Model mixed micellization

Highly insoluble molecules are in part transported in the GIT by partitioning into the mixed micelles injected into the lumen from the biliary duct in the duodenum (Fig. 2.3). Mixed micelles consist of a 4 1 mixture of bile salts and phospholipids (Fig. 7.13). In contrast, at the point of absorption in the BBB, highly insoluble molecules are transported by serum proteins. This distinction is expected to be important in in vitro assay modeling. The use of simulated intestinal fluids is appealing. 

Murakami and Kondo (1975) reported that the cationic micelle is quite effective for the pyridoxal-catalyzed elimination of S-phenylcysteine. The significant rate acceleration was explained by the binding of the Schiff s base to the micelle phase, followed by the efficient proton abstraction by hydroxide ion at the micelle surface. According to Gani et al. (1978), mixed micelles of CTAB and dodecylamine hydrochloride are good models for the site accommodating pyridoxal 5 -phosphate in glycogen phosphorylase, since the micelles can imitate well the formation of SchifT s bases in hydrophobic environments. 

In the reaction with PNPA, myristoylhistidine [29] in a cationic micelle rapidly forms acetylimidazole as a fairly stable intermediate which is readily observable at 245 nm. On the other hand, a mixed micelle of [29] and N,N-dimethyl-N-2-hydroxyethylstearylammonium bromide [30] leads to the formation and decay of the intermediate, indicating that the acetyl group is transferred from imidazole to hydroxyl groups (Tagaki et al., 1977 Tagaki et al., 1979). This can be a model of cr-chymotrypsin which catalyses hydrolysis of PNPA (non-specific substrate) by initial acylation of the histidyl imidazole followed by acyl transfer to the seryl hydroxyl group (Kirsh and Hubbard, 1972), as indicated schematically in (12). 

Among the purposes of this paper is to report the results of calorimetric measurements of the heats of micellar mixing in some nonideal surfactant systems. Here, attention is focused on interactions of alkyl ethoxylate nonionics with alkyl sulfate and alkyl ethoxylate sulfate surfactants. The use of calorimetry as an alternative technique for the determination of the cmc s of mixed surfactant systems is also demonstrated. Besides providing a direct measurement of the effect of the surfactant structure on the heats of micellar mixing, calorimetric results can also be compared with nonideal mixing theory. This allows the appropriateness of the regular solution approximation used in models of mixed micellization to be assessed. 

The derivation of a pseudo-phase separation model for treating nonideal mixed micellization is given in detail in reference 3. This leads to the generalized result... 

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.

The finding that the assumptions of the regular solution approximation do not hold for the mixed micellar systems investigated here suggests a re-examination of how the thermodynamics of mixing enter the nonideal mixed micelle model. 

The mixed cmc behavior of these (and many other) mixed surfactant systems can be adequately described by a nonideal mixed micelle model based on the psuedo-phase separation approach and a regular solution approximation with a single net interaction parameter B. However, the heats of micellar mixing measured by calorimetry show that the assumptions of the regular solution approximation do not hold for the systems investigated in this paper. This suggests that in these cases the net interaction parameter in the nonideal mixed micelle model should be interpreted as an excess free energy parameter. 

Fig. 26 A Model of the preferential dissolution process of PA at low SDS concentration. B Model for the existence of vesicle and mixed micelles at high SDS concentration...

Except for some anionic/cationic surfactant mixtures which form ion pairs, in a typical surfactant solution, the concentration of the surfactant components as monomeric species is so dilute that no significant interactions between surfactant monomers occur. Therefore, the monomer—mi celle equilibria is dictated by the tendency of the surfactant components to form micelles and the interaction between surfactants in the micelle. Prediction of monomer—micelle equilibria reduces to modeling of the thermodynamics of mixed micelle formation. 

For a binary system of surfactants A and B, the mixed micelle formation can be modeled by assuming that the thermodynamics of mixing in the micelle obeys ideal solution theory. When monomer and micelles are in equilibrium in the system, this results in ... 

If the mixed micelle model already presented is used to predict the ionic surfactant monomer concentration, and a simple concentration—based solubility product is assumed to hold between the unbound counterion and monomer, the salinity tolerance of an anionic/nonionic surfactant mixture can be accurately predicted (91). supporting this view of the mechanism of tolerance enhancement by nonionic surfactant. 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

Recently, Rubingh ll) and Scamehorn et al. (9) have shown that the activity coefficients obtained by fitting the mixture CMC data can be correlated by assuming the mixed micelle to be a regular solution. This model proposed by Rubingh for binary mixtures has been extended to include multicomponent surfactant mixtures by Holland and Rubingh (10). Based on this concept Kamrath and Frances (11) have made extensive calculations for mixed micelle systems. 

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. 

Many models have appeared in the literature describing interactions of surfactants in mixed micelles (1-14). For nonionic surfactants mixing nonideally, the key references up to 1984 have been recently summarized (15). Comparatively few models have been developed for ionic surfactants (5,6,10-12) and fewer models which acknowledge ionic/nonionic interactions are available (5-7). Since many practical surfactant mixtures involve ionic and nonionic surfactants which interact with each other and with added salts, it is important to develop explicit ionic/nonionic models. 

The purpose of this paper is to develop realistic specific models of mixed micellization which (i) can describe properties of ionic/nonionic surfactant mixtures and effects of salt (ii) lead to tractable calculations and (iii) can be used for extracting information on micelle mixing and monomer concentrations from the limited experimental data which are usually... 

KAMRATH AND FRANSES New Mathematical Models of Mixed Micellization 55... 

The nonionic/ionic model rather than the nonionic/nonionic or the ionic/ionic model should be used for more precise determination of the nonideality-of-mixing parameters in the mixed micelles in nonionic/ionic mixtures. 

Medium-chain alcohols such as 2-butoxyethanol (BE) exist as microaggregates in water which in many respects resemble micellar systems. Mixed micelles can be formed between such alcohols and surfactants. The thermodynamics of the system BE-sodlum decanoate (Na-Dec)-water was studied through direct measurements of volumes (flow denslmetry), enthalpies and heat capacities (flow microcalorimetry). Data are reported as transfer functions. The observed trends are analyzed with a recently published chemical equilibrium model (J. Solution Chem. 13,1,1984). By adjusting the distribution constant and the thermodynamic property of the solute In the mixed micelle. It Is possible to fit nearly quantitatively the transfer of BE from water to aqueous NaDec. The model Is not as successful for the transfert of NaDec from water to aqueous BE at low BE concentrations Indicating self-association of NaDec Induced by BE. The model can be used to evaluate the thermodynamic properties of both components of the mixed micelle. 

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