Surfactant divalent effects

The factors that affect phase separation discussed in this section include anion effect, divalent effect, alkaline effect, mixing effect of interstitial flow, and the synergy of mixed surfactants. 

Effect of Ca2. In many reservoirs the connate waters ontain substantial quantities of divalent ions (mostly Ca . In alkaline flooding applications at low temperatures, the presence of divalent ions leads to a drastic increase in tensions r35,36]. Kumar et al. f371 also found that Ca and Mg ions are detrimental to the interfacial tensions of sulfonate surfactant systems. Detailed studies at elevated temperatures appear to be non-existent. 

While for nonionic systems, the effect of inorganic salt on CMC is small, it is large for ionic surfactants and this difference is expected since the micelle - small ion interaction must be completely different in the two cases. The decrease in CMC for ionic systems corresponds usually to a linear relation between the logarithm of the CMC and the total counterion concentration, if a salt of the counterion is added. For divalent ions, the slope of the plot of log CMC versus counterion concentration is half of that of monovalent ions. 

The fructosyltransferase from A. aculeatus displayed activity without the addition of any metal ions. However, some effects were observed in its susceptibUity to mono- and divalent cations [33]. For ejample, Mn, K, and Co caused a 1.4—1.9-fold increase in the activity, whereas low concentrations of Hg and Zn produced 35-60% inhibition. It was also found that the enzyme was slightly activated by several non-ionic and anionic surfactants such as sodium dodecylsulphate (1.5-fold at lOmM), sodium deoxycholate (1.4-fold at ImM), and Triton X-100 (1.4-fold at 5% w/v). Moreover, it was resistant to low concentrations (1-lOmM) of reducing agents such as dithiothreitol and P-mercaptoethanol. 

Uncommon IPRs were tested recently. Polymerized acyl monoglydnate surfactant was found to be as effective as sodium dodecylsulfate for the resolution of organic amines [126]. For the analysis of pyridine-based vitamins in infant formnlas, dioc-tylsulfosuccinate produced a unique retention pattern [133], Among bizarre IPRs, tris(hydroxymethyl)aminomethane was used for the determination of cyclamate in foods. It was selected over different ion-pair reagents such as triethylamine and dibu-tylamine, based on sensitivity and time economies [134]. Hexamethonium bromide, a divalent IPR, was used successfully to separate sulfonates and carboxylates [135]. 

In addition to divalent metal cations, trivalent and tetravalent cations (i.e. ln +, Ga +, Sb +, and Sn +) were also effective as linking agents to organize [Ge4Sio]" clusters to form hexagonally ordered mesostractures. In this case, cetylpyridinium bromide was nsed as the surfactant, and formamide served as the solvent. The mesophases made with Ga + and Sb + showed intense visible photoluminescence at77K. 

There are several factors that can affect a given surfactant s performance in a reservoir environment. First, the effect of inorganic ions is significant. Most oil reservoirs have an aqueous phase of saline brine that may vary in concentration from 0.5% to upwards of 15% NaCl. Also, there are divalent ions, such as Ca" "" " and Mg "" " present in significant concentrations. Most of the experimentation, which serves as the basis for this paper, was conducted utilizing a brine of 3% NaCl with 100 ppm (mg/1) of Ca" "" ". This composition is typical of many natural reservoir brines, and those surfactants that will perform well with this brine will also do well in the majority of reservoirs. 

Consistency. Lecithins are available in both fluid and plastic (solid) forms. Fluid lecithins generally follow Newtonian flow characteristics. The viscosity profile of lecithins is a complex function of acetone-insoluble content, moisture, mineral content, acid value, and the combined effects of assorted additives such as vegetable oils and surfactants. Generally, higher AI and/or moisture content yields higher viscosity, whereas an increased AV often decreases viscosity. Certain divalent minerals, such as calcium and others, can also adjust the viscosity level. 

Variables identified as important in the achievement of the low IFT in a W/O/S/electrolyte system are the surfactant average MW and MW distribution, surfactant molecular structure, surfactant concentration, electrolyte concentration and type, oil phase average MW and structure, temperature, and the age of the system. Salager et al. (1979b) classified the variables that affect surfactant phase behavior in three groups (1) formulation variables those factors related to the components of the system-surfactant structure, oil carbon number, salinity, and alcohol type and concentration (2) external variables temperature and pressure (3) two-position variables surfactant concentration and water/oil ratio. Some of the factors affecting IFT-related parameters are briefly discussed in this section. Some other factors, such as cosolvent, salinity, and divalent, are discussed in Section 7.4 on phase behavior. Healy et al. (1976) presented experimental results on the effects of a number of parameters. 

Figure 13.6 shows the surfactant concentration remaining in a solution at different divalent concentrations. From this hgure, we can see that the Ca effect... 

The most important effect of electrolytes is the effect of Ca2+ and Mg2+ ions on dishwashing performance. Numerous patents deal with the beneficial effects of both ions on dishwashing performance. It is thought that these divalent ions form a complex with anionic surfactants. This complex allows the anionic surfactant to adsorb more readily on surfaces with a negative surface charge. 

This brings up the important point that the choice of counterion is also key for efficacy of the main anionic surfactants used in the cleaning formula. It has been known for some time that divalent metal salts of alkylbenzene sulfonate, paraffin sulfonates, and the like are better grease cleaners than the analogous sodium salts [87,88], These are more difficult to use due to their lower water stability, but they can be formulated with some of the more effective grease cutting solvents [89-90]. It has also been claimed that if ammonium salts of the anionic surfactants are used less residue is left on the surface [91]. 

The salinity effect of different salts, particularly divalent cation salts, is expressed through the term bS in the correlation for non-ionic surfactants of the polyethoxylated phenol or alcohol type. No information is available yet on the salinity effect on other non-ionics such as alkyl-polyglucosides. The salinity effect on ionic surfactant systems is a more complex issue because the surfactant itself is also a (more or less) dissociated electrolyte. Its degree of dissociation is paramount as far as its hydrophilicity is concerned. For instance sodium salts of alkyl sulphonic acids are essentially completely dissociated, hence they act as the sulphonate ion, and it is essentially the same with the salt of potassium or ammonium. The presence of multivalent anions produces an interference with the monovalent anionic surfactant ion, such as an alkyl benzene sulphonate, but it is essentially an ideal mixing rule. 

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. 

Retention in Porous Media. Anionic surfactants can be lost in porous media in a number of ways adsorption at the solid—liquid interface, adsorption at the gas—liquid interface, precipitation or phase-separation due to incompatibility of the surfactant and the reservoir brine (especially divalent ions), partitioning or solubilization of the surfactant into the oil phase, and emulsification of the aqueous phase (containing surfactant) into the oil. The adsorption of surfactant on reservoir rock has a major effect on foam propagation and is described in detail in Chapter 7 by Mannhardt and Novosad. Fortunately, adsorption in porous media tends to be, in general, less important at elevated temperatures 10, 11). The presence of ionic materials, however, lowers the solubility of the surfactant in the aqueous phase and tends to increase adsorption. The ability of cosurfactants to reduce the adsorption on reservoir materials by lowering the critical micelle concentration (CMC), and thus the monomer concentration, has been demonstrated (72,13). 

The effect of divalent cations on surfactant adsorption is shown in Figure 14, which provides a comparison of adsorption levels on several solids measured in sodium chloride brine with those measured in brines containing sodium chloride and divalent cations. The ionic strength of all brines is constant at 0.403 mol/L, thus ionic strength effects are eliminated. Evidently, the dependence of surfactant adsorption on divalent ions varies with the type of surfactant and rock. In most cases, adsorption is increased by the presence of divalent cations. Adsorption of the sul-fobetaine is less sensitive to divalent cations than adsorption of the betaine and the anionic surfactants. Adsorption of three surfactants on dolomite is not influenced very strongly by divalent cations. 

Another factor which affects IFT is hardness or the presence of divalent metal cations, because they can couple with the surfactants forming less active species in solution. Sodium silicates help maintain the lowest IFT possible, since they are very effective in sequestering and removing these ions. In most cases these divalent ions can never be completely removed from the reservoir systems because clays are present which continually reintroduce hardness into solution... 

The effect of divalent cations is not amenable to empirical expression, probably because these cations influence the degree of dissociation of the salt, and thus the hydrophilicity of the polar group, in a complex way. It is found that the calcium and magnesium salts of most anionic surfactants are (much) less hydrophilic than their sodium counterparts as a general trend. Sulfonates are less sensitive than sulfates, which are less sensitive than carboxylates [47-49]. 

Not many investigations have been dedicated to the salinity effect when the electrolyte is not sodium chloride, maybe becau.se the effect is more complex, not amenable to a simple expression in paiilcular with divalent catioiis and ionic surfactants. However some trends are available in applied publications (27,6.1), Worth noting is a systematic study of the effect of the electrolyte anion on the equivalent salinity of different sodium salts, that showed that the correlation is followed for all sodium salts and that the effective or equivalent molar salinity only depends on the valency of the anion t64). 

Although within a given valence the size of the hydrated counterion will have some effect on the miceUization of an ionic surfactant, a more significant effect is produced by changes in valence. As the counterion is changed from monovalent to di- and trivalent, the cmc is found to decrease rapidly. The divalent and higher salts of carboxylic acid soaps generally have very low water solubility and are not useful as surfactants in aqueous solution. They do find use in nonaqueous solvents because of their increased solubility in those systems, especially in the preparation of water-in-oil emulsions. 

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