Anionic surfactants precipitation

Fig. 7 Equilibria between anionic surfactant precipitating with a hardness cation and surfactant monomer and micelles.

Esumi et al [68] used dispersions of a-alumina as well to study the interaction between anionic fluorocarbon and hydrocarbon surfactants. The anionic fluorocarbon surfactants used were LiFOS and NFIOO, the anionic hydrocarbon surfactants were SDS and LiDS, and the nonionic surfactant was NP7.5. Like the flocculation behavior of iron hydroxide, a low concentration of an anionic surfactant precipitates alumina. A further addition of a surfactant, different from the first one, forms mixed bilayers and redisperses alumina. Measurements of zeta potentials, the size of adlayers, and the amounts of adsorbed surfactants indicated that mixed bilayers consisting of anionic hydrocarbon-nonionic hydrocarbon surfactants or anionic fluorocarbon-nonionic hydrocarbon surfactants are formed preferentially to hydrocarbon-fluorocarbon surfactant bilayers. 

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. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

From the characteristics of the methods, it would appear that FD-MS can profitably be applied to poly-mer/additive dissolutions (without precipitation of the polymer or separation of the additive components). The FD approach was considered to be too difficult and fraught with inherent complications to be of routine use in the characterisation of anionic surfactants. The technique does, however, have a niche application in the area of nonpolar compound classes such as hydrocarbons and lubricants, compounds which are difficult to study using other mass-spectrometry ionisation techniques. 

EXPERIMENTAL PROCEDURES AUTOMATED TURBIDIMETRIC TITRATION. A method for the automated aqueous turbidimetric titration of surfactants has been published (10) in which anionic surfactants are titrated against N-cetylpyridinium chloride to form a colloidal precipitate near the equivalence point. N-cetylpyridinium halides have a disadvantage in that they have the tendency to crystallise out of solution (15), consequently the strength of the solution may alter slightly without the knowledge of the operator, also the crystals suspended in solution may cause damage to the autotitrator. In view of these drawbacks hyamine was preferred as the titrant. 

The ability of chitosan to form complexes is of particular interest. Being slightly basic, it will readily form complexes with anionic compounds. Initially it forms into micelles with small amounts of anionic surfactants, leading to precipitation of a complex as the concentration of the anionic surfactant increases. Chitosan will complex with anionic... 

The electrolyte effect for the adsorption of anionic surfactants which leads to an enhancement of soil removal is valid only for low water hardness, i.e. low concentrations of calcium ions. High concentrations of calcium ions can lead to a precipitation of calcium surfactant salts and reduce the concentration of active molecules. Therefore, for many anionic surfactants the washing performance decreases with lower temperatures in the presence of calcium ions. This effect can be compensated by the addition of complexing agents or ion exchangers. 

The eluate from the FBBR inhibited with formaldehyde shows an initial rapid loss of LAS followed by a steady loss until day 7, after which the concentration of LAS remains stable at about half the value in the reservoir (Fig. 5.1.7). No degradative products were detected in this experiment, and therefore the loss can be attributed to sorption. However, it is important to note that precipitation of magnesium and calcium salts can also contribute to the loss of the anionic surfactant [94]. 

LDAO/SDS Interaction. Mixing of cationic and anionic surfactant solutions results In the formation of a mixed species that Is more surface active than the Individual species. The enhanced synergistic effect has been explained (2,3) by showing that a close-packed adsorption of electroneutral R R takes place (R" " and R represent the long chain cation and anion respectively). In the case of Ci2 and C14-DAO, a 1 1 LDAO/SDS molar ratio produces a minimum In surface tension and Is accompanied by an Increase In pH In the bulk solution the association seems to be of the type R R", and the absence of visible precipitate may be attributed to the solubilization of the R R" complex In the solution. In the region where LDAO Is In excess, the structure Is probably [cationic (LDAOH ) anionic (SDS)] nonlonlc (LDAO), while [cationic (LDAOH anionic (SDS)] anionic (SDS) Is formed when SDS Is In excess. Equal molar concentration results In cationic (LDAOH ) anionic (SDS) complex which should favor precipitation. However, at pH >9, there Is no Indication of precipitation (even when the total solute concentration Is 0.35 M). When the pH Is below 9, then precipitation will take place. 

JafvertC.T. and J.K. Heath (1991). Sediment-and saturated-soil-associated reactions involving an anionic surfactant (dodecylsulfate). 1. Precipitation and micelle formation. Environmental Science and Technology 25 1031-1039. 

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... 

Anionic surfactants complexation, charge neutralization, aggregation, precipitation, structure formation... 

Some carboxylate surfactants, such as long-chain fatty acids or their anionic esters with Coenzyme A, are precipitated in the presence ( fCSmd M + (Constantinides and Steim, 1986). Measurements of CMC in the presence of divalent ions should be avoided since the insoluble surfactant could introduce serious artifacts. Traces of transition-metal ions can catalyze autooxidation of some polyoxyethylene surfactants. In a recent article by Xiao (Xiao, 2006), interactions between anionic surfactant (SDS) micellar solutions and several familiar metal salt solutio b. 

Under environmental conditions, the solubility of anionic surfactants, especially of fatty acids, is affected remarkably by the presence of divalent cations. This is reflected in the dependence of the CMC on the counterion. Ca2+ and Mg2+ tend to precipitate anionic surfactants however, the solubility products of Ca(LAS)2 are relatively high (>1013 M 2, Matheson et al., 1985), so that precipitation of LAS is unlikely, given that LAS occurs in environmental waters usually at concentrations of less than lpM. 

Generally speaking, for a stable emulsion a densely packed surfactant film is necessary at the interfaces of the water and the oil phase in order to reduce the interfacial tension to a minimum. To this end, the solubility of the surfactant must not be too high in both phases since, if it is increased, the interfacial activity is reduced and the stability of an emulsion breaks down. This process either can be undesirable or can be used specifically to separate an emulsion. The removal of surfactant from the interface can, for example, be achieved by raising the temperature. By this measure, the water solubility of ionic surfactants is increased, the water solubility of non-ionic emulsifiers is decreased whereas its solubility in oil increases. Thus, the packing density of the interfacial film is changed and this can result in a destabilisation of the emulsion. The same effect can happen in the presence of electrolyte which decreases the water solubility mainly of ionic surfactants due to the compression of the electric double layer the emulsion is salted out. Also, other processes can remove surfactant from the water-oil interface - for instance a precipitation of anionic surfactant by cationic surfactant or condensing counterions. 

The expected trends are born out for the low molecular weight enzymes ribonuclease-a, cytochrome-c, and lysozyme, as shown in Figure 2. These results are presented as the percentage of the protein transferred from a 1 mg/ml aqueous protein solution to an equal volume of isooctane containing 50 mM of the anionic surfactant Aerosol 0T, or AOT (di-2-ethylhexyl sodium sulfosuccinate). As anticipated, only at pH s lower than the pi was there any appreciable solubilisation of a given protein, while above the pi the solubilisation appears to have been totally suppressed. Note, however, that as the pH was lowered even further, there was a drop in the degree of solubilisation of the proteins. This was accompanied by the formation of a precipitate at the interface between the two phases, attributed to a denaturation of the protein. 

Aqueous pH alters the protein charge property and affects the extraction efficiency. Haemoglobin (Mw 64,500, pi 6.8) is a difficult protein in terms of being able to completely extract it into reverse micelles. The representative anionic surfactant, di-2-ethylhexyl sulfosuccinate (AOT), cannot extract it, and gives rise to an interfacial precipitate. In contrast, we succeeded in the complete extraction of haemoglobin using synthetic anionic surfactants, dioleyl phosphoric acid (DOLPA), as seen in... 

Although AOT is also an anionic surfactant of the same type as DOLPA, haemoglobin cannot be transferred into the AOT reverse micellar phase, and most haemoglobin can be seen at the oil-water interface as a red precipitate. Adachi and Harada have reported that cytochrome c precipitated as a cytochrome c-AOT complex at low concentrations of AOT [7]. It was found that this precipitate is likewise the AOT-haemoglobin complex (AOT/haemoglobin = 120 1) from the results of elemental analysis [8]. These results indicate that the difference in the extraction ability of DOLPA and AOT might depend on the hydrophobicity of the surfactants provided to the hydrophilic proteins. 

Benzethonium chloride is incompatible with soaps and other anionic surfactants and may be precipitated from solutions greater than 2% w/v concentration by the addition of mineral acids and some salt solutions. 

High-performance liquid chromatography is performed using a Hewlett-Packard 1090 chromatograph equipped with a ternary-solvent delivery system, an autoinjector with a 0 -20- u.L injection loop, an oven compartment, and a diode-array UV detector. An ELS detector (Alltech Associates, Deerfield, IL) is connected in series to the UV detector. Hexane, 2-propanol, and water were used for the analysis of nonionic surfactants. Water and tetrahydrofuran (THF) are used for the analysis of anionic surfactants. No preliminary sample preparation is used other than dilution. The nonionic surfactants are diluted 1 40 (v/v) with hexane. The anionic surfactants (alkyl ether sulfates and synthetic and petroleum sulfonates) are diluted 1 20 (v/v) with water-THF (50 50). The calcium sulfonate surfactants were diluted 1 20 (v/v) with a THF-38% hydrochloric acid solution of pH 1. Hydrochloric add is required to prevent salt precipitation by converting any excess water-insoluble caldum carbonate into water-soluble calcium chloride. All diluted samples are... 

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