Precipitating surfactant system

Figure 4. Time dependence of absorbance over 1 cm path length at several wavelengths, of precipitating surfactant system of 0.070 wt % surfactant and 0.30 wt % NaCl in water, prepared by adding 3.0 wt % NaCl solution in time zero to 0.077 wt % surfactant in water (transparent). The initial total absorbance is predominantly by absorption, the increase by scattering from the separating phase.

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... 

This overview will outline surfactant mixture properties and behavior in selected phenomena. Because of space limitations, not all of the many physical processes involving surfactant mixtures can be considered here, but some which are important and illustrative will be discussed these are micelle formation, monolayer formation, solubilization, surfactant precipitation, surfactant adsorption on solids, and cloud point Mechanisms of surfactant interaction will be as well as mathematical models which have been be useful in describing these systems,... 

We may consider precipitation in these systems in the context of competitive aggregate formation between micelles and precipitate. Even systems forming ideal mixed micelles can exhibit synergisms in salinity/hardness tolerance in such systems, the more components present, the higher the tolerance. This is the reason that mixtures of isomeric surfactants generally have Krafft temperatures considerably lower than those of the individual compounds (90). 

Previous work has shown that binary surfactant systems containing Dowfax 8390 and the branched hydrophobic surfactant AOT can form Winsor III systems with both PCE and decane whereas DOWFAX 8390 by itself cannot (Wu et. al. 1999). This binary surfactant system was used in conjunction with hydrophobic octanoic acid to help with phase behavior and lessen the required concentration of CaCl2. Since this formulation is rather complicated, questions about field robustness arise. Thus, for the phase behavior studies presented here, we used the simple binary system of the nonionic TWEEN 80 and the branched hydrophobic AOT, and we optimized the NaCl concentration to give the Winsor Type III system. The lesser electrolyte concentration requirement for the binary TWEEN 80/ AOT system helps to decrease the potential for undesirable phase behavior such as surfactant precipitation, thereby increasing surfactant system robustness. 

Figure 5. Phase Diagram for hexadeeane (EACN=16) at 22°C for 2% AOT, 2% Tween 80, and NaCl. The open circles are data. The closed squares represent the phase diagram for a surfactant system which progresses through types ]4=>IIle II with neither phase separation nor precipitation.

To begin this simulation, we first need to set up an EQBATCH model. The difference between a phase behavior model and a flow model of an alkaline-surfactant system is that the matrix does not exist in the phase behavior test tube thus, there is no ion exchange on the matrix in the phase behavior model. Therefore, in the phase behavior model, we define 6 elemenfs and 14 fluid species based on Example 10.4 and remove Ihe calion exchanges only on fhe malrix. In particular, we keep fhe solid species Ca(OH)2(s) and CaC03(s). Af leasl one advantage is that we can ensure that there should not be any solid precipitation in the model, or any precipitation should be consistent with the observation in the test tube. The rest of the procedures to set up the EQBATCH model are similar to those in Example 10.4. 

In general, another possibility is that primary nucleation leaves the solution substantially supersaturated yet particle growth is slow i.e. the system takes a long time to reach equilibrium. It may be possible to resolve these issues by combining spectroturbidimetry to detect changes in state of dispersion with nmr spectroscopy to estimate amounts of dissolved or precipitated surfactant. 

Texter, J., Organic particle precipitation, in Reactions and Synthesis in Surfactant Systems, Texter, J. (Ed.), Marcel Dekker, New York, 2001, pp. 577-607. 

It is difficult to summarize all the phenomena discussed in this volume. However, major topics include ultralow interfacial tension, phase behavior, microstructure of surfactant systems, optimal salinity concept, middle-phase microemuIsions, interfacial rheology, flow of emulsions in porous media, wettability of rocks, rock-fluid interactions, surfactant loss mechanisms, precipitation and redissolution of surfactants, coalescence of drops in emulsions and in porous media, surfactant mass transfer across interfaces, equilibrium dynamic properties of surfactant/oil/brine systems, mechanisms of oil displacement in porous media, ion-... 

Normally, retention of surfaetants, which involves adsorption, precipitation, and phase trapping, has been regarded as one of the main factors for the unfavorable economics in chemical flooding. Adsorption at the solid-hquid interface should be at minimum and be the only retention mechanism for a properly designed surfactant system for a Type II( —) phase behavior. Commercial products of actual surfactants are poly-disperse in the PO and EO groups, and mixtures of them are potential flooding chemicals. 

Successful attempts have been made to modify/minimize preeipitation in polyelectrolyte/oppositely charged surfactant systems. Laurent and Scott (65) reported such an effect with the addition of simple salts and defined a critical electrolyte concentration (c.e.c.) at which precipitation is totally inhibited. (See Chapter 5 and also Section III.E below.) Likewise, Dubin et al. (66,67) have found inhibitory effects on adding nonionic surfactants to these mixed polymer/surfactant systems, presumably a result of mixed micelle formation. 

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). 

The second (and third) models for polyelectrolyte/surfactant interaction are based on the solubility and phase characteristics of the mixed systems. The general form of the solubility diagram of a polyelectrolyte/oppositely charged surfactant system, as illustrated by the Polymer JR/TEALS combination, has been referred to (57). It showed an intermediate zone of precipitation, but clarity for high (and low) concentrations of the surfactant. The line representing systems of maximum insolubility in the log polymer/log surfactant concentration plot had a 45° slope indicating constant composition of the insoluble complexes. 

As noted in the reactive surfactant systems, mixing reduced the time needed to reach the value of nanoparticle uptake. Figure 17.6 shows that for the nonreactive system, a decrease in the nanoparticle concentration with time was observed followed by a plateau after around Ih of mixing at 300rpm. The decrease in nanoparticle concentration resulted from bulk precipitation of particles with sizes exceeding the stabilization capacity of the reverse micelles. Mixing improves the rate of aggregation of these particles, and hence they leave the colloidal suspension in shorter period. 

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

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]. 

However, contrary to other polyelectrolyte-surfactant systems [39-42], the precipitate does not redissolve with an excess of surfactant, at least in the examined, very broad, interval of concentrations. The difficulty of the redissolution of complexes composed of very highly charged polymers has also been observed in some other studies [41, 43, 44]. 

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