Mixtures containing cationic surfactants

The synthesis of mesoporous silicas was performed from an isotropic reaction mixture using cationic surfactant as a structure directing agent. The decrease in pH, which causes the formation of solid particles, was achieved by hydrolysis of methyl acetate. The procedure enabled to obtain not only siliceous MCM-41 but also a less well-ordered hexagonal silica with extraordinary large surface area and silica with bimodal mesoporous structure containing the MCM-41 mesopore system and a system of mesopores with a mean diameter of 14 nm. 

Mixtures containing cationic fiuorinated surfactants and nonionic or cationic hydrocarbon surfactants have not been investigated extensively. 

TTAB) as surfactant and diethylether, n-heptane, cyclohexane, chloroform, or octanol as oil phase. Table 5 summarizes the values of N for the test mixtures in microemulsions containing different organic solvents with cationic surfactants. 

The phase behavior of anionic-cationic surfactant mixture/alcohol/oil/ water systems exhibit a similar effect. First of all, it should be mentioned that because of the low solubility of the catanionic compound, it tends to precipitate in absence of co-surfactant, such as a short alcohol. When a small amount of cationic surfactant is added to a SOW system containing an anionic surfactant and alcohol (A), three-phase behavior is exhibited at the proper formulation, and the effect of the added cationic surfactant may be deduced from the variation of the optimum salinity (S ) for three-phase behavior as in Figs. 5-6 plots. Figure 16 (left) shows that when some cationic surfactant is added to a SOWA system containing mostly an anionic surfactant, the value of In S decreases strongly, which is an indication of a reduction in hydrophilicity of the surfactant mixture. The same happens when a small amount of anionic surfactant is added to a SOWA system containing mostly a cationic surfactant. As seen in Fig. 16 (left), the values of In S at which the parent anionic and cationic surfactant systems exhibit three-phase behavior are quite high, which means that both base surfactants, e.g., dodecyl sulfate... 

A few years ago, we began a research program to develop methods of analysis which would involve the use of FAB and a high performance tandem mass spectrometer. The tandem instrument was the first triple sector mass spectrometer to be designed and built by a commercial instrument company (Kratos of Manchester, U.K.). The first mass spectrometer of the combination is a double focussing Kratos MS-50 which is coupled to a low resolution electrostatic analyzer, which serves as the second mass spectrometer U). This FAB MS-MS combination has been used to verify the structures of an unknown cyclic peptide (2), a new amino acid modified by diphtheria toxin (3), and an ornithine-containing lipid (4). A number of methods have also been worked out which rely on this instrumentation. They Include the structural determination of cyclic peptides (5), nucleosides and nucleotides (6), and unsaturated fatty acids (7) and the analysis of mixtures of both anionic (8) and cationic surfactants (9). 

In this project, we used high-purity HDTMA to manufacture the SMZ, since all of our laboratory testing had been done using this surfactant. Clearly, the bulk cost of SMZ could be greatly reduced if a less expensive surfactant were used. Surfactant mixtures which contain lower molecular-weight cationic surfactants, in addition to HDTMA, are available at lower cost. Limited experimentation with these lower-purity formulations indicated they produced SMZ that was less stable chemically and had a lower contaminant sorption capacity than SMZ prepared with pure HDTMA. The effectiveness of SMZ prepared from these alternative surfactant formulations warrants further examination, however. 

The geometrical constraints for a ternary mixture of surfactant, nonpolar solvent and water are less easily calculated for more general systems, which contain more than two components. Nevertheless, geometric considerations lead to similar behaviour as has been detailed in the previous sections. To illustrate this point, we analyse the microstructure of simple ternary microemulsions, consisting of a mixture of the cationic surfactant, didodecyl dimethyl ammonium bromide (DDAB), a range of alkanes and water. 

But some double-chain cationic surfactants form microemulsions when mixed with only water and oil over a large region of the ternary phase triangle [38, 39]. These surfactants are virtually insoluble in both water and oil and therefore are located exclusively at the oil-water interface. This aids structural analyses significantly. We shall focus on mixtures containing DDAB. Some typical phase diagrams for these mixtures are reproduced in Fig. 4.19. 

Let us consider now the case when a solution contains a mixture of two anionic (or cationic) surfactants (for example, homologues RiX and R2X with a eommon eounterion X ) with addition of inorganic electrolyte XY. In such systems the counterion concentration is given by the sum of concentrations of RiX, R2X and XY. For simplicity, the saturation adsorptions of the two homologues will be taken as equal, i.e., o)ix= o)2x=2too. After consideration of the surface-to-bulk distribution of both electroneutral combinations of ions, the surface layer equation of state for the Frumkin-type non-ideality of a mixture of two ionic surfactants can be written in a form similar to Eq. (2.35), where it is assumed that l/tO, = Corresponding... 

Figure 18 Phase behavior of systems containing a mixture of anionic and cationic surfactants with oil, water, and alcohol. (From Ref 99.)...

These data indicate that microemulsions can be made with mixtures containing a small amount of cationic surfactant and a large amount of the anionic surfactant, or conversely. 

As explained in detail elsewhere [99], it seems that the anionic-cationic surfactant mixtures would contain a certain amount of catanionic equimolecular compound, which is much less hydrophilic than its components, and an excess of either the anionic or cationic surfactant. In other words, a mixture containing 10 mol of cationic surfactant and 90 mol of anionic surfactant would really behave as a mixture of 80 mol of anionic surfactant with 10 mol of the catanionic equimolar component. There is evidence that this anionic-cationic... 

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