Surfactant head-group

Examples of common hydrophobes include alkylphenol, fatty alcohol, paraffin, olefins, and alkylbenzene. Examples of common hydrophiles include ethylene oxide (EO), sulfates, sulfonates, phosphates, and carboxylic acid groups. Depending on the charged nature of the head group, surfactants are classified as ... 

Surfactants are most commonly classified according to their polar head group. Surfactants, according to the composition of their head, are classified into following four groups ... 

Figure 19.18. In the absence of strong specific interactions between a hydrophilic surface and the surfactant head-groups, surfactant molecules will self-assemble into discrete micelles at the surface...

Falcone et al. used the anionic sodium l,4-bis(2-ethylhexyl) sulfosuccinate (AOT) and the cationic surfactant, benzyl-n-hexadecyldimethylammonium chloride (BHDC) to prepare IL microemulsions [48]. The ILs chosen were l-butyl-3-methylimidazolium trifluoromethanesulfonate ([C mim][CF3S03]) and l-butyl-3-methylimidazolium trifluoroacetate ([C mim][CF3COJ see Scheme 13.2). DLS experiments reveal the formation of microemulsions. Besides, the FTIR results suggest that the ionic interactions (with the surfactant polar head groups, surfactant counterions, or IL counterions) are substantially modified upon confinement. These interactions produce segregation of IL s ions, altering the composition of the microemulsion interfaces. 

Head group (surfactant). A term referring to the portion of a surfactant molecule that imparts solubility to the molecule. Generally used in the context of water solubility. 

Soap is one example of a broader class of materials known as surface-active agents, or surfactants (qv). Surfactant molecules contain both a hydrophilic or water-liking portion and a separate hydrophobic or water-repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the aUphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air—water interfaces. It is this special chemical stmcture that leads to the abiUty of surfactants to clean dirt and oil from surfaces and produce lather. 

Within a series with a fixed hydrophilic head group, detergency increases with increasing carbon chain length, reaches a maximum, and then decreases. This behavior frequentiy reflects a balance between increased surface activity of the monomer and decreased monomer concentration with increased surface activity. Similar effects are seen in surfactants in biological systems. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

Larger aggregates seldom have spherical geometry, but tend to form cylindrical micelles. In this case, the diameter of the cylinders can usually be adjusted such that the head groups can cover their optimal head group area Uq, and the interaction free energy per surfactant reduces to the constant The size distribution for cylindrical micelles is then exponential in the limit of large N,... 

The interfacial activity is determined by the sterical properties of the molecule. At the interface the spatial demand A0 of the hydrophobic part of the molecule is higher because of the second chain of the internal sulfonate compared with the terminal sulfonate. Thus, the surface concentration of the surfactant molecules is lower. That means that the hydrocarbon chains are laterally oriented and therefore cover the interface between the solution surface and air more completely. Because the ratio of the spatial demand of the head group to the volume of the alkyl chain governs the radius of the micellar surface, it... 

The Stauff-Klevens equation is valid for the critical micelle concentration of homologous surfactants with the same ionic head group ... 

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