Surfactants, Cosmetic excipients, UNIT I Classification cosmetic products
Surface Active Agents (Surfactants)
Surfactants are necessary ingredients for cosmetics, toiletries and personal care products for enhancing their action and performance. Surfactants can be defined as the substances that are short-chain fatty acids, having one part that has an affinity for nonpolar media and one part that has an affinity for polar media. Surfactants are often used in dosage forms as emulsifying agents, solubilizing agents, suspensions stabilizers or wetting agents.
The market for cosmetic surfactants has grown extensively during the past few years. This could be attributed to the increased use of cosmetics associated with the improved living standards and cultural shifts worldwide. The majority of the surfactants used in cosmetics are derived from substances naturally present in the body and are hence safe.
Dimeric surfactants are a class of low-cost modern surfactants which have been reported to have greater utility and versatility in the formulation of cosmetics as compared to the conventional surfactants, due to the lower critical micelle concentration, and higher solubilising capacity, good aqueous solubility as well as antimicrobial activity unique to them. Besides this, properties like the ability to form foam and being mild to human skin enhance their application in the formulation of cleansers, both for facial use and the body.
Surfactants are made of two distinct portions, namely a polar head and a nonpolar tail. When placed in the aqueous solvents, the surfactants at a concentration known as critical micellar concentration orient themselves in such a way that their polar heads face the polar phase. This orientation helps in decreasing the contact angle between the aqueous phase and the nonpolar portion of the surfactant. This phenomenon is known as micellisation and the aggregates thus formed are termed micelles. The concentration of the surfactant decides the shape of these aggregates, which varies from lamellar to cylindrical
Classification of Surfactants
Depending on the type of charge present in the head, surfactants are classified as non-ionic, anionic and cationic.
If the head of the surfactant is not charged, it is termed as a non-ionic surfactant., whereas if the head carries a positive or a negative charge it is known as a cationic or anionic surfactant respectively. In case the surfactants have both positive and negative charges, they are known as amphoteric surfactants.
They are the most common and widely used surfactants used in the formulation of cosmetic products. In the presence of water, they dissociate to produce an amphiphilic anion. Their excellent cleaning and hair conditioning properties make them an ingredient of choice in the formulation of shampoos. The anionic surfactants can however be partially deactivated in the presence of calcium and magnesium ions present in water, which limits their application as formulation ingredients.
Examples of anionic surfactants include:
These surfactants dissociate in water to produce an amphiphilic cation and an anion. They are quaternary ammonium compounds possessing excellent bactericidal activity besides their surfactant property and thus when added in formulations serve the dual purposes of a preservative and of a surfactant. Because of their antibacterial property, they are mainly used for cleaning burns and wounds. Although cetrimide is the most commonly used cationic surfactant used, a few other examples of cationic surfactants used include:
Primary ammonium N+H3
Secondary ammonium N+(R)H2
Tertiary ammonium N+(R)2H
Quaternary ammonium N+(R)3
These surfactants contain a non-dissociable hydrophilic group and therefore do not ionize in the presence of water. Example of the non-dissociable hydrophilic group includes alcohol, phenol etc. They are classified as polyoxyethylene esters, polyol esters and poloxamers. They have excellent emulsifying and oil removing properties, besides being resistant to deactivation by ions like calcium and magnesium imparting hardness to the water.
Examples of non-ionic surfactants include:
Polyoxyethylene (an ‘ethoxylate’) (OCH2CH2)nOH
Surfactants containing both positively (almost always ammonium) and negatively charged groups (vary example can be carboxylate, sulphate, sulphonate) and thereby exhibiting dissociations of both anionic and cationic nature are termed amphoteric or zwitterions. Depending on the pH of the aqueous media in which they are placed, they behave either as anionic, cationic or as non-ionic surfactants.
A few examples of anionic surfactants are given below: Amine oxide N+(R)3O−
Other types of Surfactants
It consists of a permethylated siloxane group coupled to one or more polar groups. The most common examples are polydimethylsiloxane-polyoxyalkylene graft copolymers but they also include the small-molecule trisiloxane (superwetter) surfactants. They are used in a wide range of applications in which conventional hydrocarbon surfactants are ineffective. They are surface-active in both aqueous and non-aqueous systems and they lower surface tension to values as low as 20mNm−1. They are usually liquids even at high molecular weights. The trisiloxanes promote the spreading of aqueous formulations on hydrophobic surfaces such as polyethylene.
The most important emulsifying agent in this group is acacia. This stabilizes o/w emulsions by forming a strong multimolecular film around each oil globule and so coalescence is retarded by the presence of a hydrophilic barrier between the oil and water phase.
Purified lecithins are natural surfactants derived from egg yolk or soya bean oil. They are used extensively as o/w emulsifiers in parenteral and oral emulsions. They are composed of complex mixtures of neutral and negatively charged phospholipids of which the major components are phosphatidylcholine and phosphatidylethanolamine.
Examples of steroidal emulsifying agents derived from animal sources include wool fat (lanolin), wool alcohols (lanolin alcohols), beeswax and cholesterol. They are generally complex mixtures of cholesterol, long-chain alcohols and related sterols. Purified derivatives are still widely used in traditional dermatological emulsions, such as creams, as w/o emulsifiers, and for their emollient properties. They are prone to oxidation and hydrolysis, and anti-oxidants may need to be incorporated into the emulsion formulated using them.
Reference: e-PGPathshala NME-ICT
Final Year B Pharm Notes, Syllabus, Books, PDF Subjectwise/Topicwise