ON THE USE OF ELECTROMETRIC, SPECTROPHOTOMETRIC AND RADIOTRACER TECHNIQUES IN THE STUDY OF SURFACTANTS AND HEAVY METAL SOAPS

Abstract

Certain solutes, even when present in very low concentrations, have the unusual property of altering the surface energy of their solvents to an extreme degree. The effect is invariably a lowering rather than an increase of the surface energy. Solutes having such properties are known as surface-active agents or simply surfactants. The best known and often quoted surfactant is soap which is an alkali metal salt of higher fatty acids. Within the past two decades,however, the newer synthetic surfactants, with their obvious advantage over ordinary soaps with regard to their stability to hard or acidic water, have received an increasingly wider recognition. The earliest surfactants to be developed were probably the "sulphonated oils", used as dyeing and wetting assistants. The development of synthetic surfactants reached its most active phase in the oeriod between the two World Wars. The tendency of surfactants to be adsorbed at interfaces or surfaces is most frequently due to aproperty for which G.S.Hartley has coined the name "amphipathy", i.e., the occurrence 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, toother 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 to these two types of groups as " lyophilic" and "lyophobic", respectively. Classification of the surfactants:- Surfactants may be classified under two general heads: (i) Aqueous surfactants (ii) Non-aqueous surfactants. Aqueous surfactants : In contrast to non-aqueous surfactants, physical chemistry of surfactants soluble in water has been thoroughly investigated. These surfactants may be further divided under following heads : (a) Anionic surfactants If the elongated, low-affinity(hydrophobic) portion of the surfactant molecule is included in the anion, the surfactant is called anion active or simply anionic. This class of surfactants includes soaps, Twichell's reagents (alkyl-aryl sulphonates), alkane sulphonic acids, alkyl sulphates etc.. (b) Cationic surfactants The cationic surfactants give a cation containing the elongated hydrophobic portion of the surfactant molecule on dissociation in water. This class consists of quaternary ammonium compounds, salts of long chain orimary, secondary and tertiary amines etc.. 3 (c) Non-ionic surfactants They do not ionise in aqueous solution. This class contains non-ionisable hydrophilic groups, usually containing a number of oxygen, nitrogen or sulphur atoms in non-ionising configurations. This class mainly includes surfactants derived from ethylene oxide. Behaviour of the surfactants in their aqueous solutions: The physical chemistry of aqueous solutions of ionic surfactants is particularly fascinating since it involves two widely separated fields of strong electrolytes and simple solutes on the one hand and soluble macromolecular colloids on the other. The existence of hydrophobic and hydrophilic portions in single molecule of the surfactants imparts them properties markedly different from ordinary solutes. When a surfactant is put in water, water molecules, due to very strong cohesive force between them, tend to push out hydrophobic portions out of the solution and simultaneously this tendency is opposed by the solubilising influence of hydrophilic portion. The suit is that in dilute solutions, such molecules ncentrate at surfaces/interfaces with hydrophilic oortion oriented towards the aqueous phase. This orientation brings about decrease in surface/interfacial tension. However, in concentrated solutions all the re CO 4 solute molecules cannot remain at the sirface/interface. In that case, the surfactant molecules ;egin to aggregate is such a way that hydrophobic portion? remain in the interior of the aggregate and hydrophilic portions on the exterior. All the pioneering workers ramely, McBain, Hartley, Lottermoser, Wright, Tartar, Ralston, Hoerr, Harkins etc. agree on the presence of. aggregates in aqueous solutions of surfactants. These aggregates of tfKTsurfactant ions were first termed b/ McBain(l) as "ionic micelles". The micelles in aqveous solutions of surfactants are not formed at any arbitrary concentration but begin to form in large amounts only when adefinite concentration range is reached. The concentration above which micelle formation star:s is referred to as critical micelle concentration (c.m.c). The c.rn.c. is not a specific concentration value but a narrow concentration range within which the constitution of the surfactant in solution changes from molecularly dispersed state to an equilibrium between molecules and aggregates. A number of methods, viz., electrical conductance (2-5), surface tension(6-8), freezing point (9,10), osmotic pressure(ll), vapour pressure(l2), solubility( 13,14), viscosity(15,l6), solubilization*!?), partial molal volume(18), refractive index(l9) etc. ' been employed to determine the c.m.c. values of sr 5 The size and Shape of Micelles: ** Although there is general agreement on the presence of micelles in aqueous solutions of surfactants, there is disagreement as to their kinds, shapes and structure. Various types of micelles have been postulated to explain a large amount of physico-chemical data collec ted on aqueous solutions of surfactants. However, there have been two leading school of thought in this field, one represented by McBain and the other by Hartley. Other workers, while agreeing with general outlines of one or the other theory, differ on some minor points. According to Hartley(20-22), the ionic surfactants below c.m.c, are completely dissociated and unaggregated. At the c.m.c, aggregation begins abruptly with the formation, at first, of relatively small micelles which grow rapidly over a very limited concentration range to a size which for a given surfactant remains approximately constant. Hartley believes that the micelles are liquid and essentially spherical and that their interior approximates to the random distribution state of liquid paraffin, but with the hydrophilic end of the ion constrained to remain at the surface of the micelle. His spherical micelle contains, in addition to a large number of paraffin chain ions, a considerable number of counterions held on the surface of the micelle. Hartley thus postulates only one type of micelle of approximately constant size at all concentra tions above c.m.c. 4 solute molecules cannot remain at the sirface/interface. In that case, the surfactant molecules ^egin to aggregate is such a way that hydrophobic portion? remain in the interior of the aggregate and hydrophLic portions on the exterior. All the pioneering workers ramely, McBain, Hartley, Lottermoser, Wright, Tartar, Ralston, Hoerr, Harkins etc. agree on the presence 0/ aggregates in aqueous solutions of surfactants. These aggregates of the surfactant ions were first termed b/ McBain(l) as "ionic micelles". The micelles in aqteous solutions of surfactants are not formed at any arbitrary concentration but begin to form in large amounts only when a definite concentration range is reached. The concentration above which micelle formation stares is referred to as critical micelle concentration (c.m.c). The c.m.c is not a specific concentration value but a narrow concentration range within which the constitution of the surfactant in solution changes from molecularly dispersed state to an equilibrium between molecules and aggregates. A number of methods, viz., electrical conductance (2-5), surface tension(6-8), freezing point (9,10), osmotic pressure(ll), vapour Pressure(l2), solubility( l3,14), viscosity(l5,l6), solubilization(17), partial molal volume(18), refractive index(l9) etc. have been employed to determine the c.m.c. values of surfactants. On the other hand, McBain(23-25) visualises two types of micelles. The one is highly conducting, spherical ionic micelle of not more than 10 like ions retaining their charges, formed in dilute solutions even before c.m.c. is reached; the other is large, poorly conducting micelle with little or no ionic charge, referred to as "neutral-colloid", formed just beyond the c.m.c. The "neutral-colloidal micelle" has been identified by McBain{25) with the lamellar micelle postulated on the basis of X-ray diffraction patterns(26-29). The lamellar micelle is described as being composed of alternate layers of water and double amohipathic molecules. Harkins and co-workers(30-32) agree with Hartley's concept of one type of micelle, but from their X-ray results consider it to have some regularity of structure and picture it as cylindrical or disc-shaped. Hartley(20) believes the disc shape to be improbable and has indicated(33) that the results of X-ray diffraction patterns are capable of being interpreted on the basis of the spherical micelle, v^ The size, shape and charge of the micelles have been studied mostly by light scattering techniques. The most powerful single method for estimating micellar size is the light scattering technique originally developed by Debye(34) and subsequently described by a number of workers(35-38). The presence of salts decreases the c.m.c. and increases the micellar size (39,40). 7 The charges of the micelle have been estimated by light scattering(35-39), electrophoretic mobility(41,42), colligative properties(43,44), osmosis(45) etc.. Effect of Environmental Factors on Micelle Formation: The c.m.c. of any given surfactant and the properties of micelles are markedly influenced by such environmental factors as the presence of salts, the nature of the solvent, the presence of solubilized material, tem perature, etc.. The effect of alcohols(46,47), hydrocarbons (48-50), dioxane and glycol(51), inorganic salts(52-55) on the micelle formation of surfactants has been thoroughly investigated. Solubilization: Solubilization for an aqueous system may be defined as the spontaneous dissolving of a normally water insoluble substance by a relatively dilute aqueous solution of a surfactant. The phenomenon of solubilization is of interest from practical as well as the theoretical view point. The major factor which controls the amount of aterial solubilized in a given system is the chemical composition of the surfactant and solubilizate. Various ethods.viz., turbidimetry(56,57), spectrophotometry^,59), X-ray diffraction(51,60) etc. have been employed to study solubilization. X-ray diffraction has very clearly distinguish ed between two different types of solubilization. In one type, the spacings representing the thickness of micelle m m 8 layers increase in proportion to the quantity of material solubilized. In this case the hydrocarbon is visualized as occupying the space between the hydrophobic ion tails. The second type of solubilization is represented by the fatty alcohols in dodecyl sulphate. Here the micelle thickness does not change significantly. This indicates that fatty alcohols penetrate into the micelle and become oriented in the same way as the ions or molecules which were originally present in the micelle(5l,60). Apart from these two types of solubilization, the solubilization of dimethyl phthalate has been explained on the basis of its adsorption on the exterior of polar groups of the micelles