Admicelle formation

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 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. 

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. ... 

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 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). 

The adsorption of binary mixtures of anionic surfactants in the bilayer region has also been modeled by using just the pure component adsorption isotherms and ideal solution theory to describe the formation of mixed admicelles (3 ). Positive deviation from ideality in the mixed admicelle phase was reported, and the non-ideality was attributed to the planar shape of the admicelle. However, a computational error was made in comparison of the ideal solution theory equations to the experimental data, even though the theoretical equations presented were correct. Thus, the positive deviation from ideal mixed admicelle formation was in error. 

If ideal solution, theory for mixed admicelle formation is assumed (/. = 1.0), six independant equations can be written tor a binary system. The set of equations consists of Equation 3 written for components 1... 

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... 

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

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) ... 

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. 

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..

The adsorption of binary mixtures of anionic surfactants of a homologous series (sodium octyl sulfate and sodium dodecyl sulfate) on alpha aluminum oxide was measured. A thermodynamic model was developed to describe ideal mixed admicelle (adsorbed surfactant bilayer) formation, for concentrations between the critical admicelle concentration and the critical micelle concentration. Specific... 

Except at concentrations near the CAC, the amount of surfactant adsorbed in the Henry s law region is small in comparison to the amount of surfactant present in the admicelles. This implies that nearly all of the adsorbed surfactant molecules are associated on the mineral surface in the form of admicelles. It is important to keep in mind that Equation 8 is valid only between the CAC and the CMC. Above the CMC, equations could be included to account for the formation of micelles, by including monomer-micelle equilibrium equations (1 1). ... 

Salgaonkar, L.P. and Jayaram, R.V., Polyaniline film formation in hexadecyl trimethyl ammonium bromide admicelles on hydrous zirconia surface, J. Colloid Interf. Sci., 291. 92, 2005. 

Boufi ef al. have recently tackled in a systematic fashion the formation of admicelles around cellulose fibres using cationic surfactants [6-10] or fatty-esterified surfaces [11]. The purpose of this strategy is to exploit these admicellar sleeves as hydrophobic reservoirs for water-insoluble organic molecules. The first study of this series [6] made use of the admicelles to adsorb alkenyl monomers, which were then polymerized, thus generating a hydrophobic... 

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