Surfactant precipitating

AOS 2024 adsorption increases continuously on going from fresh water to 1% NaCl to 4% NaCl. Aqueous 1% NaCl was studied as an analog to aqueous 1% sodium sulfate. The last two entries of Table 19 show that increased AOS 2024 adsorption in the presence of sodium sulfate could mitigate any reduction in calcium ion-promoted surfactant precipitation. However, the larger than usual experimental uncertainty in the sodium sulfate results means that the... 

It has been reported by Celik and Somasundaran T381 that the interaction of divalent (and trivalent) cations with sulfonate surfactants causes surfactant precipitation followed by dissolution of the precipitate at higher concentrations. The precipitate redissolution phenomenon is not observed with monovalent ions. Indeed, some surfactant precipitation in the spinning drop tube was observed above concentrations corresponding to the first minimum of Figure 8 it is not known whether redissolution took place at higher concentrations resulting in the second tension minimum. 

In reverse, the surfactant precipitates from solution as a hydrated crystal at temperatures below 7k, rather than forming micelles. For this reason, below about 20 °C, the micelles precipitate from solution and (being less dense than water) accumulate on the surface of the washing bowl. We say the water and micelle phases are immiscible. The oils re-enter solution when the water is re-heated above the Krafft point, causing the oily scum to peptize. The way the micelle s solubility depends on temperature is depicted in Figure 10.14, which shows a graph of [sodium decyl sulphate] in water (as y ) against temperature (as V). 

Surfactant-polymer flooding, 13 628 Surfactant precipitation, in volumetric sweep efficiency, 13 621 Surfactant propagation, in enhanced oil recovery, 13 629... 

The solution is then transferred into the coupling vessel equipped with a mechanical stirrer and, possibly in the presence of a surfactant, precipitated with acetic acid, hydrochloric acid, or phosphoric acid. The coupling component may also be precipitated indirectly i.e., the appropriate mixture of acid and emulsifier is filled into the kettle first and the alkaline solution of the coupling component is then added gradually to the clear solution by gravity flow. The clarified solution of the diazonium compound is then introduced into or onto this coupling suspension. 

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,... 

There have been two general approaches to studying surfactant precipitation measurement of the Krafft... 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

Studies of surfactant precipitation have concentrated on predicting the precipitation phase boundaries (i.e., the amount of added electrolyte necessary to cause... 

Systems in which surfactant precipitate is present in substantial quantities in equilibrium with micelles and monomer are of interest. For example, in a technique for improving mobility control in oil reservoirs, surfactant is purposely precipitated in the permeable region of a reservoir to plug it (44). When substantial precipitate is present, crystals of different composition can be in simultaneous equilibrium. Experimental study and modeling of these systems where several Ksr- relationships are simultaneously satisfied will be a challenging task. 

The size of surfactant precipitate structures may vary widely, from colloidal precipitate, too small to be seen visually, to large crystals. This is an important property, because some deleterious effects of surfactant precipitation could be minimized or beneficial effects of precipitation could be negated if the precipitate were of colloidal dimensions. Examples of the importance of precipitate structure and size include the aforementioned oil field process and recovery of surfactant from surfactant-based separations via precipitation-fiItering. Effects of using surfactant mixtures on precipitate structures would be useful to know. 

For ionic surfactants micellization is surprisingly little affected by temperature considering that it is an aggregation process later we see that salt has a much stronger influence. Only if the solution is cooled below a certain temperature does the surfactant precipitate as hydrated crystals or a liquid crystalline phase (Fig. 12.4). This leads us to the Krafft temperature1 also called Krafft point [526]. The Krafft temperature is the point at which surfactant solubility equals the critical micelle concentration. Below the Krafft temperature the solubility is quite low and the solution appears to contain no micelles. Surfactants are usually significantly less effective in most applications below the Krafft temperature. Above the Krafft temperature, micelle formation becomes possible and the solubility increases rapidly. 

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. 

Typical surfactant-water-phase diagrams are shown in Fig. 3.4 for single-chained ionic, and non-ionic surfactants respectively. Below a "Krafft" temperature characteristic of each surfactant, the chains are crystalline and the surfactant precipitates as a solid. Increased surfactant concentration (Fig. 3.4) results in sharp phase boundaries between micellar rod-shaped (hexagonal), bilayer (lamellar) and reversed hexagonal and reversed micellar phases. (The "cubic" phases, bicontinuous, will be ignored in this section and dealt with in Chapters 4,5 and 7.)... 

When we introduced phase behavior tests earlier, we mentioned aqueous stability tests. The main objective of aqueous stability tests is to eliminate the surfactant precipitation problem. As we already know, the solubility of surfactant decreases with salinity. During aqueous stability tests, the surfactant solution becomes opaque up to some salinity, showing the surfactant starts to aggregate or even precipitate. When divalent or multivalent ions exist in the solution, the salinity needed to start precipitation is much lower. 

If the surfactant concentration is increased, the solution will also become opaque, as shown in Figure 7.41. In the figure, the reduction in light transmittance through the solution represents the degree of surfactant precipitation in... 

Somasundaran, R, Cehk, M.S., Goyal, A., Manev, E.D., 1984b. The role of surfactant precipitation and redissolution in die adsorption of sulfonate on minerals. SPEJ (April), 233—239. Somasundaran, R, Hanna, H.S., 1977. Physico-chemical aspects of adsorption at sohd/hquid interfaces, I. Basic principles. In Shah, D.O., Schechter, R.S. (Eds.), Improved Oil Recovery by Surfactant and Polymer Hooding. Academic Press, pp. 205—251. 

Salinity Salinity plays at least two important roles, namely it maintains the integrity of the reservoir and it balances the physicochemical environment so that surfactant formulation stays close to optimal. Thus, ultra-low interfacial tension and oil solubilisation are very sensitive to salinity. Mixing of the surfactant slug with connate water may alter the surfactant formulation mainly due to dilution and to the incorporation of new electrolytes to the formula. Adsorption and desorption of electrolytes, particularly divalent cations, onto or from solid materials such as clay, will also change the salinity of the aqueous phases to some extent and may cause surfactant precipitation, which is however not always an adverse effect [151]. In order to attenuate the undesirable salinity effects on formulation, surfactants able to tolerate salinity changes [109], high salinity [150] and the presence of divalent ions [112] maybe used. 

A low salinity water drive could be injected to induce divalent-ion desorption from the porous medium through ion exchange and their subsequent washing away to avoid the main surfactant precipitation as a calcium salt. 

Cation Exchange, Surfactant Precipitation, and Adsorption in Micellar Flooding... 

In the course of attempts to determine adsorption isotherms of anionic surfactants on selected clays two other phenomena requiring separate investigation were noted, namely, salting-out of surfactants by NaCl, and surfactant precipitation as calcium or magnesium salts by multivalent cations displaced from clays. 

These SEM, then, show clearly the typical geometric and mineraological heterogeneity of reservoir rocks. They also give one an idea of the shapes and sizes of clay and mineral crystals, and they suggest intimacy of contact between micellar fluid and clay crystals. Finally, in some cases they suggest the possibility of surfactant precipitation, e.g., dolomite in the San Andres core sample. 

Mississippi montmorillonite in the ratio of 10 ml of liquid per gram of clay. After equilibration and centrifugation to separate the solids, it was noted that there were two layers of solids the lower one was tan in color and due to the clay the upper solid layer was an off-white color and it was later shown to consist mostly of surfactant precipitated as calcium and magnesium sulfonates. 

Experimental Results with Purified Surfactants. A similar series of experiments was carried out with desalted and deoiled alkylbenzene sulfonates. Aside from the apparently smaller solubility of the purified surfactants, the results were essentially the same as for the crude surfactants. The calcium ion concentration necessary to initiate precipitation from SDBS solutions was about 200 ppm. For purified SPBS it was 10 to 20 ppm, and for purified TRS 10-410 it was about 5 ppm. The onset of precipitation was relatively insensitive to temperature in the range 25-60°C, and little affected by the addition of 3 wt.% of either n-butanol or 2-butanol. It should be noted that precipitation occurred in all cases in spite of the fact that the surfactant concentrations were well above the CMC. In short, fixation of calcium ions by micelles did not prevent surfactant precipitation in these experiments. 

Experiments like the one just described can best be summarized by extracting from each alkane scan the values of n. and y. . A plot of n. vs. y. can then be made and is ound toi Be independent o nsalini yn( within the range from 0.25 wt% to the point where the surfactant precipitates) and surfactant molecular weight (15,16). The shape of the plot is little affected by addition of an alcohol cosurfactant (14) and is the same for mixed and monoisomeric surfactants of the same structure (14-16). 

Moreover, these surfactants precipitate at moderate salinities, making them unsuitable for foam applications in many Canadian reservoirs currently subjected to hydrocarbon-miscible flooding. Although surfactants that remain soluble and form effective mobility-control foams in near-saturated salt solutions have been identified (2), these surfactants adsorb at moderate to high levels under some conditions. Therefore, consideration of adsorption is extremely important in the selection of foamforming surfactants. 

Surfactant adsorption on saltlike minerals, such as calcite and dolomite, is a more complex process and is less understood than adsorption on oxide surfaces. These minerals are relatively soluble and when in contact with an aqueous medium develop an interfacial region of complex composition (41—43). In addition to the two mentioned mechanisms of adsorption, covalent bonding, salt formation between surfactant and lattice ions at the solid surface, ion exchange of surfactant with lattice ions, and surface precipitation have been suggested as adsorption mechanisms (36, 43—47). The dissolution products of sparingly soluble minerals may interact with the surfactant, precipitate or adsorb at the solid surface, or lead to mineral transformations that affect surface composition and electrochemical properties (46, 48—52). All these factors can be expected to influence surfactant adsorption. 

An increase in electrolyte concentration reduces the solubility of anionic surfactants in the aqueous phase and increases their tendency to accumulate at the solid—liquid interface. An increase in temperature offsets the loss in solubility to some degree For the DPES—AOS on Berea sandstone, the slopes of the lines in Figure 13a decrease as the temperature increases, and this finding lends support to the hypothesis that surfactant adsorption is related to surfactant solubility. Adsorption of surfactants that are less salt-tolerant than the DPES—AOS, such as the AOS and the IOS, increases much more steeply with salinity. Both surfactants adsorb negligibly at salinities of 0.5 mass % NaCl, but adsorb similarly to the DPES—AOS at a salinity of 2.3 mass %. At moderate salinities (on the order of 3 mass %), these surfactants precipitate, which severely limits their applicability to foam-flooding in many reservoirs that are currently being flooded with hydrocarbon solvents. 

Some hydrophobic cations, such as tetrabutyl ammonium, appear to have the opposite action on the CMC. OH acts as normal anion whereas practically has no effect up to ca 0.5 M and it rise the CMC at higher concentrations in view of the ether oxygen protonation of the ethylene oxide chain. As a result of electrolyte addition, strong specific binding of ions may make surfactants sometimes more soluble and, in contrast, it may cause coacervation. Surfactant precipitation or strong aggregation occurs in many cases. 

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