Precipitates, surfactants

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,... 

The adsorption densities ( r ) on minerals (C< CMC) of the salt type are in some cases higher because of precipitation of the ionic surfactant with multivalent cations in the bulk phase. Measurements were carried out to determine the fraction f the precipitated surfactant by divalent cations Ca and Ba " leading to a decrease in its equilibirum concentration. They showed a shift of the adsorption maximum towards lower values of r, even after a correction of the adsorption density due to the precipitation. On the other hand, a direct co-adsorption of the precipitated surfactant on a mineral surface cannot be excluded. 

Many surfactants have been suggested as candidates for CO2 foam. However, at high salinity and temperature in the presence of oil, most surfactants foam poorly due to partitioning and emulsion formation and fail to control mobility during CO2 injection. This behavior is analogous to that observed in chemical (microemulsion) oil recovery (5-1). As the salinity, hardness and temperature increase, surfactants form water/oil emulsions, precipitate surfactant-rich coacervate phases, and partition into the oleic phase. CO2 decreases further the solubility of surfactant in the aqueous phase. 

Significance For Equilibrium Adsorption Measurements. While these results appear to have little relevance for micellar slugs, they are quite relevant for equilibrium adsorption measurements. They show that precipitation of surfactants can be expected to occur if the calcium ion concentration exceeds the limit required to initiate precipitation (SDBS 200 ppm and TRS 10-410 < 10 ppm). Unless the adsorbent is strongly colored and the surfactant concentration is substantial, i.e., > 0.1 wt.%, one may experience difficulty in detecting the presence of precipitated surfactant. 

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.

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. 

Indirect spectrophotometric methods are in most cases based on the formation of ion pairs that are extractable into organic solvents. These methods are often used in combination with volumetric and gravimetric methods, as many precipitated surfactant complexes can be dissolved in the appropriate solvents and analyzed colorimetrically. The spectrophotometric determination of the end-point in two-phase titration is often carried out. 

Other CTAs and inhibitors also tried (about 0.05 M) were anionic mer-captoacetate and mercaptosuccinate and cationic isopropylamine, 1-dimeth-ylamino-2-propanol, cysteamine hydrochloride, and copper sulfate. The pH was adjusted as necessary to yield the desired ions. None of the ionic CTAs reduced the measured PLMA MW, although mercaptoacetic acid at a pH of 2.2 (mixed anionic and neutral species) reduced the molecular weight almost to that obtained with mercaptoethanol. The ionic CTAs may increase the micelle aggregation number (electrolyte effect). Cu(II) precipitated surfactant, but the other solutions were clear. 

Mesoporous silica MCM-41 was prepared following the synthesis procedure reported previously [8]. MCM-41 material was synthesized, using tetraethyl orthosilicate (TEOS) as a silica source, octadecyltrimethylammonium bromide (C18TMAB) as a template and ammonia as the catalyst. The precipitated surfactant-silica mesophase was filtered, washed with distilled water and dried at 323K. Part of as-synthesized samples was next calcined at 823K in air atmosphere and additionally thermally treated in oxygen to remove the carbon deposits located inside the channels. 

Endo-exo ratios of the micelle-catalysed reactions have been determined by adding 0.25 mmol of 5.1c and 0.5 mmol of 5.2 to a solution of 5 mmol of surfactant and 0.005 mmol of EDTA in 50 ml of water in carefully sealed 50 ml flasks. The solutions were stirred for 7 days at 26 C and subsequently freeze-dried. The SDS and CTAB containing reaction mixtures were stirred with 100 ml of ether. Filtration and evaporation of the ether afforded the crude product mixtures. Extraction of the Diels-Alder adducts from the freeze-dried reaction mixture containing C12E7 was performed by stirring with 50 ml of pentane. Cooling the solution to -18 C resulted in precipitation of the surfactant. Filtration and evaporation of the solvent afforded the adduct mixture. Endo-exo ratios... 

In the removal of contaminating ions such as (PO or Fe " a precipitate such as Ca2(P0 2 Fe(OH)2, after oxidizing ferrous ion to ferric, is formed and the soHd is removed. The addition of surfactants is usually not essential (nor desirable) since most waters contain natural surfactants that would render the soflds sufficiently hydrophobic for flotation to occur. Such surfactants derive from the degradation of organic matter, and humic substances abundantly available in nature (30). 

Sequestration forms the basis for detergent and water-treatment appHcations of polyphosphates. Sequestration of hardness ions by sodium tripolyphosphate used in detergent formulations prevents the precipitation of surfactants by the hardness ions. Sodium polyphosphate glass (SHMP) may be added to water system to prevent the formation of calcium or magnesium scales by reducing the activity of the hardness ions. However, if the ratio of cation to polyphosphate is too high at a given pH, insoluble precipitates such as may result instead of the soluble polyphosphate complexes. The... 

The resulting solutions contain high dissolved soHds content in the range of 30 wt % or more. Special surfactant technology (25) is sometimes used to avoid precipitation of Al(OH)3 or at least to extend the shelf life of the caustic Hquor. 

In acidic media, amine oxides and anionic surfactants form precipitates the CMC is much greater than in neutral or alkaline media. Change in CMC parallels change from ionic to nonionic form. Amine oxides are stable in formulated detergent products and do not act as oxidizing agents. Composition and function of representative commercial amine oxides are given in Table 26. 

In some cases, dye-forming moieties attached to a polymeric backbone, called a polymeric coupler, can replace the monomeric coupler in coupler solvent (51). In other reports, very small particles of coupler solubilized by surfactant micelles can be formed through a catastrophic precipitation process (58). Both approaches can eliminate the need for mechanical manipulation of the coupler phase. 

In most cases, these active defoaming components are insoluble in the defoamer formulation as weU as in the foaming media, but there are cases which function by the inverted cloud-point mechanism (3). These products are soluble at low temperature and precipitate when the temperature is raised. When precipitated, these defoamer—surfactants function as defoamers when dissolved, they may act as foam stabilizers. Examples of this type are the block polymers of poly(ethylene oxide) and poly(propylene oxide) and other low HLB (hydrophilic—lipophilic balance) nonionic surfactants. 

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