Sodium Long-chain surfactants

The molecular structures of the two Sodium Long-chain AlkylBen-zeneSulphonate surfactants (SLABS) are given in Fig. 19.1. 

Carboxylates (9.4, where R is the long-chain hydrophobe and X the cation) represent the oldest type of surfactants, since they could be obtained from naturally occurring fats and oils long before the advent of the petrochemical industry sodium heptadecanoate (9.5), for example, incorporates the cetyl group as hydrophobe. Sodium stearate, sodium palmitate and sodium oleate are the simplest carboxylates generally used as surfactants. Alkylaryl compounds (9.6) are also known. 

Stabilization in water (surface tension 72 mN/m) can be achieved through the use of surfactants [77] such as SDBS [78] or sodium cholate [79], or long chain polymers such as polyvinyl alcohol. While this allows a non-toxic and easily removed solvent to be used, the presence of the surfactant or polymer molecules can be detrimental to the subsequent applications [80]. 

In a previous publication ( ), results were presented on the micellar properties of binary mixtures of surfactant solutions consisting of alkyldimethylamine oxide (C12 to Cig alkyl chains) and sodium dodecyl sulfate. It was reported that upon mixing, striking alteration in physical properties was observed, most notably in the viscosity, surface tension, and bulk pH values. These changes were attributed to 1) formation of elongated structures, 2) protonation of amine oxide molecules, and 3) adsorption of hydronium ions on the mixed micelle surface. In addition, possible solubilisation of a less soluble 1 1 complex, form between the protonated amine oxide and the long chain sulfate was also considered. 

A surfactant molecule is an amphiphile, which means it has a hydrophilic (water-soluble) moiety and a hydrophobic (water-insoluble) moiety separable by a mathematical surface. The hydrophobic tails of the most common surfactants are hydrocarbons. Fluorocarbon and perfluorocarbon tails are, however, not unusual. Because of the hydrophobic tail, a surfactant resists forming a molecular solution in water. The molecules will tend to migrate to any water-vapor interface available or, at sufficiently high concentration, the surfactant molecules will spontaneously aggregate into association colloids, i.e., into micelles or liquid crystals. Because of the hydrophilic head, a surfactant (with a hydrocarbon tail) will behave similarly when placed in oil or when put in solution with oil and water mixtures. Some common surfactants are sodium or potassium salts of long-chained fatty acids (soaps), sodium ethyl sulfates and sulfonates (detergents), alkyl polyethoxy alcohols, alkyl ammonium halides, and lecithins or phospholipids. 

Since phosphates and sulfates with long chain alkyl substituents form micelles at concentrations above their CMC, the hydrolysis of these esters can be subject to micellar catalysis thereby providing a simplified system in which micelle formation and structure are not alfected by the presence of a foreign solubilizate. The hydrolysis of such surfactants must be considered, however, in investigations of their effects on reaction rates. Fortunately, the rate constants for the neutral hydrolysis of esters such as sodium dodecyl sulfate are extremely slow at 90° = 296 days at pH = 8-63), and the acid-catalyzed hydrolysis of the same ester is some three orders of magnitude faster and thus is still negligible in most cases (Kurz, 1962). 

It can be seen that in each of the three types of surfactants there is a chemical grouping, which would be considered nonpolar in nature. This is usually a long-chain fatty acid (saturated hydrocarbon). The polar end of the molecule may be the sodium salt of an acid, the chloride salt of a quaternary nitrogen, or the free hydroxy group of a glycol ether. 

Oil-in-water creams, for topical use, generally contain mixed emulsifiers/surfactants one of which is a water soluble surfactant with a high HLB, the other being an amphiphile, usually a long chain fatty alcohol (e.g., of chain length C14 to Cig) or acid (e.g., palmitic or stearic). The water soluble surfactant may be anionic (e.g., sodium lauryl sulphate), cationic (e.g., cetri-mide), or non-ionic (e.g., cetomacrogol. Tweens). 

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