Common Surfactant Hydrophobic Groups

Natural Fatty Acids. Obtained primarily from the hydrolysis of triglycerides such as animal and vegetable fats and oils, the most useful members of the group have from 12 to 18 carbon atoms. The most commonly employed members of the family have hydrocarbon chains that are fully saturated, although some unsaturated examples are employed (especially oleic acid). Because most vegetable oils contain high proportions of unsaturated fatty adds, they usually require extensive processing in order to separate the useful 

CHoCHCH.O(CH CH ) X CH3 Polyoxypropylene (n = degree of oligomerization, X = oligomerization initiator)  

FIGURE 3.4. The most c mmonly encountered hydrophobic materiak used in the commercial manufacture of surfactants. 

Paraffins. These hydrocarbon materials are obtained from petroleum distillates boiling higher than gasoline. They are generally saturated materials with 

10-20 carbon atoms. The mixture will normally contain branched isomers, some cyclic materials, and aromatic groups (benzene, toluene, naphthalene, etc.). Since saturated hydrocarbon materials in this family are relatively unre-active, their conversion to surfactants usually involves a preliminary reaction such as chlorination that, by its somewhat random nature, results in the production of a more complex mixture of isomers. The production of a relatively pure product requires further purification and fractionation, increasing the cost of such materials. Paraffin-based materials are therefore commonly encountered as complex mixtures of isomers and homologues. 

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Amphiphilic tertiary phosphines have their phosphorus donor atom located somewhere in the hydrophobic part of the molecule and should have at least one long alkyl or alkyl-aryl chain carrying a polar head group (Scheme 4. 10). Some of them, such as the sulfonated derivatives, are quite well soluble in water, others, such as Ph2P(CH) COOH (n = 3, 5, 7, 9, 11) are practically insoluble, however, can be easily solubilized with common surfactants (SDS, CTAB etc.). 

Some more modern semi-synthetic metal-working oils are actually O/W microemulsions [193], Such microemulsions may switch readily to O/W macroemulsions when diluted with water at the time of application. Once applied, the surfactants need to adsorb onto metal surfaces with their hydrophobic groups oriented away from the surfaces in order to reduce friction and ensure wetting of the metal by hydrocarbons present in the metal-working liquid or emulsion. Rosen and Daha-nayake [193] list the commonly used surfactants for this application. 

The prerequisites to be fulfilled for a structural preservation of a preformed LLC assembly are as follows (i) the sohdification has to be irreversible, (ii) the resulting sohd product should not compete with the surfactant head groups for water, as this would result in substantial changes of the composition with the consequence of phase changes, (iii) no macroscopic demixing must occur, which indicates that the sohd substance has to be compatible either with the hydrophobic, or (more commonly) with the hydrophilic domains of the phase, (iv) the presence of reactants or the release of by-products should not affect the surfactant phase structure, and (v) the synthesis has to occur at moderate temperature or at least far below the boiling point of the least volatile component (usually water). 

The most common method of stabilizing latex particles is by addition of mixed anionic and nonionic surfactants. The key property of a siufactant is that it possesses chemically dissimilar groups one hydrophobic and one hydrophilic. The hydrophobic group is physically adsorbed onto the polymer latex particle while the hydrophilic portion extends into the aqueous phase. It is the hydrophilic groups that provide the stabilization. 

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