Ionic surfactant systems, salinity effect

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. 

The investigation technique is the unidimensional formulation scan as in the phase behavior. studies discussed in the previous chapter. For the sake of simplicity, the scanned variable is often taken as the salinity for ionic surfactant systems, and as surfactant EON or temperature for nonionic systems, but it should be well understood that other formulation variables would produce exactly the. same effects. In the reasoning, the formulation will be referred to a.s SAD, the deviation from optimum formulation, whatever the variable used to produce the scan. 

For an ionic surfactant such as AOT, increasing salinity leads to a 2-3-2 transition. The AOT-brine-propane system conforms to this classical behavior in several ways. For example, increases in the salinity of the aqueous phase increase the proportion of AOT in the propane phase. At high pressures, the size of the reverse micelles (as reflected by JVo) decreases as salinity increases. The pressure at which the 2-3 transition occurs decreases as salinity increases, an indication of increasing surfactant affinity for the propane phase. One interesting aspect of the AOT-brine-propane system is that the amount of NaCI required to effect phase changes appears to be higher than is the case in conventional liquid solvents. For propane at 310 bar and 37°C, about 1.1 wt% salt is required to drive AOT into the propane phase. For the heptane-brine-AOT system at the same concentration, only 0.5 wt% salt is required to achieve the same effect [36]. This result suggests that propane is a weaker solvent for AOT than heptane. 

If the actual formulation change was not due to a variation in oil ACN or EACN. the equivalent change in ACN units is readily found by applying the cotrelalion. For instance, a AACN — +10 units is equivalent to a hvefold decrease in salinity so that A In S = -0.16 x 10 = —1.6. a change in surfactant EPACNUS = Ao/lf = —10. which i.s equivalent to remove 4.4 carbon atoms from the ionic surfactant tail, or to increase AEON = +1.6 EO group in the polyeihcr chain. It also corresponds to an increase of about I60°C for ionic. surfactants and a A7 change of about -20 to -40 C (decrease) for nonionic systems, or to the removal of u lipophilic alcohol effect/(A) or 0(A) = -1.6 equivalent to about 1.5 (respectively 2.5) % of ri-pentanol or 0.4 (respectively 1.4) % n-hexanol, for ionic (respectively nonionic) systems. 

This effect of temperature has been recognized first on nonionic surfactant systems, and the temperature is still the choice variable to study nonionic surfactant systems phase behavior [51,56-58]. According to the Winsor approach, it is clear that the temperature is likely to change not only Acw but all other interactions as well, a situation that does not simplify the interpretation of experimental data. Moreover, the temperature range that can be experimentally handled without complication is not very wide, because it matches the liquid state of water. This is why the electrolyte concentration (salinity) and ethylene oxide number (EON) have been often preferred by experimentalists as scanning variables for ionic and nonionic systems, respectively, as their effect is easier to predict and interpret. 

With an ionic system, such displacement results from an increase in salinity of the aqueous pha.se, a reduction of the oil EACN either by using a shorter alkane or by adding polar or aromatic components, the addition of some lipophilic alcohol starting with n-pcntanol up to a few vol less with hexanol or higher alcohols. Finally the surfactant may be mixed with a less hydrophilic surfactant of the sante family (e.g.. with a longer tail). If dte system contains nonionic surfactants an increase in temperature would decrea.se the hydrophilic interaction quite rapidly while the electrolyte effect would be alnntsl negUgihIc. with the exception of polyvalent cation salts. 

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