Hemimicellization, characteristic

The adsorption isotherm obtained for dodecylsulfonate (CDS) on alumina is given in Figure 2. This isotherm is similar to that obtained in the past for sulfonate/alumina systems (4). This isotherm behaves in an s-shaped manner (6) revealing its four characteristic regions of adsorption 1) Electrostatic interaction 2) Lateral association (hemimicellization), 3) Electrostatic hindrance and 4) micellization. 

It is sometimes found convenient to designate two subclasses of liquid films—liquid-expanded (Li) and liquid condensed (L2)—based on subtle differences in respective ti-A curves. The Li curve is one which typically extrapolates to a limiting value of is (sometimes zero) at a molecular area of about 0.5 nm. In contrast to the bulk liquid analog, such films exhibit a significant degree of compressibility but show no signs of island or hemimicelle formation that is, it appears to maintain the characteristics of a uniform phase. In many cases, Li films show a transition to a gaseous film at low pressures and perhaps to an L2 film as the available area per molecule is decreased. 

In dilute solutions, oxyethylated alcohols are present as monomers whose characteristic structural components are a hydrophilic part (ethylene oxide chain) and a hydrophobic part (aliphatic chain). With an increase in concentration, the monomers undergo spontaneous self-aggregation, forming micelles in the surface phase (hemimicelles) at lower concentrations, while at higher concentrations forming micelles in the bulk phase. Figure 17.4 shows a schematic example of a spherical micelle. 

In 1956, Fuerstenau coined the term hemimicelle to describe the two-dimensional aggregates that he proposed were responsible for the rise through several orders of magnitude in the observed absorption of surfactant molecules on certain mineral oxide surfaces from aqueous solutions. The study on coadsorption of pinacyanol chloride (cyanine dye) with sodium p-(l-propylnonyl)-benzenesulfonate (dissolved in water) on AI2O3 surface reveals that surfactant adsorption is independent of the concentration of pinacyanol chloride, and the surfactant solution remains clear at total surfactant concentration ([SurfJx) below CMC, whereas the solid surface becomes dyed blue, the color characteristic of... 

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