Micelles ideal

The idealized reverse micelle sketched in figure C2.3.1 is an aggregate of a double-tail surfactant. In such systems the solvent is more compatible with the lyophobic part of the surfactant than with the headgroup. This preference... 

As has probably become obvious already, the study of micelles has been one of the big topics in simulations of systems with surfactants. We have cited many of the related publications in the previous sections. Here, we shall discuss some special aspects of micelle simulations in order to illustrate the use of idealized chain models for this type of problem. 

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

When the variation of any colligative property of a surfactant in aqueous solution is examined, two types of behavior are apparent. At low concentrations, properties approximate those to be expected from ideal behavior. However, at a concentration value that is characteristic for a given surfactant system (critical micelle concentration, CMC), an abrupt deviation from such behavior is observed. At concentrations above the CMC, molecular aggregates called micelles are formed. By increasing the concentration of the surfactant, depending on the chemical and physical nature of the molecule, structural changes to a more... 

Solubility and dissolution are processes that take place in the gastric and the luminal fluids, not on the surface of epithelial cells. Measurement of solubility ideally needs to take place at pH 1.7 (stomach) and pH 5-8 (small intestinal tract). Ideally, the screen media should resemble intestinal fluids and contain bile acid-lecithin mixed micelles. Fast and reliable techniques for assessing solubility in... 

The critical concentration at which the first micelle forms is called the critical micelle concentration, or CMC. As the concentration of block copolymer chains increases in the solution, more micelles are formed while the concentration of nonassociated chains, called unimers, remains constant and is equal to the value of the CMC. This ideal situation corresponds to a system at thermodynamic equilibrium. However, experimental investigations on the CMC have revealed that its value depends on the method used for its determination. Therefore, it seems more reasonable to define phenomenologically the CMC as the concentration at which a sufficient number of micelles is formed to be detected by a given method [16]. In practical terms, the CMC is often determined from plots of the surface tension as a function of the logarithm of the concentration. The CMC is then defined as the concentration at which the surface tension stops decreasing and reaches a plateau value. 

Fig. 6.10 Spherical cross-section of an idealized anionic (a) normal micelle and (b) reverse micelle ( ) polar head group ( ) counter ion ( vw) the hydrocarbon chain.

Hyperbranched polyesteramides, partially or fully functionalized with fluoroalkyl chains, are expected to be valuable additives for numerous applications. A fully functionalized derivative could be considered to be a unimolecular micelle (like the polyethyleneoxide functionalized analogue) suitable for, e. g., extractions in organic media. An idealized example is shown in Fig. 27. 

Titration results for the mixed erne s of the SDS/CgE4 and C12E2S/C8E4 systems as a function of their relative mole fraction in solution are shown in Figures 2 and 3, respectively. Here, the experimentally determined points are compared with calculated results from the nonideal mixed micelle model (solid line) and the ideal mixed micelle model (dashed line). Good agreement with the nonideal model is seen in each case. 

Figure 2. Cmc s of mixtures of SDS and CgE4 in distilled water (at 25°C). The plotted points are experimental data, the solid line is the result for the nonideal mixed micelle model with B = -3.3, and the dashed line is the result for ideal mixing.

Calorimetric measurements can be used to obtain heats of mixing between different surfactant components in nonideal mixed micelles and assess the effects of surfactant structure on the thermodynamics of mixed micellization. Calorimetry can also be successfully applied in measuring the erne s of nonideal mixed surfactant systems. The results of such measurements show that alkyl ethoxylate sulfate surfactants exhibit smaller deviations from ideality and interact significantly less strongly with alkyl ethoxylate nonionics than alkyl sulfates. 

The ease that certain protein mixtures can be separated using reverse micelle extraction was clearly demonstrated by Goklen and Hatton [46], Goklen [31], and Jarudilokkul et al. [25], who investigated a series of binary and ternary protein mixtures. In two cases, they were able to quantitatively extract cytochrome c and lysozyme from a ternary mixture of these proteins with ribonuclease A. Woll and Hatton [24] investigated the separation of a mixture of ribonuclease A and concanavalin A, and showed that the system behaved ideally and that there was no interaction between the proteins. 

The first observation of depletion flocculation by surfactant micelles was reported by Aronson [3]. Bibette et al. [4] have studied the behavior of silicone-in-water emulsions stabilized by sodium dodecyl sulfate (SDS). They have exploited the attractive depletion interaction to size fractionate a crude polydisperse emulsion [5]. Because the surfactant volume fraction necessary to induce flocculation is always lower than 5%, the micelle osmotic pressure can be taken to be the ideal-gas value ... 

After the mixture had been dried, the remaining residue (CD-capped gold nanoparticles + compound 3) was found to express insolubility in dry CHCfi but the solubility was restored when water was used in the equilibration of the chloroform. In our judgment, this finding clearly makes it apparent that there must be some water necessary for the efficient phase transfer of the nanoparticles into CHCfi. This finding leads to the conclusion that the idealized structure that has been proposed for the nanoparticles after they have been transferred to the chloroform phase (Scheme 2) has some aspects that must be similar to the structure of reverse micelles. We come to the conclusion that these nanoparticle-centered assemblies are similar in a conceptual way to gold-filled reverse micelles. (124 words)... 

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