Anionic-cationic surfactant systems precipitation

Figure 3 Precipitation phase diagram for the anionic-cationic surfactant system sodium decyl sulfate (SDS)-DPC at two different temperatures.

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... 

Reference has already been made to the interesting finding by Laurent and Scott (65) that precipitation of various polyanion/cationic surfactant systems can be totally inhibited by the addition of a sufficient amount of simple salt. This work allowed the definition of a critical electrolyte concentration (c.e.c.), which was found to vary from system to system. Clearly, electrostatic screening effects are again involved. This phenomenon has been confirmed and examined in some detail by Lindman and co-workers (see next section). Less work has been carried out in this respect on polycation/anionic surfactant systems and, at least in some systems involving cationic cellulosic polymer/SDS combinations, resolubilization by salt addition was found not to be facile (59,103). 

Phase equilibria of systems containing oppositely charged ionic surfactants have been the subject of extensive experimental and theoretical investigations [39-61]. Competition between various molecular interactions (van der Waals, hydrophobic, electrostatic, hydration forces, etc.) may result in a variety of micro-structures, mixed micelles, vesicles, and catanionic surfactant salts. Mixing aqueous solutions of anionic surfactant with an equivalent amount of cationic surfactant (alkyl chains with more than eight atoms) results in precipitation of... 

The electrostatic effects on the phase behavior of the CTAB and SOS mixture with added salt have been studied [26]. The phase behavior of this surfactant system changes markedly when an electrolyte is added (Fig. 5). At certain compositions, there is a vesicle-to-micelle transition with increasing salt concentration, and surface charge density measurements show that aggregate composition changes with added electrolyte. A thermodynamic cell model for micellization of mixtures of anionic and cationic surfactants which provides an accurate account of surfactant inventory, micelle composition, and counterion binding (as probed by electrical conductivity) has been developed. Model predictions for the phase equilibria between spherical micelles and a crystalline precipitate phase are in agreement with experimental data [24,26]. 

In addition to mixtures of single-chained anionic and cationic surfactants, mixtures of single-chained anionic surfactants with double-chained cationic surfactants have also been studied. The phase equilibria of the SDS-DDAB-water system have been studied by water deuteron nuclear magnetic resonance (NMR) and polarization microscopy methods at 40°C [27]. Based on particle size measurements, the possibility of vesicle formation has been realized from this study. Spontaneous vesicle formation in the aqueous mixture of didodecyldimethylammonium bromide and sodium dodecyl sulfate has been investigated with differential interference microscopy, transmission electron microscopy, glucose-trapping experiments, -potential measurements, and surface-tension measurements [28]. A solution of DDAB with a small amotmt of SDS is a lamellar phase. Adding more SDS induces surfactant precipitation. Further addition of SDS causes DDAB-SDS precipitate to disperse and results in the vesicle formation. The DDAB and SDS mixtures yield... 

Continuous dyeing of PAC-cotton plush with cationic and direct dyes by the pad steam process plays an important role. The choice of dyes must take into account liquor stability, reservation of PAC or CEL fiber, and solubility. Precipitation of cationic and anionic dyes present in the pad liquor at relatively high concentrations cannot be avoided solely by dye selection. Suitable auxiliary systems have been developed. Differently charged dyes are kept in solution separated from each other in two phases by the combination of anionic and nonionogenic surfactants. With the help of fixing accelerators, good penetration of PAC fibers can be achieved in 10-15 min with saturated steam at 98-100°C. 

In our model system there are four salts that can precipitate A, A X, A A and X X. A and A are, respectively, a cationic and an anionic surfactant X and are the corresponding counterions. The system is represented as a trigonal bipyramid. We have calculated the equilibrium between a lamellar phase and cryst line phases A A, A X and A X with different weight fractions of A A. ... 

A potentiometric determination of saccharin was proposed by Fatibello-Filho et al. [86]. In this method, saccharin was potentiometrically measured using a silver wire coated with a mercury film as the working electrode. With this, the main difficulty was the presence of a precipitate (mercurous saccharinate) that could adsorb on tube walls and the electrode surface. To avoid these undesirable effects, a relocatable filter unit was placed before the flow-through potentiometric cell and a surfactant was added to the carrier solution (Figure 24.12). The same investigation team reported the construction and analytical evaluation of a tubular ion-selective electrode coated with an ion pair formed between saccharinate anion and toluidine blue O cation incorporated on a poly(vinyl chloride) matrix [87]. This electrode was constructed and adapted in a FIA system. The optimum experimental conditions found were an analytical path of 120 cm, an injection sample volume of 500 pL, a pH of 2.5, a flow rate of 2.3 mL/min, and a tubular electrode length of 2.5 cm. 

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