Dispersions mixed micellization

In another study of the physical behavior of soap-LSDA blends, Weil and Linfield [35] showed that the mechanism of action of such mixtures is based on a close association between the two components. In deionized water this association is mixed micellar. Surface tension curves confirm the presence of mixed micelles in deionized water and show a combination of optimum surface active properties, such as low CMC, high surface concentration, and low surface concentration above the CMC. Solubilization of high Krafft point soap by an LSDA and of a difficulty soluble LSDA by soap are related results of this association. Analysis of dispersions of soap-LSDA mixtures in hard water shows that the dispersed particles are mixtures of soap and LSDA in the same proportion as they were originally added. These findings are inconsistent with the view that soap reacts separately with hard water ions and that the resulting lime soap is suspended by surface adsorption of LSDA. The suspended particles are responsible for surface-active properties and detergency and do not permit deposits on washed fabric unlike those found after washing with soap alone. 

For still larger quantities of water, one obtains the isotropic phase (IV), formed of mixed micelles. By extending the frontier line, WN, up to its intersection with side L-NaC of the triangle of the Figure 3, it can be seen that in order to get this micellar dispersion, at least one molecule of Na cholate is needed for two molecules of lecithin. 

These statements lead to the conclusion that the limiting proportion of 1 gram of Na cholate associated to 1 gram of lecithin is simply imposed by the size of a certain form of mixed micelle which can remain in equilibrium with an excess of Na cholate in micellar solution. Thus, it clearly appears that association is governed by the necessity of securing the proper hydrophilic-lipophilic balance of the mixture of two components. Here, as in the case of other amphiphilic substances, by the progressive increase in proportion of the more hydrophilic amphiphile. the association can reach complete micellar dispersion in water. 

In this paper, a molecular thermodynamic approach is developed to predict the structural and compositional characteristics of microemulsions. The theory can be applied not only to oil-in-water and water-in-cil droplet-type microemulsions but also to bicontinuous microemulsions. This treatment constitutes an extension of our earlier approaches to micelles, mixed micelles, and solubilization but also takes into account the self-association of alcohol in the oil phase and the excluded-volume interactions among the droplets. Illustrative results are presented for an anionic surfactant (SDS) pentanol cyclohexane water NaCl system. Microstructur al features including the droplet radius, the thickness of the surfactant layer at the interface, the number of molecules of various species in a droplet, the size and composition dispersions of the droplets, and the distribution of the surfactant, oil, alcohol, and water molecules in the various microdomains are calculated. Further, the model allows the identification of the transition from a two-phase droplet-type microemulsion system to a three-phase microemulsion system involving a bicontinuous microemulsion. The persistence length of the bicontinuous microemulsion is also predicted by the model. Finally, the model permits the calculation of the interfacial tension between a microemulsion and the coexisting phase. 

As the mixed micelles are very small (10-20 nm) the initial swelling must be very limited. To be able to absorb more of Z, the initial small droplets must be furnished with more emulsifier, and even more importantly, with more fatty alcohol (Z,). This may be achieved by coalescence of initial droplets or by absorbtion of mixed micelles from the surroundings. The assumption that the emulsification takes place by a diffusion process seems to be supported by experiments with mixed systems of ionic emulsifier and fatty alcohol and various dispersed phases, showing that a necessary condition for a rapid emulsification is that the compound to be emulsified have slight water solubility. Furthermore, it has been observed that if even small amounts of Zj are added to Z, before addition to the water-mixed emulsifier system, the extent of emulsification is reduced and the resulting emulsion becomes less stable. 

CDC are defined only by their size (most scientists agree on sizes below 1 pm others set 0.5 pm as the upper limit). CDC are very heterogeneous in all other aspects (e.g., thermodynamic stability, chemical composition, and the physical state, including solid, liquid, or liquid-crystalline dispersions) [ 1 ]. The most prominent examples are nanoparticles, nanoemulsions, nanocapsules, liposomes, nanosuspensions, (mixed) micelles, microemulsions, and cubosomes. Some CDC have reached the commercial market. Probably the best known example is the microemulsion preconcentrate of cyclosporine (Sandimmun-Neoral), which minimized the high variability of pharmacokinetics of the Sandimmun formulation. In addition, intravenous injectable CDC have been on the commercial market for many years. Examples include nanoemulsions of etomidate (Etomidat-Lipuro) and diazepam (Diazepam-Lipuro) [2-4], mixed micelles (Valium-MM, Konakion), and liposomes (AmBisome) [5]. 

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. 

Effluent profiles obtained from a core-flood performed with a mixture of two surface-active components (C12 and C18) separated from a commercially available sulfobetaine are shown in Figure 24 (115). The points represent experimental data, and the lines were obtained by simulating the core-flood with a convection—dispersion—adsorption model that is based on the surface excess concept and takes into account monomer—micelle equilibrium (115). Because the mixture contains different homologues of the same surfactant, the ideal mixed micelle model... 

Figure 8.5. Plot of the system constants (solvation parameter model) against composition for a mixed micelle electrolyte solution containing 50 mM sodium N-dodeconyl-N-methyltaurine and different amounts of the non-ionic surfactant Brij 35 (polyoxyethylene [23] dodecyl ether) (Left). Plot of the system constants for a mixed micelle buffer containing 50 mM sodium N-dodecanoyl-N-methyltaurine and 20 mM Brij 35 against the volume fraction of acetonitrile added to the electrolyte solution (Right). System constants m = difference in cavity formation and dispersion interactions r = difference in electron lone pair interactions s = difference in dipole-type interactions a = difference in hydrogen-bond basicity and b = difference in hydrogen-bond acidity. (From ref. [218] Royal Society of Chemistry).

A similar approach to mixed micelles was also proposed by Nagarajan [18, 56], where most of the contributions to the standard free energy difference between the single dispersed and the aggregated states were expressed as linear functions of individual contributions. For example, the transfer free energy contribution was presented as... 

Non-ionic surfactants do not exhibit Krafft points. Rather the solubility of non-ionic surfactants decreases with increasing temperature and the surfactants begin to lose their surface active properties above a transition temperature referred to as the cloud point. This occurs because above the cloud point, a separate surfactant rich phase of swollen micelles separates the transition is visible as a marked increase in dispersion turbidity. As a result, the foaming ability of, for example, polyoxyethylenated non-ionics decreases sharply above their cloud points. The addition of electrolyte usually lowers the cloud point while the addition of ionic surfactant usually increases the cloud points of their non-ionic counterparts, this increase being dependent on the composition of the mixed micelle. 

The composition of the adsorbed mixed monolayer of binary component systems in equilibrium with the singly dispersed components can be evaluated using Rosen s equations (Li et al, 2001, Zhou Rosen, 2003). From analogy, using the derivation of Rubingh s equations for mixed micelles, the mole fraction of component 1,, in the mixed monolayer... 

In simple sphingomyelin-Triton X100 systems, the sphingomyelin is dispersed as bilayers below a surfactant concentration of 0.2 mM. Above this concentration mixed micelles formed [301, 302]. Studies of the transfer of cholesterol solubilized in sodium dodecyl sulphate to PC liposomes has shown that 90 % of... 

The structure of the microdomains which form in surfactant solutions is shown schematically in Fig. 5.1. Side chains and surfactant molecules combine to form aggregates resembling mixed micelles dispersed throughout the bulk aqueous network. The composition of these microdomains has been investigated using a fluorescence probe technique adapted from the surfactant literature [2]. Results are summarized in Table 5.1 and Fig. 5.2. The total aggregation number, Nj, of the clusters is equal to the sum of the number of surfactant molecules and alkyl side chains N and respectively) in each... 

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