Mixed micelles composition

The mass action model (MAM) for binary ionic or nonionic surfactants and the pseudo-phase separation model (PSM) which were developed earlier (I EC Fundamentals 1983, 22, 230 J. Phys. Chem. 1984, 88, 1642) have been extended. The new models include a micelle aggregation number and counterion binding parameter which depend on the mixed micelle composition. Thus, the models can describe mixtures of ionic/nonionic surfactants more realistically. These models generally predict no azeotropic micellization. For the PSM, calculated mixed erne s and especially monomer concentrations can differ significantly from those of the previous models. The results are used to estimate the Redlich-Kister parameters of monomer mixing in the mixed micelles from data on mixed erne s of Lange and Beck (1973), Funasaki and Hada (1979), and others. 

Figure 9 Plot of the frequency dependence of the composite symmetric CH2 stretching band and micelle aggregation numbers (19) vs. mixed micelle composition in 0.3 M DTAC/SDS mixed micelles (T = 23°C). Reprinted from ref. 47. Copyright 1990 American Chemical Society.

Palous, J.L., Turmine, M., Latellier, P. Mixtures of nonionic and ionic surfactants determination of mixed micelle composition nsing cross-differentiation relations. J. Phys. Chem. 1998, 702(30), 5886-5890. 

A. (1986a). Preparation of liposomes via detergent removal from mixed micelles by dilution. The effect of bilayer composition and process parameters on liposome characteristics, Pharm. Weekbl. 

Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

The term mixed micelle refers to those micelles composed of two or more surface active agents. The sizes of micelles in a solution obey a distribution function that is characteristic of their chemical composition and the ionic nature of the solution in which they reside. 

The bile salts and their ability to form mixed micelles is discussed in some detail in order to foster a better understanding of their applications. It is highly important for the electrophoretic characterization of the micellar phase, and therefore for the calculation of the distribution coefficients, to have a thorough understanding of the mode of micelle formation and structural changes achieved by alteration of the surfactant concentration and micelle composition as well as to develop strategies for micelle optimization. 

Further information regarding the composition of mixed micelles can be obtained by coupling to mass spectrometry. However, the use of surfactants in electrospray ionization will always be fraught with difficulties because contamination of the interface with nonvolatile tensides causes undesirable effects. Stable mixed micelles can be measured only by employing nonequilibrium conditions (only buffers without micellar components). Two further variants of MEKC-MS have been developed and successfully used in recent years, but these are not readily employed for the electrophoretic characterization of micelles, since either a partial filling technique or surfactants of high molecular mass have to be used (37). 

Schwarz et al. used ESI-MS detection to characterize the composition of binary (bile salt/phosphatidylcholine) and ternary (bile salt/phophatidy 1-choline/fatty acid) mixed micelles that were used in micellar affinity capillary electrophoresis (43,44). The detrimental effects of the surfactants turned out to be tolerable for short-time qualitative determinations. 

The effects of dilution of the micellar surface charge on the rate of alkaline hydrolysis of a betaine ester surfactant have been investigated for a mixture of decyl betainate and a nonionic surfactant with a similar CMC. It was shown that the relation between micellar composition and the hydrolysis rate essentially parallels the relation between micellar composition and counterion binding to mixed micelles made up of ionic and nonionic surfactants [20]. 

In order to illustrate the eFFect oF micellar nonidealities oF mixing on total surFactant monomer concentrations and micelle compositions in a system at the CHC, consider a hypothetical binary surFactant pair, A and B. Assume CMCa = 1 mli and CMCb = 2 mil. For a equimolar mixture oF A and B as monomer, the values oF Cn and micelle compositions are tabulated in Table I at various values oF W/RT. 

A brief accounting of the thermodynamics of mixed micelle formation is given here primarily to clarify certain important issues which appear to have been previously overlooked. The necessity for measuring the monomer and micellar composition will be demonstrated. 

Figure 1. Inventories and micelle compositions for azeotrope micell-ization of mixed ionic surfactants, c /cj = 3.0, A = - 3.0, B = 0.0, j3i = 2 0-7 AZ 0.8113.

On the other hand, we showed that the coii5>osition of surfactant in a mixed adsorbed film can be estimated thermodynamically from experimental results without introducing such a supposition (9-11). Further, the composition of a mixed micelle was calculated assuming that the micelle behaves thermodynamically like a macroscopic bulk phase whose thermodynamic quantities are given by the excess thermodynamic quantities similar to those used for the adsorbed film (i8). Therefore, we can now compare the composition of surfactant in the mixed adsorbed film with that in the mixed micelle at the critical micelle concentration (CMC). 

In this paper, dodecylammonium chloride (DAC) and decylainmoniuin chloride (DeAC) are chosen to reveal the fundamental behavior of surfactants in the mixed adsorbed film and micelle. The surface tension of their aqueous solution is measured as a function of their concentrations in the neighborhood of the CMC and the comparison between the mixed adsorbed film and the mixed micelle is made in terms of the composition evaluated. 

C FNa-CioSNa and C-7FNa-Cla5Na systems. These are the mixed anionic-anionic surfactants systems. The surface tension (interfacial tension) - concentration relationships are shomn in Fig, 1 to 3. There are surfactant compositions at which uniform or homogeneous mixed micelle do not exist in these two systems due to the "mutual pho-bicity" between FC- and HC-chains of the surfactants (4,7) Therefore the molecular interaction parameterof the two surfactants in micelles can not be calculated from the Surface tension curves because this cmc has no longer the physical meaning of mixture cmc. However, we can obtain the /3(t values from the surface tension curves by means of equation 13, Table 1 and 2 show the results. 

As for the close packed surface layer it is assumed that the micelles do not contain any water, i.e. x + x = 1. Assuming ideal mixing of the two surfactant types in the mixed micelle, i.e. f. = f2 = 1, Equation 23 shows that the cmc is the arithmetic mean or the two single surfactant erne s with the surfactant composition in the micelle as the variable. 

In order to check how the introduction of the surfactant-surfactant interaction affects the calculated cmc and the surfactant composition in the micelles, we used Equations 25 and 26 to obtain an average value of x 2 which was found to be -1.6. The dashed lines in Figures 1 and 2 show that introducing a non-zero value of x 2 not affect the calculated cmc or the surfactant composition in the micelles very much. This is because the dominating term in Equation 28 involves the difference in the cmc of the two surfactants. Thus, to obtain an assessment of the importance of surfactant-surfactant interaction in the mixed micelles, one should choose surfactants with their cmc s close to each other. 

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