Anionic surfactant molecules

The clustering of low molecular weight anionic surfactant molecules to form micelles can also be represented by the following reaction ... 

One type of interaction that can occur in solution is chemical in nature, as that between a positively charged molecule, such as the herbicide paraquat, and negatively charged molecule, such as the anionic surfactants. However, work at Davis (57) has shown that not all such interactions are detrimental to the effectiveness of the herbicide. It appears that even though the chemical interaction has occurred, it is not in itself sufficient to prevent the toxicity of paraquat. This seems to depend on the structural nature of the surfactant molecule and the concentrations of the surfactants and herbicides used. Table I shows that paraquat-14C in solution with surfactants of the anionic type is adsorbed or oriented toward the surface layers of the mixture. Presumably this type of result indicates chemical ionic adsorption of paraquat-14C into the multilayered or monolayered solution—air interface of anionic surfactant molecules. 

In flg. 3. lb the preference of the hydrophobic tails of the (anionic) surfactant molecules for the oil phase gives rise to the double layer. Such double layers are for instance encountered in some emulsions. They may also occur at the air-water interface then the driving force for their formation is the expulsion of the hydrocarbon tails from the aqueous phase. We speeik of ionized monolayers and return to them in Volume III. 

Ad tion of a surfactant wiA a similar charge to that of the particle can also be used as an aid to stabilization [46]. Since even negatively charged particles have some hydrophobic surface regions the evidence suggests that these can adsorb some anionic surfactant molecules [60]. 

Alternatively, retention can be predicted from solute log Po/w values. The most suitable organic solvent to be used as modifier of the mobile phase should be chosen according to the polarity of the eluted compound. For SDS, a low propanol content ( 1%, v/v) is useful to separate compounds with logPo/w< — 1> such as amino acids. A larger amount of propanol ( 5-7%) is needed for compounds in the range -1 < log Po/w < 2, such as diuretics and sulfonamides. Other alcohols (<10% butanol or <6% pentanol) are required for apolar compounds with log Po/w >3, such as steroids. This rule of thumb is, however, not always valid propanol is too weak for cationic solutes, such as phenethylamines (0 < log Po/w < 1-7) or j -blockers (1 electrostatic attraction to the anionic surfactant molecules adsorbed on the stationary phase makes a stronger solvent necessary. 

In the conditioning process, under suitable alkaline conditions, both ionization of functional groups at the bitumen surface [33, 105] and adsorption of the natural anionic surfactant molecules at the bitumen/ aqueous interface [100,101,104] occur. Descriptions of the experimental techniques, including microelectrophoresis, employed to study the effects are given elsewhere [100,102,104,106]. Figure 14 shows how addition of NaOH in the process increases the concentrations of surfactant in the aqueous phase, which in turn increases the extents of surfactant adsorption at all of the aqueous phase interfaces present in the system gas/ aqueous, bitumen/aqueous, and solid/aqueous. The adsorption increases until monolayer coverage is achieved and thereafter either levels off or continues into multilayer adsorption. 

The adsorption of anionic surfactant molecules directly affects the electrophoretic mobilities of dispersed bitumen droplets, gas bubbles, and fine solid particles. These electrophoretic mobilities are directly linked to the Zeta potentials at the surfaces and therefore to the surface electric charges on the drops, particles or bubbles. Reference [100] shows how to convert the mobilities into Zeta potentials or surface charges. Although, as will be seen later, the shapes of the processibility curves can vary considerably, the various surface charges are always quite negative... 

Fig. 2.12 Molecular structure of a typical soap film, containing anionic surfactant molecules plus metal ions and water molecules.

In the presence of 286 ppm CaCOj the onset of the adsorption isotherm starts at much lower equilibrium concentrations, reaching a fivefold higher plateau value (Fig. 6). As a result, calcium ions improve the adsorption of anionic surfactant molecules on the kaolin surface. However, under these circumstances the precipitation of Ca-surfactant complexes is noticeable. The adsorption... 

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