Interactions hemimicelles

Anionic Surfactants. PVP also interacts with anionic detergents, another class of large anions (108). This interaction has generated considerable interest because addition of PVP results in the formation of micelles at lower concentration than the critical micelle concentration (CMC) of the free surfactant the mechanism is described as a "necklace" of hemimicelles along the polymer chain, the hemimicelles being surrounded to some extent with PVP (109). The effective lowering of the CMC increases the surfactant s apparent activity at interfaces. PVP will increase foaming of anionic surfactants for this reason. 

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. 

Organic adsorbates that are more hydrophobic exhibit different adsorption behavior, particularly at higher concentrations. Long-chain fatty acids adsorb to oxide surfaces in part through surface complexation, as shown by electron spin resonance spectroscopy (32). At higher concentrations at the surface, however, favorable interactions between sorbed molecules (hemimicelle formation) appear to dominate and result in greater than monolayer adsorption (40, 41). Because humic substances (like the fatty acids) are amphiphilic, both surface complexation and hydrophobic interactions may be involved in the adsorption of humic substances on oxide surfaces. 

Stability), the area per molecule is determined by the cross-sectional area of the sulphate group, which is in the region of 0.4 mn With nonionic surfactants consisting of an alkyl chain and a poly(ethylene oxide) (PEO) head group, adsorption onto a hydrophobic surface is determined by the hydrophobic interaction between the alkyl chain and the hydrophobic surface. For the vertical orientation of a monolayer of surfactant molecules, the area per molecule will depend on the size of the PEO chain, which is in turn directly related to the number of ethylene oxide (EO) units in the chain. If the area per molecule is smaller than that predicted from the size of the PEO chain, the surfactant molecules may associate on the surface to form bilayers and hemimicelles (as discussed in detail in Chapter 5). This information can be related directly to the stability of the suspension. 

Mineral-surfactant equilibria. Mineral-surfactant equilibria determine adsorption of various surfactants on minerals, hemimicellization, interactions among dissolved mineral species and reagents and electrochemical interactions. The optimum conditions for mineral-surfactant interactions as well as flotation can be calculated from data for such parameters as the solubility product and complex formation constants and species distribution diagrams with the helps of plots of logC-pH, AG°-pH, log/3, iih-pH, etc. 

Using the noncoulombic model, one can investigate the details of surfactant adsorption at the solid-water interface. The van der Waals interactions of the hydrocarbon tails oF the surfactant Ions lead to the abrupt condensation of tbe surfactant on the surface as the concentration of surfactant in solution increases. The formation of such a condensed film (called a hemimicelle) appeals to be neoessaty in tbe development... 

Hemimicelle An aggregate of adsorbed surfactant molecules that may form beyond monolayer coverage, the enhanced adsorption being due to hydrophobic interactions between surfactant tails. Hemimicelles have the form of surface aggregates or of a second adsorption layer with reversed orientation, somewhat like a bimolecular film. 

FIGURE 14.5. In some polymer-surfactant interactions there is evidence for the formation of micelle-like or hemimicelle aggregates of surfactant molecules along the polymer chain—something like a string of pearls. ... 

Despite the role of electrostatic interactions, these results also note the importance of cooperative effects of both the water structure and surfactant hemimicelle formation at the interface (Figure 10). 

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