Admicelles

Figure C2.3.16. Adsorbed micelle stmctures (a) bilayer admicelle (b) spheroidal, globular adsorbed micelle.

Admicelle formation and associated CAC (Critical admicelle concentration) as proposed by Scamehom (10) and Harwell (1 1) were not introduced here for a practical reason a feasible and fast method of CAC measurement does not seem to exist at the moment. The difficulties related to such delicate determinations appear well from observation of the detailed adsorption isotherms of pure sulfates mixtures published by Roberts et alii (10). 

Adhesive force, non-Brownian particles, 549 Admicelle formation, 277 Adsorption flow rate, 514 mechanism, 646-647 on reservoir rocks, 224 patterns, on kaolinite, 231 process, kinetics, 487 reactions, nonporous surfaces, 646 surface area of sand, 251 surfactant on porous media, 510 Adsorption-desorption equilibria, dynamic, 279-239 Adsorption plateau, calcium concentration, 229... 

When sur-f actants adsorb on metal oxide sur-f aces (e.g., minerals), at low concentrations, the adsorbate molecules are widely dispersed enough that no signi-ficant interactions between adsorbed sur-f actants occurs. Above a certain critical concentration, dense sur-factant aggregates form on the surface (72). These are called admicelles. For ionic surfactants, the admicelles are bilayered structures (72). Above the CMC, the total adsorption of surfactant can increase or decrease slowly. 

When two similarly structured anionic surfactants adsorb on minerals, the mixed admicelle approximately obeys ideal solution theory (jUL - Below the CMC, the total adsorption at any total surfactant concentration is intermediate between the pure component adsorption levels. Adsorption of each surfactant component in these systems can be easily predicted from pure component adsorption isotherms by combining ideal solution theory with an empirical correspond ng states theory approach (Z3). ... 

When an ionic/nonionic surfactant mixture adsorbs on a metal oxide surface, the admicelle exhibits negative deviation from ideality (74). This means that the adsorption level is higher than it would be if the admicelle were ideal, at a specific surfactant concentration below the CMC. Above the CMC, the adsorption level is dictated by the relative enhancement of micelle formation vs. admicelle formation. In this region, the level of adsorption can be viewed as the result of the competition between micelles and admicelles for surfactant. In analogy, the surface tension above the CMC can be viewed as competition between the monolayer and micelles for surfactant. 

The relative tendency for surfactants or surfactant mixtures to form a micelle compared to a monolayer is approximately the same. However, the relative tendency to form an admicelle can be substantially different from that to form micelles or monolayers. This is because there can be specific interactions between the solid surface and the surfactants as well as intersurfactant interactions in the aggregate. The surfactant technologist can take advantage of this to design... 

The thermodynamics of mixing upon formation of the bilayered surface aggregates (admicelles) was studied as well as that associated with mixed micelle formation for the system. Ideal solution theory was obeyed upon formation of mixed micelles, but positive deviation from ideal solution theory was found at all mixture... 

Surface aggregates formed by ionic surfactant adsorption on oppositely charge surfaces have been shown to be bi layered structures (1.) and are called admicelles<2) in this paper, though they are sometimes referred to as hemimicelles. The concentration at which admicelles first form on the most energetic surface patch is called the Critical Admicellar Concentration (CAC) in analogy to the Critical Micelle Concentration (CMC), where micelles are first formed. Again, in much of the literature, the CAC is referred to as the Hemimicellar Concentration (HMC). 

The thermodynamics of formation of admicelles composed of two or more surfactants is the focus of this... 

Scamehorn et. al. (20) also presented a simple, semi—empirical method based on ideal solution theory and the concept of reduced adsorption isotherms to predict the mixed adsorption isotherm and admicellar composition from the pure component isotherms. In this work, we present a more general theory, based only on ideal solution theory, and present detailed mixed system data for a binary mixed surfactant system (two members of a homologous series) and use it to test this model. The thermodynamics of admicelle formation is also compared to that of micelle formation for this same system. 

Consider the pure surfactant adsorption isotherms shown in Figure 1. At concentrations between the CAC and the CMC, there is a unique concentration level correspond ng to each adsorption level for each pure component. Since this concentration corresponds to formation of admicelles on specific patchs on the surface, for component i, we call the concentration CACi" the variable CAC (no superscript) will be reserved to refer to the concentration which corresponds to admicelle formation on the most energetic patch on the surface. We will only consider binary mixtures of surfactants, so the subscript i can refer to either component A or B. For a surfactant mixture, the total surfactant concentration required to reach a specified adsorption level is defined as CACm. ... 

The mixed admicelle is very analogous to mixed micelles, the thermodynamics of formation of which has been widely studied. If the surfactant mixing in the micelle can be described by ideal solution theory, the Critical Micelle Concentration (CMC) or minimum concentration at which micelles first form can be described by (21) ... 

At a speci-fic adsorption level, we can view the sur-f actant monomers as being in equilibrium with admicelles on speci-fic sur-face patches, just as the monomer is in equilibrium with the micelles at a monomeric concentration o-f the CMC. There-fore, CAC is... 

Mixed Admicelles. The total sur-factant adsorption o-f the two pure sur-factants and mixtures thereo-f on alumina are shown in Figure 3. The mixtures are at constant surFactant ratio in the Feed or initial solution, but not necessarily in the Final equilibrium solution. The concentration on the abscissa is the equilibrium concentration. The individual surFactant adsorption isotherms For the pure surFactants and in the mixtures are shown in Figures 4 and 5. The experiments were run at the same swamping electrolyte concentration as were the CMC data. 

From Figures 3 and 6—9, the predicted total adsorptions For surFactant mixtures are higher than observed values. ThereFore, the mixed admicelles showed positive deviation From ideality at all compositions. This remarkable behavior has not been observed beFore because data oF the accuracy and range reported here has not (to our knowledge) previously been reported. Observation oF the expected ideal behavior For the CMC data indicate that this is probably not due to a peculiarity oF the surFactants used. 

The explanation For this eFFect lies in the steric constraints oF the two-dimensional admicelle. In the admicelle, the thickness oF the hydrophobic portion oF the admicelle is probably dictated by the length oF the longest alkyl chain present (i.e., the dodecyl chain). In the mixed admicelle, the decyl chain is almost completely surrounded by other alkyl chains and so is exposed to approximately the same environment as in a pure CioS0 admicelle. However, the two additional methylene groups on the dodecyl chain are not in as hydrophobic an environment as in the pure Ci SO admicelle, since neighboring decyl chains cannot interact... 

Large negative deviations from ideality are well known when mixed micelles are formed between ionic and nonionic surfactants (11—15.21.24) Negative deviations from ideality have been reported for mixed ionic/nonionic admicelle formation (26), although the degree of nonideality was not quantified. Since this work has pointed out the similarities and differences between mixed micelles and admicelles, the study of these systems should elucidate this relationship even further and will be the subject of future publications. 

The formation of mixed aggregates on solid surfaces (admicelles) has only in the last decade been addressed in systematic fashion. The surface has only been scratched in this important topic. 

The same thermodynamic quantities needed for mixed micelle formation (already discussed) are also needed for mixed admicelle formation. Luckily, the monomer-admicelle equilibrium data can be fairly easily and unambiguously obtained (e.g., see Chapter 15). This should be combined with calorimetric data for a more complete thermodynamic picture of the mixed admicelle. As with micelles, counterion bindings on mixed admicelles also need to be obtained in order to account for electrostatic forces properly. Only one study has measured counterion binding on single-component admicelles (3 .), with none reported for mixed admicelles. 

In the real world, soluble organic materials may be present in the aqueous surfactant system. As with micelles, it is important to know how these affect mixed admicelle formation and how well these organics are solubilized in the admicelle (adsolubi1ized). 

Admicelles—Continued definition, 201 formation on heterogeneous surfaces, 332... 

Figure 17.4 shows a surfactant sorption isotherm from low to high (>CMC) concentrations of the surfactant. It can be divided into three parts (Figure 17.5). In Region 1, individual surfactant molecules are in equilibrium with the surfactant molecules adsorbed to the solid sorbent. In Region 2, the surfactant concentration in the water has exceeded the CMC. That is equivalent to saturation of the air/water interface with surfactant molecules. Subsequent addition of surfactant molecules leads to increased sorption due to formation of sorbed surfactant aggregates (Region 2). In Region 3, the aggregates in solution (micelles) are in equilibrium with the sorbed aggregates, the so-called admicelles..

Although the partitioning of HOCs to surfactant micelles has been well studied, HOC partitioning to sorbed surfactants (e.g., hemimicelles, admicelles) has received much less attention. Holsen et al. (1991) examined the sorption of several HOCs on ferrihydrite coated with sodium dodecyl sulfate (SDS) and found that the HOCs with the lowest water solubility... 

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