STUDIES ON THE PROPERTIES & PHYSICO-CHEMICAL BEHAVIOUR OF SOME ANIONIC & CATIONIC SOAPS

Abstract

The name 'surface active agents * has been given to those substances which even in small quantity,alter the conditions prevailing at the interfaces. Different termenologies have been used from time to time to represent these compounds. In Industry the term'syndet' is widely used which is a contraction for 'synthetic detergent1. 'Surfactant' a shorter term for surface active agent, represent an organic molecule or an unformulated compound having surface active properties while'syndet'represents primarily a detergent formulation. Hartley has given the name'amphipathy',which shows by its meaning that the molecule or the ion contains two groups,one that has the sympathy for the phase in which the molecule or the ion is dissolved and the other group which is antipathetic to the medium. In the colloid chemistry these types of groups are known as'lyophilic' and 'lyophobic' respectively and hence these types of compounds are also known as'colloidal electrolytes'/ * McBain called all of them by the name of 'soap'.* The soaps are natural as well as synthetic products. The compounds like glu^egg white,the natural gums etc. . are natural products. Synthetic surface active agents,as we understand,are substances which have been specially • synthesised inorder to obtain the compound which has surface active properties,is a modem development,which has received its most active phase in the period"between two World Wars. Classification. • The synthetic surface active agents are classified as follows: Oil-soluble surfactants:- They are of three types (i) long chain polar compounds,which lower the oil-water interfacial tension and are adsorbed on the polar surfaces; (ii) fluorocarbon compounds,whose polar groups are sufficiently soluble in oil and lower the surface tension of oils;and (iii) silicones,they are generally used as insoluble components of the system and serves antifoeming agents. Water soluble surfactants:- Most of the interest of the workers is centered towards this class of soaps. These are generally classified depending upon the state of the soap in solution. (i) Anionic surfactants:- In which on ionisation the hydrophobic portion contains negative charge. The most important among them are,Twichell's reagents (alkyl-aryl surphates and sulphonates), ester sulphonates,amide sulphonates,aliphatic sulphates etc.. (ii) Cationic surfactants;- In which on ionisaticjn the long * chain contains positive charge. This class generally consists long chain primary,secondary and tertiary amines, quartemary ammonium compounds,alkyl pyridinium chlorides.etc, (iii) Non-ionic surfactants:- They do not Ionise in solution. These are fatty alkanol amides, ethylene oxides 3 derived non-ionic surfactants,sugar esters etc.. • . Civ) Amphoteric or ampholytic surfactants:- They ionise in solution,with a long chain ion carrying either a positive or negative charge,depending upon the pH of the solution. They behave like amino acids. Their surface active properties are pH dependent. They are N-alkyl taurines,amino-sulphonic acids etc.. Amongst the newer ones are Imidazolin derivaties. Uses and industrial applications. Almost all types of soaps have been put to use in industry,technology and allied fields. A very large proportion of the total surfactant production is used in washing textile fabrics. There are three major areas in which soap are used for washing fabrics. These are: (a) in mills where the fabrics are made and finished; (b) in home; and (c) in commercial laundries. The alkane sulphonates (Mersolates) were at one time very widely used in Europe for house hold detergents. The soaps have also been used in cleaning the* *" hard surfaces,generally includes,metal surfaces $glass, ceramic and other non-metallic inorganic surfacesjhard and relatively durable organic surfaces such as paints, • plastics,linoleum; surfaces of senstitive materials,such as food stuffs,which are easily penetrated or injured. In textile and dry cleaning industries,they are used for several textile processing operations such as dyeing,wetting,dispersing,emulsification and other similar gross effects(l). The use of soaps for various non- • 9 detersive applications in the field of medicine and hygine has increased appreciably within recent years. Anionic surfactants such as lauryl sulphate and non Ionic surfactants such as the Tweens and Spans have been widely adopted for preparing skin lotions in which the effective medicament is suspended or emulsified. One of the most interesting applications of soaps in medicine is the use of an aerosol spray of sodium lauryl sulphate solution as an inhalant for children suffering with severe bronchial congestion. This treatment appears to loosen the excessive mucous secretion and to provide moisture more effectively than stream inhalations(2). The cationic germicides such as benzalkonium chloride can be incorporated into ointments based on methyl cellulose,starch polyglycol,gelatin,pectin or Tween emulsions but their activity is reduced or destroyed when anionic emulsifiers or bases are used(3). Surfactants are used internally as ingredients in many different dispersed or emulsified medicaments in whic"h *"~ the surfactant itself play no important biological role but merely serves as a carrier for the active drug. Alkylaromdtic sulphonates have been used as carriers for* phenothiazine in anthelmintic preparations(4). Penicillin is itself a surface active,and has a relatively high critical micelle concentration,and its surface activity is negligible at the extremely low concentrations where it is still biologically active(5). * . The fields of cosmetic technology which have derived most benefit from the recent advances in surfactants include: hand and skin cleansers and bath preparations; emulsion products,lotions, and creams;dentifrices;shaving preparations;and shampoos. In metal and mineral technology,surfactants are used in flotation,electroplating and surface finishing of metals. The alkyl aryl sulphonates,including the long chain alkyl benzene sulphonates and the short chain alkyl napthalene sulphonates,as well as the fatty alcohol sulphates, have become more or less standard antipitting additives for nickel plating baths (6). For bright copper plates thiourea in combination with napthalene disulphonic acid have been used(7). Bright alloys of copper-tin or coppertin- zinc, i.e. bronze or brass plates,have been obtained by using the ampholytic surfactant,dodecyl trimethyl bentaine in an alkaline cyanide(8,9). In building and construction industries surfactants"" are being consumed on a large scale. Specific areas in which surfactants are widely used include the preparation and use of asphalt bonding materials,concrete,plaster board, * and earth,clay,and ceramic products. Soaps and sulphonated anionic surfactants have been used as retarders in the preparation of Plaster of Paris(10). In the construction of dirt roads,aircraft runways,floors,and other load-bearing earth surfaces it is desirable to render the soil stable and compact. Cationic soaps are very effective stabilizers for certain types of soil and soil-aggregate(ll). Fatty soaps and rosin soaps,folowed by an alun treatment,have been used to increase the water resistance of dirt roads. Hydrophobic and hydrophillc surface active substances have been shown to be effective in increasing the penetration of Portland cement suspensions into sand(l2). The use of soaps in the manufacture of fertilizers was first announced by Seymour(l3). The major beneficial result from the soap is the improvement of the physical character of the fertilizer by reducing the hardness of the fertilizer,and by increasing the resistance to caking. The alkyl sulphonates,such as Santomerse,Oronite Detergent, and Kreelon are used commercially. The acid-stable nonionics ( alcohol or mercaptan-ethylene oxide condensates, alkylphenyl polyethylene glycol ether,or alkylphenoxy polyoxyphenyl ethanol) are the most effective surfactants for the treatment of fertilizers(14). In insecticides, fungicides and herbicides the demand of soaps has " **"" appreciably Increased. In the industries of leather,fur, paper, synthetic filaments and films, synthetic rubers, polymers,plastics, paints,petroleum and chemical-processing industries etc. the soaps have wide applications. Besides the aforesaid applications soaps have used in fire fighting and fire preventation recently. One* of the well established methods for fighting fires is to lay down over the smouldering material a thick blanket of aqueous foam. This blanket serves to prevent reignition of combustible material, even though it rarely has sufficient cooling power to extinguish a vigorous flame. Properties of soaps. As has already been stated that the soap molecule contains hydrophobic and hydrophilic groups in a single molecule. When such a substance is put into water,the water molecules will try to repel the hydrophobic group and simultaneously has a loving tendency towards the hydrophilic portion. The least energy of water will be wasted in pushing away the hydrophobic portion if the system exhibit the smallest possible area of contact between the water molecules and the non-polar hydrocarbon chain of the soap molecule. There is such minimum contact when soap molecules go to the surface and orient with their polar group directed towards the water. This tendency of the soap molecules decreases the surface tension and the interfacial tension. This is what actually happens in the dilute solutions but in the concentrated solutions all the hydrophobic portion cannot remain at the surface. In that case the hydrophobic portion starts to aggregate • and these aggregates are surrounded and hidden by polar groups. This aggregation of the soap molecules is called a micelle. A number of investigators like McBain(l5), Hartley(16),Wright(17) and Tartar(l8) etc., have all agreed on the presence of aggregates of the soap .solutions. These aggregates or micelles do not form at any • arbitary concentration but they start forming only after passing an optimum concentration. The concentration at which micelle begins to form is known as critical micelle concentration (cm. c.). The c.m.c. is not a specific concentration value, but it is a concentration range in which the dispersed soap molecules changes to an equili brium between molecules ( or ions) and aggregates. When practically any physical property of an aqueous soap solution is considered, it exhibits more or less abrupt change in the magnitude over a narrow concentration range ( c.m.c). This abruptness in the physical property has been utilized in the determination of the c.m.c. The values of the c.m.c. derived from various physico-chemical properties may not necessarily be identical since they will depend on the abruptness of the change of the property. Any method that measures the deviation from ideal behaviour can be used to determine the c.m.c. of a surfactant. A number of physical methods,viz., conductivitjp— (19-23), surface tension(24) freezing point(25),osmotic pressure(26), vapour pressure(27),solubility(28),viscosity (29), etc, have been employed from time to time by workers. Because of its simplicity the spectral-change method involving the colour change of the dyes has been recently used for estimating the c.m.c of soap solutions. This method has been critically studied by Mukerjee and Mysels(30). Tracer technique(3l) and electrophoretic mobility(32) • methods have also been employed in investigating this property. An indirect evidence of micelle formation is the phenomenon of solubilization. When such substances which are not very soluble in water,dissolve to an appreciable extent in the micellar soap solution,the phenomenon is known as solubilization. Direct evidence that solubilized materials are present within the micelles is provided by X-ray measurements(33).McBain (34) established that for solubilized systems,the components are in thermodynamically stable equilibrium. In their monograph on "Solubilization",McBain and Hutchinson(35) treat solubili zation as a distribution phenomenon between two phases: One is aqueous phase and the other is micellar phase. This phenomena Is discussed later. Soap micelles in non-aqueous. The oil soluble soaps forms their micelles in hydrocarbon solvents(36-43).(Micelles formed in non-polar solvents are considered that the polar groups are directed towards the interior of the micelles). These micelles are capable to solubilize water. Matton and Mathew (39) found that pure sodium bis(2-ethylhexyl) sulphosuccinate formed spherical micelles in n-dodecanc Kaufman and Singleterry( 41,43) investigated micelle • formation by a number of salts of dinonyl naphthalene sulphonate. Oil soluble non-ionic surfactants and 10 . amine-fatty acid soaps,form micelles at concentrations -2 • of about 10 • molar and above and the size of the micelles varies substantially with concentration(44-47). Shape of the micelle. There is a general agreement on the presence of micelles in soap solution but a controversy still exist about their shape. McBain(48) was of the opinion that in aqueous soap solutions,two types of aggregates which are very dissimilar in structure,are in equilibrium with each other. One ( the ionic micelle) is highly charged and spherical and the other (the lamellar micelle) has a small charge. Hartley(49) on the other hand,postulated only one type of micelle,spherical, essentially liquid in structure and is large compared with McBain's ionic micelle and contains in addition to the amphipathic Ions a considerable number of counter ions. Debye(50) has proposed a cylindrical micelle with curved ends which are approximately Hartley spheres. An ellipsoid of rotation was proposed by Hughes (51) aifd ^. Halsey has suggested that the micelle is rod like(52). Viscosity(53,54) and birefringence (55) measurements suggest the spherical micelles in* 5% soap concentration but beyond this concentration upto 20$ the micelles are asymmetric or rod like and at still higher concentrations the micelles are microcrystallites of the lamellar kind(56)