Solute micelle interaction

Lastly, the use of micellar mobile phases allows a convenient means of studying micelle - solute interactions (i.e. determination of binding constants) (1,10 4,105) as well as determination of surfactant CMC values (from breaks in the log k gQ vs. log C, plots)... 

A detailed examination of relative second-order rate constants in micellar and aqueous pseudophases is outside the scope of this discussion, but for anionic reagents it seems that (Tables 3 and 4) deviates from unity when the substrate is very hydrophobic and the anion more hydrophilic, suggesting that, on the average, they are not located in the same region of the micelle. However, more evidence will be needed on micelle-solute interactions for this question to be answered. 

Although SDS adsorption enhances the selectivity of the stationary phase toward the vanillin compounds, SDS micelle-solute interactions also contribute to the selectivity of this separation. For example, SDS micelles interact more strongly with vanillin than with isovanillin, as evidenced by the greater K m binding constant for vanillin, and this interaction is responsible, at least in part, for the baseline resolution of these two compounds. Nevertheless, the successful separation of the vanillin compounds with the 0.02 M SDS mobile phase is primarily due to solute-stationary phase interactions, which is also the reason why the separation of the vanillin test mixture is more favorable at lower SDS concentrations (see Fig. 7.7). 

The development of micellar liquid chromatography and accumulation of numerous experimental data have given rise to the theory of chromatographic retention and optimization methods of mobile phase composition. This task has had some problems because the presence of micelles in mobile phase and its modification by organic solvent provides a great variety of solutes interactions. 

Recent development of the use of reversed micelles (aqueous surfactant aggregates in organic solvents) to solubilize significant quantities of nonpolar materials within their polar cores can be exploited in the development of new concepts for the continuous selective concentration and recovery of heavy metal ions from dilute aqueous streams. The ability of reversed micelle solutions to extract proteins and amino acids selectively from aqueous media has been recently demonstrated the results indicate that strong electrostatic interactions are the primary basis for selectivity. The high charge-to-surface ratio of the valuable heavy metal ions suggests that they too should be extractable from dilute aqueous solutions. 

In the absence of micelles, all neutral molecules reach the detector in time /0. Micelles injected with the sample reach the detector in time rmc, which is longer than t0 because the micelles migrate upstream. If a neutral molecule equilibrates between free solution and the inside of the micelles, its migration time is increased, because it migrates at the slower rate of the micelle part of the time. The neutral molecule reaches the detector at a time between f0 and /nlc. The more time the neutral molecule spends inside the micelle, the longer is its migration time. Migration times of cations and anions also are affected by micelles, because ions partition between the solution and the micelles and interact electrostatically with the micelles. 

Micellar interactions Write net equations for the following (a) carbonate-benzenesulfonate RMs solution with strong acid SH, (b) soft-core phenolate RMs and strong acid SH, and (c) soft-core carboxylate reverse micelles solution with carboxylic acid. 

If the standard operating conditons do not provide the required separation, selectivity can be modified by changing a number of variables, including the nature of the surfactant and the aqueous phases. Altering the hydrophilic end of the surfactant has a dramatic effect since this is the end of the micelle that interacts with the solutes. Alternatively, a second surfactant can be added to form a mixed micelle. The addition of a nonionic surfactant to an ionic one decreases the migration time window so that the migration time of all the analytes decreases nonionic chiral surfactants are often added to MECC buffers for the separation of enantiomers. 

I. Benjamin, Chemical reactions and solvation at liquid interfaces a microscopic perspective, Chem. Rev. (Washington, D. C.), 96 (1996) 1449-75 I. Benjamin, Theory and computer simulations of solvation and chemical reactions at liquid interfaces, Acc. Chem. Res., 28 (1995) 233-9 L. R. Martins, M. S. Skaf and B. M. Ladanyi, Solvation dynamics at the water/zirconia interface molecular dynamics simulations, J. Phys. Chem. B, 108 (2004) 19687-97 J. Faeder and B. M. Ladanyi, Solvation dynamics in reverse micelles the role of headgroup-solute interactions, J. Phys. Chem. B, 109 (2005) 6732 10 W. H. Thompson, Simulations of time-dependent fluorescence in nano-confined solvents, J. Chem. Phys., 120 (2004) 8125-33. 

P = 2RTM l is called the second virial coefficient it yields the same qualitative information about interaction as A[nq]2 in Eq. (4.6). Membrane osmometry seldom requires an accuracy to more than P cf. Doi and Edwards (1986) define a dilute solution as one in which P = 0—the ideal condition for accurately measuring Mn. Te is that temperature where P = 0 (Alberty and Silby, 1992). The fact that P provides information about solute-solute interactions, micellization and demicellization studies are made possible by the use of Eqs. (4.29) and (4.30). 

These results on amino acid solubilisation in reversed micelle solutions have indicated clearly that such systems could be useful for the recovery, separation and concentration of small, charged biological molecules from aqueous media. Furthermore, they have shed some light on the role that hydrophobic interactions will play in the solubilisation of more complex molecules such as proteins, which have a distribution of polar and nonpolar amino acid residues over their surfaces. 

In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

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