PHYSICO-CHEMICAL STUDIES ON THE MICELLAR PROPERTIES OF SURFACTANTS AND THEIR INTERACTION WITH DYES

Abstract

Surface active agents9weU known for their specific behaviour of altering the surface energy of their solvents, have been obtained from natural products by extraction or modification since prehistoric times. These products have, however,always found limited applicability due to their Instability in th© acid solution or their tendency to undergo hydrolysis in the aqueous medium. The industrial and technical developments which came in the wake of the first world war,focussed attention on the disadvantages of these soaps and a vigorous search was made to have soaps or surfactants which besides being free from these disadvantages may be more effective and industrially useful. The discovery of synthetic surface active agents Is thus of recent origin as compared to those obtained from natural products. Most of the interest in surface chemistry has centered about the behaviour of surfactants in water although examples derletiag the use of organic solvents for bringing them into solution are not wanting. Generally si eaking the water soluble surface active agents can be classed under two heads, the Ionic and non-ioale soaps. Of the two,the former have undergone further subdivision into anionic and eationie soaps, depending upon the charge which the large hydrophobic part attains when brought into the solution. On the other hand non-ionic soaps,although not lonisable,possess a sufficient number of Ionising polar groups,e.g«, «0-,-0H, • CO "H -,- COO, which make them soluble in water. And yet there is another class of compounds named amphoteric surfactants which ionise in solution with the long chain carrying either positive or negative charge,depending upon the pH of the solution. Besides there are surfactants which fall under polymers and fluorooarbons. Non-ionic surface active agents have greatly attracted the attention of industry with the result that they are finding increasing applications end appreciation in many fields. The advantages of non-ionic agents are based on such fundamen tal concepts of surface chemistry as the modification of the property of each compound by simply changing the length of polyoxyethylene group, the ease with which they can be mixed and formulated,their stability towards pH and chemical inertness and their non-vulnerability towards high concentrat ions of electrolytes. Surfactants have been put to use in industry, technology and allied fields. They are finding increasingly great use in medicine and hygeine as powerful bactericides, as agents for preparing skin lotions,ointments and emulsions of the sex harmones (1) • A very large proportion of the total surfactant production Is used in textile industry. The Importance of surfactants in the field of cosmetics, metal end mineral technology,in paper,leather,synthetic rubber, pelymer,plastie,palnt,petroleum industries is now well established. Although the building and construction 3 Industries seldom claim the use of surfactants for soil stabilisation,the possibility of their being used as soil stabilisers can be explored (2). Their usefulness in agricul ture to improve physical properties of fertilisers and soils (a) and in bringing about quick germination of *m&* is also being gradually recognised. The selection of surfactants for specific application is governed by some of the distinction features like adsorp tion and orientation of molecules at the interfaces,micelle formation above a certain concentration known as critical micelle concentration ( c.m.e.) , solubiiisation of water insoluble substances by micelles etc.. Truly speaking amongst colloids it is the surfactants which possess the unique distinction of exhibiting reversible thermodynamical equilibrium between colloidal aggregates and surrounding environments. Many physical properties of the surfactants exhibit more or less an abrupt change over a narrow concentration range. Thit abruptness in property hat been utilised in the determination of c.nuc. A number of methods based on electrical conductance ( 4-3 ), osmotic pressure (9)fvapour pressure (10) .viscosity (U) ,e.m.f. (12) .diffusion (13,14) , freeaing point (16) .solubility (16,17),surface tension (18*20) , refractive index (21) , polarography (2$ ,light scattering (29) measurements have been employed from time to time by various workers to study the micellar properties 4 ef surfactant solutions. Of these the light scattering method is of special importance since it affords the determi nation of both the aggregation number and cm. a. of the surface active substances (ionic and non-ionic). Spectral dye method for the determination of cm. c,first initiated by Hartley (24) , has met with great success (26-27). This method has now been extended to non-ionic surfactants also (28-29). Another method,although an indirect one,has been used to provide evidence for micelle formation. It is based on the ability of the surfactant solution to dissolve or solubillze water insoluble dyes at the e.a.c. (30,31). This method can be equally well employed in the ease of both aqueous (32) and non-aqueous solutions (33-36) of surfactants. all surface chemists subscribe to the existence of micelles in solution of surface active agents "but opinions differ as to their klnds,shapes and mechanism of formation (36-39) . McBaln considers two kinds of mleeiles,ene a highly conducting,spherioal one revealing Its existence before c.ra.c. while the other, a non-conducting lamellar micelle making its appearance beyond c.m.c.. Contrary to McBain*s views Hartley (40,41) acknowledges only one kind of micelle, namely the spherical one. On the other hand Debye (42) has proposed a eyllnderloal micelle while others (43,44) have Indicated the presence of rod like micelle giving impression of a stack of coins. 5 Light scattering technique Is probably the most powerful tool which can be employed to fathom the mysteries of micelles. It affords the determination of mieellar molecular weight (45-49) , amount of charge on the micelle (50), and dissymmetry (51). This method singly provides such a hoard of information which other methods like diffusion (62-64) , X-ray (56,56) , ultracentrifuge measurements (57) and sedimentation data (58) may not be able to give when used collectively. Recently Sehott (59) calculated the mieellar molecular weight and aggregation number of some ionlo and non-ionic agents by solubilization, fueh effects as the decrease in cm.a. end increase in mieellar size (60,61) on the addition of salts to surfactant solutions can be studied by this technique. Thermodynamics! considerations of Debye (45) based on the free energy of the micelle rather than of the entire system aroused great controversy (62,63). Reich (62) pointed out that Debye*s treatment was not conclusive and could not provide explanation for such simple facts as the format ion of micelles by non-ionic surfactants. Other workers like Philips (64) made simple assumptions that the micelles are mono-dispersed, with all of the micelles having essentially the same size and charge while Hoeve and Benson (65) criticized the simplicity of such statements. Later on, «'a>n:aw& et.,-~. , (6G) \\xt forward | theory which SsWiCmfStl 6 tuch factort as heat of hydration and configurational • entropy terms of the hydrophobic chains in discussing the formation of associated colloids. This theory could give the quantitative prediction for cm.c,mieellar weights,and other properties but could not ressonably explain temperaure effects and clouding phenomenon. ?tainsby and Alexander (67) have calculated the change in heat content and also in entropy at mleelilastion from the temperature dependence of the cm.c. of surfactant. Reeently,the heat of micellisatioi in aqueous solutions of various Ionic detergents has been determined by direct calorimetric measurements of heats of dilution ( 68-71 )