Micelle adsorption

In application-related problems the question may also be formulated in terms of minimizing the necessary additional work. From knowledge of the interfacial properties of surfactant mixtures the surface activity, tendency to form micelles, adsorption, etc., can be increased. The following effects may pertain ... 

The CMC of TRS-10-410 and its NaCl concentration dependence are not known with the precision of that for SDBS, but the CMC is probably somewhat less than 0.5 wt.% (1.2x10 moles/1). From Figure 2, one can observe that the shoulder of the adsorption isotherm occurs near the CMC, and that there is no evidence of micelle adsorption on silica gel. 

There is little or no evidence of micelle adsorption in these systems, and, indeed, it is not to be expected owing to the coulombic repulsion between the negatively charged silica or clay and the micelles. Surfactant anion adsorption at positive (cationic) sites followed by further surfactant adsorption induced by lateral attraction between the hydrocarbon tails to form... 

Adsorption of nonionic surfactants on porous solids has been studied by Huinink et al. in a series of p ers [ 149,150]. They elaborated a thermodynamic approach that accounts for the major features of experimental adsorption isotherms. It is a very well known fact that during the adsorption of nonionic surfactants there is a sharp step in the isotherm. This step is interpreted as a change from monomer adsorption to a regime where micelle adsorption takes place. Different surfactants produce the step in a different concentration range. The step is more or less vertical depending on the adsorbate. The thermodynamic analysis made by Huinink et al. is based on the assumption that the step could be treated as a pseudo first order transition. Their final equation is a Kelvin-like one, which shows that the change in chemical potential of the phase transition is proportional to the curvature constant (Helmholtz curvature energy of the surface). 

Solubilization as a surface reaction This is the major solubilization mechanism for oils that are practically insoluble in water [156,158,160,162-170]. The uptake of such oils cannot happen in the bnlk of the aqueous phase. The solubilization can be realized only at the oil-water interface. The mechanism may include (1) micelle adsorption, (2) uptake of oil, and (3) desorption of the swollen micelles [168-170]. Correspondingly, the theoretical description of the process involves the rate constants of the three consecutive steps. 

This character, called amphiphilic, produces two characteristic sets of behavior, adsorption on the interfaces and auto-association in the form of micelles that extend into the oily surroundings as illustrated in Figure 9.8. 

The examples in the preceding section, of the flotation of lead and copper ores by xanthates, was one in which chemical forces predominated in the adsorption of the collector. Flotation processes have been applied to a number of other minerals that are either ionic in type, such as potassium chloride, or are insoluble oxides such as quartz and iron oxide, or ink pigments [needed to be removed in waste paper processing [92]]. In the case of quartz, surfactants such as alkyl amines are used, and the situation is complicated by micelle formation (see next section), which can also occur in the adsorbed layer [93, 94]. 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

An expression for the number of particles formed during Stage I was developed, assuming micellar entry as the formation mechanism (13), where k is a constant varying from 0.37 to 0.53 depending on the relative rates of radical adsorption in micelles and polymer particles, r is the rate of radical generation, m is the rate of particle growth, is the surface area covered by one surfactant molecule, and S is the total concentration of soap molecules. 

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent ia which it is dissolved to a much greater extent than is expected from its concentration effects. This marked effect is the result of adsorption at the solution s iaterfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation ia the bulk of the solution, and orientation of the surfactant ions or molecules ia the micelles, which are caused by the amphipathic stmcture of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubiUty balance of the molecule. An efficient surfactant is usually relatively iasoluble as iadividual ions or molecules ia the bulk of a solution, eg, 10 to mol/L. 

Excess collector can also reduce the separation by forming micelles in the bulk which adsorb some of the colhgend, thus keeping it from the surface. This effect of the micelles on Ki for the colhgend is given theoretically [Lemhch, Principles of Foam Fractionation, in Periy (ed.). Progress in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1] by Eq. (22-44) [Lemlich (ed.). Adsorptive Bubble Separation Techniques, Academic, New York, 1972] if F, is constant when C, > C-... 

The function of emulsifier in the emulsion polymerization process may be summarized as follows [45] (1) the insolubilized part of the monomer is dispersed and stabilized within the water phase in the form of fine droplets, (2) a part of monomer is taken into the micel structure by solubilization, (3) the forming latex particles are protected from the coagulation by the adsorption of monomer onto the surface of the particles, (4) the emulsifier makes it easier the solubilize the oligomeric chains within the micelles, (5) the emulsifier catalyzes the initiation reaction, and (6) it may act as a transfer agent or retarder leading to chemical binding of emulsifier molecules to the polymer. 

To simplify our discussion, we will consider two specific cases spherical micelles in a selective solvent and selective adsorption on to a solid surface from a selective solvent. 

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

Clearly Fig. 7 must actually have a maximum at high asymmetry since this corresponds to negligible anchor block size and therefore to no adsorption (ct = 0). The lattice theory of Evers et al. predicts this quantitatively [78] and is, on preliminary examination, also able to explain some aspects of these data. From these data, the deviation from power law behavior occurs at a number density of chains where the number of segments in the PVP blocks are insufficient to cover the surface completely, making the idea of a continuous wetting anchor layer untenable. Discontinuous adsorbed layers and surface micelles have been studied theoretically but to date have not been directly observed experimentally [79]. 

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

Because of their preferential use as detergents, the main interest in the physicochemical properties of the salts of a-sulfo fatty acid esters is related to their behavior in aqueous solution and at interfaces. In principle these are surface-active properties of general interest like micelle formation, solubility, and adsorption, and those of interest for special applications like detergency, foaming, and stability in hard water. 

When the CMC determination is made by surface tension measurements, the resulting curve appears without minimum as a single surfactant. It is probable that an inversion takes place through the adsorption of the LSDA onto the surface of the Ca soap micelle, so that complete precipitation does not occur [23]. Zhang and Xiao [32] are of the opinion that the dispersion comes from the union of LSDA with the free ionic soap molecules. The particles from the soap-LSDA mixture are far larger than the corresponding soap molecules in soft water and therefore result in turbidity in hard water. 

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