Sodium hexadecyl sulphate

Effect of hexadecane as additive In a series of papers Hallworth and Carless (7,8,9,TO) have investigated the effect of the nature oT the internal phase on the stability of oil in water emulsions as well as the effect of addition of long chain fatty alcohols with sodium dodecyl sulphate or sodium hexadecyl sulphate as the ionic emulsifier. They found that light petroleum and chlorobenzene emulsions prepared only with sodium hexadecyl sulphate were much less stable than those produced using the longer chain paraffins, white spirit and light liquid paraffins. 

Hallworth and Carless (1 ) discuss several possibilities for the effect of light liquid paraffin on the stability of emulsions with light petroleum or chlorobenzene as the main components. They seem to prefer an explanation previously advanced by them and several other authors for the effect of fatty alcohol, namely that the increased stability is due to the formation of an interfacial complex between the additive and sodium hexadecyl sulphate. The condenced mixed film will resist coalescence primarily by virtue of its rheological properties. With mixed films of the present type, the importance of the film viscoelasticity lies in its ability to maintain electrical repulsion between approaching droplets by preventing lateral displacement of the adsorbed ions. The effective paraffinic oil has chains at least as long as those of the alkyl sulphate and will be associated by van der Waals forces with the hydrocarbon chain of the alkyl sulphate at the interface. 

Adsorbent materials are used medicinally to absorb toxins in vivo. The enterotoxin of Pseudomonas aeruginosa is implicated in the aetiology of diarrhoea adsorbents such as kaolin are used to decrease the toxicity of such agents by removal of the toxin by adsorption. Armstrong and Clarke have investigated the adsorption sites on kaolin by measurement of electrophoretic mobility in the presence of surfactants and other materials as a function of pH [58]. In particular they studied the uptake of gentian violet on to koalin treated with anionic and cationic agents. Kaolin pretreated with cetrimide showed a marked decrease in adsorptive capacity. On the other hand, when pretreated with sodium hexadecyl sulphate the adsorptive capacity was increased (Fig. 9.22). 

Figure 9.22 Variation with pH of the uptake of a 0.05 % (w/v) solution of Gentian Violet on natural kaolin and on kaolin treated with anionic and cationic materials. Kaolin, pH 7.4 ( >) and 5.7 ( ) washed kaolin, pH 7.4 (V) and 5.4 ( ) kaolin plus sodium hexadecyl sulphate, pH 7.5 (O) and 5.6 ( ) and kaolin plus cetrimide, pH 7.6 (A) and 5.4 (A). From Armstrong and Clarke [58] with permission.

Much of the early work on amphiphiles was undertaken on soaps and lipids based on fatty acids, and the corresponding non-systematic chemical names of these parent compounds and their derivatives are still commonly encountered. For convenience, Table 4.1 lists the systematic and trivial names of fatty acids, along with their structures. The names of derivatives are based on these for example sodium dodecyl sulphate is (still ) sometimes referred to as sodium lauryl sulphate. Other non-systematic names also exist to cause further confusion For example, hexadecyl (Cie chain) compounds are often termed cetyl derivatives. The use of the term fatty here and elsewhere is used to indicate an alkyl chain with 12 or more carbon atoms, i.e. a hydrocarbon that forms fats. 

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