Mixed micelles kinetics

Consequently, new investigations dealing with reactions in micellar solutions composed of functional surfactants, or mixtures of inert and functional surfactants, continue to appear in the literature. An interesting study of acid-catalyzed hydrolysis of 2-(p-tetradecyloxyphenyl)-l,3-dioxolane (p-TPD) in aqueous sodium dodecyl sulfate (SDS) solutions has been reported [13]. In this case,/ -TPD behaves as a non-ionic functional surfactant and apparently forms non-ideal mixed micelles with the anionic surfactant (SDS). Based on the observed kinetic data, the authors propose that, at elevated temperatures, the thermodynamic non-ideality results in the manifestation of two populations of micelles, one rich in SDS and the other rich in/>-TPD. 

To check the validity of this concept, the simplest and most straight-forward approach seemed to be to carry out rate studies of emulsion polymerizations with recipes of identical micellar sizes. Since, as mentioned previously, each specific value of surfactant ratio, r, of the mixed surfactant stands for a specific size of the mixed micelles, the experimental approach boils down to run several series of kinetic studies with different surfactant ratios between series, but with varying surfactant concentrations within each series. The standard recipe for such experiments was described in the Experimental Section. This standard recipe is essentially identical with the one used by Kamath ( ), Wang i9) and Letchford (15) with the exception of eliminating KOH. 

Equations (4) and (5) are the relations derived by Aniansson for mixed micelles and they relate the relaxation times to kinetic constants and other parameters as follows ... 

The results of this study show that ultrasonic absorption techniques, when used in conjunction with other aggregate properties, provide important kinetic information about the partitioning process of alcohols tetween ionic micelles and the bulk phase. The effect of hydrocarbon chain length of the alcohol on the exit rates of monomer surfactant and alcohol components from the mixed micelles, aggregate size distribution, and the electrostatic stabilization of the head group region of the micelles will be considered. As well, previously unreported data for water- 1-butoxyethanol-DTAB will be used to extend conclusions based on previous work [2,3]. 

The experimental techniques used to obtain the properties necessary to derive the kinetic constants of interest from the ultrasonic relaxation times have been previously described in detail [2,3]. Briefly, the degree of micelle ionization (P) and the binding constant (Ka) of an alcohol to mixed micelles were obtained from specific conductivity measurements as a function of surfactant concentration at various fixed alcohol compositions. The binding constant was determined from the slopes of the curves above the cmc, as proposed by Abu-Hamdiyyah et al... 

Data on absorption of non-micellar lipids in the presence of bile salts is available from the study )y Knoebel [79]. The lymphatic transport of absorbed oleic acid and site of uptake from the intestinal lumen was measured in bile fistula rats. It was found that the concentration of bile salts in a continuous intraduodenal infusion did not affect the steady-state level of lipid appearing in the lymph until the bile salt concentration was as low as 1 mM, which represented a molar ratio of 20 1 of lipid to bile salt. In the case of infusates with relatively low concentrations of bile salts it was found that a larger part of the available surface area of the small intestine was utilized. The main conclusion is that lipids are equally well absorbed in vivo from non-micellar dispersions of lipids and bile salts as from solutions where the lipids are completely solubilized by bile salt mixed micelles. However, a detailed analysis of kinetics of uptake from non-micellar phases in vitro with isolated intestinal segments has not yet been done. 

Additional information <5, 6> (<6> kinetic analysis using Triton X-100/ phosphatidylinostitol-mixed micelles [28]) [7, 28]... 

Buxeda, R.J. Nickels, J.T. Belimis, C.J. Carman, G.M. Phosphatidylinositol 4-kinase from Saccharomyces cerevisiae. Kinetic analysis using Triton X-100/phosphatidylinositol-mixed micelles. J. Biol. Chem., 266, 13859-13865... 

In Fig. 16, experimental results of the time-dependent intensity after mixing proteated and deuterated PS-PB micelles in DMF under KZAC conditions [101] are shown. As can be seen the intensity decreases with time, directly showing that the micelles mix and kinetic processes are active. By analyzing the evolution of the scattered intensity and appropriate modeling, the mechanism and pathways can be determined from these experiments. In the following section, the technicalities will be described in more detail. 

We now turn to the kinetic results. Aniansson (29) has recently reported an extension to mixed micelles, made out of two detergents, of his theoretical treatment of the kinetics of simple micellar systems (8,25). The expressions given for and T2 can be used to qualitatively assess the effect of alcohol on these relaxation times, since the alcohol molecules can be considered to form mixed micelles with the detergent. The ratio C /cmc, where is the micelle concentration, appears in the predominant term in Aniansson s equation for 1/Tj. At constant detergent concentration, close to the cmc, an addition of alcohol brings about an increase of C and a decrease of cmc. The ratio C /cmc, and thus l/Tj should increase with the alcohol concentration, as is indeed observed. 

A quantitative analysis of the above kinetic results on the basis of Aniansson s theory of mixed micelles is not possible at the present time because it requires the knowledge of such quantities as the total micelle molecular weight at every concentration of alcohol and detergent, and the micelle composition. Work is now in progress in our laboratory in order to determine these quantities. 

Since the shape of an oil droplet is an indication of IFT as measured by sessile drop method, the oil droplet flattening time reflects the rate of change in IFT. The results clearly show that the presence of alcohol increases the rate of achieving the final value of interfacial tension. This implies that the surfactant molecules come to the interface much faster in the presence of alcohol. Zana (20) has shown that the kinetics of micellization is more rapid in the presence of alcohol. This is presumably due to loose packing of mixed micelles containing surfactant and alcohol. Thus, it appears that the kinetics of micellization could influence the rate at which molecules saturate the surface by the breakdown of micelles to provide monomers for adsorption. 

One of the rare works on the kinetics of the formation of mixed micelles of Pluronics and surfactants also treats the system L64/SDS [58]. In temperature-jump experiments, the authors identified three different relaxation times for the L64/SDS mixture. The fastest ps) is associated with the binding of additional L64 unimers to the micelles. The two slower relaxation processes are interpreted as structural rearrangement of the mixed micelles and micelle clustering. These findings are qualitatively in agreement with the first kinetic investigation of these mixtures by Hecht and Hoffmann [59]. 

Frese et al. [38] studied the adsorption kinetics of mixed anionic/cationic surfactant systems, finding that net charge-neutral mixed micelles were formed. They found that the dynamic surface tension was higher than expected for small-surface ages, possibly due to an activation barrier associated with the mixed-micelle complexes. 

Deems, R. A., B. R. Eaton, and E. A. Dennis. 1975. Kinetic Analysis of Phospholipase A2 Activity toward Mixed Micelles and Its Implications for the Study of Lipolytic Enzymes. Journal of Biological Chemistry 250 (23) 9013-9020. 

A theory for the stepwise association and dissociation of surfactant micelles was developed a few years ago. Its application to a large quantity of experimental data has provided a consistent interpretation of these data and enabled the extraction of basic kinetic and equilibrium parameters for these systems In the following extension to mixed micelles the concepts and assumptions used are closely analogous to those of the previous treatment to which the reader is referred for more details. For simplicity the treatment is limited to two-component micelles but the extension to larger number of components is quite straightforward. 

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