PHYSICO-CHEMICAL STUDIES ON THE INTERACTIONS OF SURFACTANTS AND PROTEINS

Abstract

Solutes,which even in very low concentratlons,p reduce a marked lowering in the surface energy of their solvents and tend to be adsorbed at the Interfaces are called surface active agents or 'surfactants' In a contracted form,while their unusual effect is termed surface activity. Surface active agents have been used as detergents since prehistoric times and obtained from natural products either by extraction (e.g.,saponin ) or by modification (e.g.,soaps ). The surface activity of any particular substance is exhibited in its various surface properties such as surface tension,foaming, wetting etc.. Possibly, the oldest of surface active agents used as detergent are soaps which are the alkali salts of long chain fatty adds. In spite of their excellent general cleansing action,the soaps suffer from limited applicability owing to their instability to hard or acidic waters and tendency to undergo hydrolysis in aqueous medium. Attempts were therefore made to develop substitutes free from these defects, during the past about fifty years,hundreds of detergents or surface active agents have been synthesized, evaluated, end put to a wide variety of uses. Current developments are aimed largely at producing surface active agents to meet specific requirements for specialized applications. 3^ Most surface active agents are characterized by a molecular structure which is essentially linear. One end of the linear structure Is composed of lyophllic groups having affinity (sympathy ) fbr the solvent and the other end is composed of lyophobic groups which are antipathic to the solvent and tend to be repulsed by it. This structural peculiarity, termed ' amphipathy' by Hartley', distinguishes these soap-like surface active agents from such macromolecular substances as starch, vegetable gums, proteins etc. which, too, promote foaming, emulsification, detergency, and other effects associated with 'surface activity'. CLASSIFICATION OF SURFACE ACTIVE AGENTS •www im'— •' ftsm• mSmmmmm—mwSmmmm mmmmtmmmmmisMmsiwi wmmm I • i wmm The large majority of the knovn synthetic surfactants are water-soluble substances. These have been classified as anionic, catlonic, or nonlonlc according to the sign of the charge on the elongated, hydrophobic part of the molecule responsible for surface activity. Anionic Surfactants These are principally the salts of sulphuric and sulphonic derivatives of straight paraffin chains or of alkylated aromatic residues, or of more complex hydrocarbons. In the latter, the carbon chain may be unsaturated, branched, mixed, or may contain heterocyclic rings. The soaps, by contrast, are the salts only of the higher fatty acids. Catlonic ^rfactants These are also called inverted or reverse soaps and 3 a, are chiefly the higher alkyl quaternary ammonium salts, Including the alkyl pyridinium halides. Since most surfaces possess a negative charge, the catlonic surfactants are among the most effective germicides. Their bactericidal action has been found to be most pronounced when the alkyl radical is cetyl. Nonlonlc Surfactants The surfactants of this class have non-ionizable hydrophobic end groups, usually containing a number of oxygen, nitrogen, or rarely sulphur atoms in non-ionlzeble configurations. Most of the nonlonlc surfactants are polyoxyethylene or polyoxypropylene derivatives but certain other types such as sugar esters, fatty alkanolamides, fatty amine oxides etc, also belong to this class. The nonlonlc surfactants are chemically inert and stable towards pH changes, and therefore, particularly well suited for making formulations for specific purposes. An additional advantage is th»t their properties can be modified by simrly changing the length of the polyethylene ( o^polypropylene ) oxide chain. Some surfactants such as alkyl amino carboxyllc acids (e.g., cetyl aminoacetic acid, CiaHas.NH.CHa.CO0H ) , N-alkyl taurines, amino sulphonic acids, certain imldazolin derivatives etc, may behave as anionic, nonlonlc, or catlonic surfactant depending upon the pH of the solution. These have been called ampholytlc surfactants. However, this class of surfactants is I not Important and few of these have earned any widespread use. THF MIC^LTZATTON PROCESS The existence in the same molecule of two moieties, one of which is compatible with the solvent system and the other one incompatible, endows the surfactants with peculiar characteristics not met with In other solutes. In aqueous solutions of surfactants, the strong cohesive forces between the water molecules cause the hydrophobic moiety of the surfactant molecule to be squeezed out from close contact with them. This effect is opposed by the influence of the hydrophilic group which has strong affinity for water. As a result of these opposing influences, the molecules spread at the interface in such a manner that the hydrocarbon chain ( the hydrophobic portion ) is oriented remote from the water and the hydrophilic portion is oriented towards water. This orientation leads to a decrease in the free energy of the system which is reflected in a lowering of surface tension. In the bulk of the solution, however, there are no surfaces for adsorption and at a certain concentration the molecules or ions spontaneously associate to form aggregates in which the hydrocarbon chains are inward, remote from the water and the hydrophilic ends on the exterior, so organized that water does not penetrate inside. This arrangement satisfies the tendency of the hydrocarbon chain to avoid contact with water on the one hand and the tendency of the hydrophilio s portion for hydration on the other. These aggregates are in reversible thermodynamic equilibrium with the solvent and molecularly dispersed solute and are called micelles. The phenomenon of micelle formation is referred to as mlcelllzation.