Hexadecyltrimethylammonium bromide solubilization

An interesting example of extension of the size of micelles is provided by the system composed of hexadecyltrimethylammonium bromide acting as a surfactant, to which n-alkane acting as solubilizing agent was added [29]. 

It seems likely that the cationic CPC micelles, which have a large positive charge at or near the micellar surface, interact attractively with the n-molecular orbital system of benzene, and that this interaction contributes to the fact that the solubilization constant for benzene in CPC is approximately twice as large as that in SDS micelles. A preferential interaction between cationic surfactants and aromatic solutes has been reported by several groups of investigators (25-27), and recent work in our laboratory shows that 1-hexadecyltrimethylammonium bromide micelles also solubilize benzene more effectively than do the anionic alkylsulfate surfactant micelles (28). Thus, the tendency of benzene molecules to solubilize near the surface of the cationic micelles, at low XB values, may lead to a partial saturation of surface "sites" by benzene, diminishing the ability of additional benzene molecules to bind near the surface. Such an effect could be responsible for the initial increase in activity coefficient that occurs, particularly in the CPC solutions, as Xg increases. 

The initial descending leg is interpreted as the solubilization process. If micelles are present, the added oil will be solubilized. Also, the added oil may be solubilized in the hexadecyltrimethylammonium bromide-cetyl alcohol complex. That the sol-... 

The parameters P(l) and P(2) define the solubilization stage. In the absence of micelles, there is no solubilization of the oil phase. Therefore, in this case, there are no values of P(l) and P(2), e.g., as for the 1 6 hexadecyltrimethylammonium bromide-cetyl alcohol molar ratio. 

The parameter P(2), the solubilization constant, has a finite value when micelles are present in the mixed emulsifier system. The smaller the value of P(2), the smaller the concentration of micellar hexadecyltrimethylammonium bromide and the better the balance between the hexadecyltrimethylammonium bromide and cetyl alcohol in the crystalline rodlike particles. 

The parameter P(3) represents the conductance at the end of the solubilization stage. Table III shows that the concentration of hexadecyltrimethylammonium bromide in the crystalline rodlike particles increases with increasing cetyl alcohol concentration. Therefore, this parameter represents the number of crystalline rodlike particles in the system prior to emulsification. 

A partition of the transparent isotropic water-in-oil type solubilization area has been proposed as well by Smith and others (20,21) who investigated the phase and structural behavior of oil-continuous systems composed of water, hexane, 2-propanol, with or without addition of hexadecyltrimethylammonium bromide or perchlorate. The techniques used were conductometry, ultracentrifugation and, later, NMR. Smith and coworkers (20) put into evidence kinks on conductivity curves as the system composition was varied by increasing 2-propanol content. Plotting in the phase diagram... 

The Cotton effect of the probe when incorporated in micelles of both hexadecyltrimethylammonium bromide and sodium dodecyl sulphate more closely resembles that of water than heptane, suggesting an aqueous environment for the solubilized probe. 

The effect of solubilizate on micellar size has been examined in only a few systems. In a light-scattering study of solubilization by hexadecyltrimethylammonium bromide Hyde and Robb [150] showed that the incorporation of increasing amounts of the non-polar molecules, decane, octane and cyclohexane, causes a pronounced increase in the micellar molecular weight. This is due to increases in the numbers of solubilizate and surfactant molecules in each micelle. However, the solubilization of the polar molecule, octanol, although increasing the micellar weight causes a decrease in the number of surfactant molecules in each micelle. 

Tamura and Aida reported the unique influence of shape and volume of inner micellar space on product distribution. They irradiated ACN in aqueous sodium octyl sulfate, decyl sulfate, dodecyl sulfate, and hexadecyltrimethylammonium bromide under pressures up to 150 MPa. Pressure enhanced the dimerization reaction in all micellar systems due to the formation of a van der Waals dimer in the ground state. Plots of the consumed ACN vs. concentration of micelles exhibited a minimum under constant concentration (9.7 mmol kg" ) of ACN. Thus, the number of micelles in the solution increases with increase of surfactant concentration and, as a result, the number of ACN molecules solubilized per micelle decreases. This leads to a decrease of probabihty of collision between ACN molecules that undergo bimolecular reaction. Viscosity measurements indicate that spherical micelles start to aggregate and form rod-shaped micelles with larger volumes at ca. 9 wt% of micelle, which exactly coincides with the minimum concentration of micelles. At concentrations of micelles higher than the minimum, the number of ACN molecules included in a micelle increases due to aggregation of spherical micelles into rod-shaped micelles, leading to enhanced dimerization of ACN. 

As an example we show in Fig. 2.13 measurements by Brun et al.43) using a cation exchange electrode. Larsen et al.44,4S) used bromide and chloride specific electrodes for solubilization studies in 0.1 M hexadecyltrimethylammonium chloride or bromide solutions, 76-77% of the counterions are bound, a figure changing only slightly... 

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