ELECTRONIETRIC AND INTERFACIAL STUDIES ON THE MICELLAR BEHAVIOUR OF SURFACTANTS AND DYES

Abstract

The name surface active agents has been given to those substances, which when present even in very email concentration cause a remarkable lowering of the surface tension of the solvent. The best known surfactants are soaps, which are salts of long chain fatty acids. These substances besides being used as general cleansing agents find great use in industry and technology. Considerable attention was foeussed on the technical and commercial potentialities of the surface active substances during the first world war and a vigorous search was made to have soaps or surfactants which unlike ordinary soaps may not suffer from instability in acid solution and tendency to undergo hydrolysis in aqueous medium. The first synthetic product, Turkey Red Oil, was prepared by the aotion of sulphuric acid on castor oil and was predominantly a mixture of sulphuric acid esters. Many other products were made in this way from a variety of natural fats. Various terminologies have been used from time to time to represent these compounds. In industry the term •syndet1 is widely used which Is a contraction of the term synthetic detergent. •Surfactant1 a shorter term for surface active agent represents an organic molecule or an unformulated compound having surface active properties while •syndet1 represents primarily a detergent forraulation. The tendency of the surfactants to be adsorbed at 2 Interfaces is moat frequently due to a property for which G.S. Hartley has coined the name •amphlpathy*, i.e., the occurence in a single molecule or ion, with a suitable degree of separation, of one or more groups which have affinity (sympathy) for the phase in which the molecule or Ion is dissolved, together with one or more groups which are antipathetic to the medium (i.e., which tend to be expelled by It). It is convenient from analogy in colloid chemistry to refer these two types of groups as *lyophilie* and *lyophobio* respectively. Classification of the surface active agentsi Surfactants may be classified under two heads (1) water soluble and (li) water insoluble surfactants. w»*«r fPtoblf ftujgaetantffi Most of the interest in the surface chemistry has been centered about the behaviour of surfactants In aqueous solutions although examples of the water insoluble type surfactants Involving the use of organic solvents to bring them into solution are by no means scarce. The water soluble surfactants can be classed under two heads, the ionic and the nonionie surfactants. The ionic surfactants may be further classified under following headsI (*) Anionic surfactantst In whioh on ionlaation the long chain hydrophobic portion (hydrocarbon part) contains negative charge. The 3 most important among them are Twichell*s reagent (alkylaryl sulphonates), alkane sulphonio acids, alkyl sulphates etc. (b) CatIonic surfactantsI In which on ionization the long chain hydrophobic portion, contains positive oharge. This class generally consists of long chain prim ry, secondary and tertiary amines, quaternary ammonium compounds, alkyl pyrldinium halldee etc (o> Amphoric surfacjantfi Amphoteric or ampholytic surfactants ionise in solution with the long chain carrying positive or negative oharge depending upon the pH of the solution, e.g., H-alkyl taurines, imldasolin derivatives etc. Besides, there are surfactants which fall under polymers and fluorocarbons. smswsswli sswswssssswslP They do not ionize in aqueous solutions. This class contains non-ionizable hydrophilio groups, usually containing a number of oxygen, nitrogen or sulphur atoms in nonionising configurations. This class mainly includes surfactants derived from ethyleneoxide. These agents have the general formula R( [CHj>CHpO]mH)n, where m and n are each unity or greater, and R ie an organic residue which usually, but not always contains a hydrophobic group such as a long paraffin chain, an aromatic residue or an 4 alkyl aromatio group. There are several advantages of this class of surfactants. The most important advantage is that the solubiliziag effect is independent of ionization and the properties of the surfactants can be modified by simply changing the length of polyethyleneoxide chain. The nonionie surfactants are less affected by the presence of electrolytes, they are chemically inert, and stable towards pH change and therefore, they are particularly well suited for preparing mixtures and formulations for specific purposes. The Increasing importance of polyethoxylated and other nonionie surface active agents is largely due to these advantages, and also to the use of a considerable number of cheap, readily available raw materials as intermediates in the synthesis of this type of surface active compounds. Behaviour of surfactants In aqueous solutions and the critical, rajceile concentration; The surface active agents possess a duality and asymmetry of properties. The existence of hydrophobic and hydrophillo portions in a single surfactant moleoule Imparts them properties which are markedly different from ordinary solutes. It is the same combination of properties which is responsible for the characteristic surface or interf&oial tension depression and the powerful emulsify ing, solubilizing, wetting and foaming abilities of their solutions. When a surfactant is put into water, the introduction of hydrocarbon part into water requires work be done to separate water molecules which is counterbalanced by strong affinity of the polar group with the water molecules. The oondltlon that the free energy of the system may decrease requires that the area of contact between water molecules and nongfolar hydrocarbon part should be minimum. It is achieved when surfactant molecules concentrate at the surface with their polar groups directed towards the water and their hydrocarbon chain directed outwards. Therefore, the surface aotive agents have a very strong tendency for adsorption and orientation at the interface from aqueous solution and free energy of the system decreases as a result of adsorption. However, in the bulk of the solution, there are no surfaces available for adsorption, the surfactant molecules or Ions begin to aggregate, at some concentra tion, in such a way that hydrophobic portions remain in the interior of the aggregate and the hydrophilio portions are grouped on the exterior. The term •ionic micelle* was first coined by MeBain (1) for theee aggregates. All the other polneer workers namely Hartley, Lottermoser, Tartar, Wright, Hoerr, Ralston etc. agree on the presence of such aggregates In the aqueous solutions of surfactants. The micelle formation in aqueous solution of 6 surfactant does not take place at any arbitrary concentration, but the micelles only begin to form in large amounts when a definite concentration range is reached. The concentration at which micelle formation becomes apparent is referred to as critical micelle concentration (o.m.c). Strictly speaking, o.m.c. is not a unique concentration value but rather a narrow range of concentration within whioh the surfactant ions or molecules in aqueous solution are dragged from molecularV dispersed state to form aggregates and an equilibrium between moleoularly dispersed and aggregated form is established. Any method that measures the deviation from the ideal behaviour in solutions oan be employed to determine the cm.c values of the surfactants. A number of methods, viz., electrical conductivity(2-5), surface tension (6-9), transference number (10-ir), solubilization (IS), partial molal volume (14,16), refractive index (16,17), freezing point (18), osmotic pressure (19), viscosity (£0,21) etc, have been used by various workers to determine the cm.c values of surfactants. Another rapid method, based on the observations of Hartley (2?) and Shappard and Geddes (23) that the fluorescenoe of some dye stuffs changes considerably, depending on whether the dye stuff is in the bulk or is adsorbed by the micelles, has been very suoessfully 7 used to determine the cm.c values of ionic surfactants (24,25). This method has also been extended to determine cm.c of nonionie surf otanta using pinacyanol chloride (26), erythroein (IT), Iodine (28,29), Benzopurpurin 4B (30) etc. as indicators. The use of polarographic method for determining cm.c. needs special mention. Colichman (31) has found tht*t the limiting diffusion current of certain reducible substances exhibit a sharp change in the presence of surfactants at their cm.c. values. The maximum suppreseion point (?4SP) has also been used to determine the cm.c of the surfactants, though in many esses the maximum suppression point seems to be different from o.m.c (32). Recently the potentiometrie method (33-35) using metal soap electrodes have been successfully employed for the determination of cm.c of both oatlonlo and anionic surfactants. This method has the advantage over other methods in determining the values of other thermodynamical constants besides cm.c. A number of unclassified methods e.g., Wien effect (36), high frequency conductivity (37) membrane electrodes (38) etc have also been used for determina tion of o.m.c, but they are mostly of aeademio interest only.