic group numbers)- ∑( lipophilic group numbers)+7

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Hydrophilic groups Group number Lipophilic groups Group number -so3Na+ 38.7 -CH- 0.475 -coo-Na+ 19.1 -CH2- 0.475 HYDROXYL(FREE) 1.9 =CH- 0.475

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3.Each group or atom has been assigned as constant. .HLB for polyoxy ethylene chains HLB=E+P/5 E= % by wt. of ethylene oxide chains P=% by wt. of poly hydric alcohols

HLB VALUES: 

HLB VALUES 1-3:anti foaming agents 3-8:w/o emulsifying agents 7-9:wetting&spreading agents 8-16:o/w emulsifying agents 13-16:detergents >16: solublising agents

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Depending on their charge characteristics thesurface -active molecules may be anionic cationic , zwitterionic ( ampholytic ) or non-ionic.

Anionic surfactants: : 

Anionic surfactants : These ionises in aqueous soln. to yield a large anion which responsible for action& a small cation . commonly Carboxylates , sulphonates & sulfates Sodium Lauryl Sulphate BP It is a mixture of sodium alkyl sulfates , the chief of which is sodium dodecyl sulfate, C12 H25 SO4 – Na+ It is very soluble in water at room temperature It is used pharmaceutically as skin cleaner, having bacteriostatic action against gram-positive bacteria, and also in. medicated shampoos It is a component of emulsifying wax.

Cationic surfactants: : 

Cationic surfactants: These are ionises in aqueous soln. to yield a large cation which responsible for action & a small anion. Cetyl tri methyl halides The quaternary ammonium cationic surfactants are important and pyridinium pharmaceutically because of their bactericidal activity against a wide range of gram-positive and some gram-negative organisms. They may be used on the skin, especially in the cleaning of wounds. Their aqueous solutions are used for cleaning contaminated utensils.

Non-ionic surfactants : 

Non-ionic surfactants These are not ionised in aqueous soln. Polyoxyethylene sorbitan fatty acid esters Sorbitan esters are supplied commercially as Spans and are mixtures of the partial esters of sorbitol and its mono- and di -anhydrides with oleic acid. They are generally insoluble in water (low hydrophile–lipophile balance (HLB) value) are used as water-in-oil emulsifiers and as wettingagents .

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Polysorbates are complex mixtures of partial esters of sorbitol and its mono and di -anhydrides condensed with an approximate number of moles of ethyleneoxide . They are supplied commercially as Tweens . The polysorbates are miscible with water, as refl ected in their higher HLB values, and are used as emulsifying agents for oil-in-water emulsions.

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Reduction of surface and interfacial tension When surfactants are dissolved in water they orientate at the surface so that the hydrophobic regions are removed from the aqueousenvironment . The reason for the reduction in the surface tension when surfactant molecules adsorb at the water surface is that the surfactant molecules replace some of the water molecules in the surface and the forces of attraction between surfactant and water molecules are less than those between two water molecules, hence the contraction force is reduced

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. For a homologous series of surfactants: An increase in the length of the hydrocarbon chain ( hydrophobic)increases the surface activity . This relationship between hydrocarbon chainlength and surface activity is expressed by Traube’s rule, which states that ‘in dilute aqueous solutions of surfactants belonging to any one homologous series, the molar concentrations required to produce equallowering of the surface tension of water decreases threefold for each additional CH2 group in the hydrocarbon chain of the solute’.

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An increase of the length of the ethylene oxide chain (hydrophilic) of a polyoxyethylated non-ionic surfactant results in a decrease of surface activity.

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Surfactants will also adsorb at the interface between two immiscible liquids such as oil and water and will orientate themselves The surface activity of a particular surfactant depends on the balance between its hydrophilic and hydrophobic properties.

APPLICATIONS: 

APPLICATIONS PHARMACEUTICAL ADJUVANTS They are added to the drugs in order to increase the product characterstics in the design of dosage forms in a variety of ways likesolublisingagents,wettingagents,detergents,suspending,foaming,emulsifying agents INFLUENCES THE DRUG ACTION These are at low conc. Enhances the penetration of hexylresorcinol into pinworm,ascaris .

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ANTIBACTERIAL ACTIVITY These are alter the integrity and lost the essential elements by leak Mainly cationic surfactants show activity But non ionic surfacants help in metabolism of organisms & facillitate their gowth Natural suractants enhances the absorption of antibiotics

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FOAMING AGENTS: They can be used in shampoos & lathering shaving cream so it can produce stable foam when mixed with air pockets enclosed with thin film of liquid . ANTI FOAMING AGENTS They can be used in fermentation process to prevents the foams in production . eg.,octnol,ether,castor oil &silicones

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SOME ARE DRUGS: Phenithiazine & procaine- tranqulizer,localanaesthetic Acridines -antibacterial Streptomycin-antibiotic Veratum alkaloids-anti cancer

Bibliography : 

Bibliography A.Martin , J.swarbrick and A.Cammarata,physical pharmacy ,3 rd edn.,varghese publishing house,bombay,1991. M e aulton , pharmaceutics,churchill livingstone,Edinburgh,1996 E A RAWLINS ,BENTLY S TEXT BOOK OF pharmaceutics,8 TH EDN.,LONDON,1992 INTERNET