Solubility of surfactants

The Krafft point has practical implications for the solubility of surfactants. Only above the Krafft temperature can concentrated surfactant solutions be prepared. Otherwise, on cooling a hot surfactant solution a sudden precipitation may occur. A linear correlation between the Krafft temperature TK (°C) and the carbon number nc of sodium alkanesulfonates C10-C22 is given by the following equation ... 

The high water-solubility of surfactants and their, often more polar, metabolites prevents direct application of gas chromatographic separation (GC) with appropriate detection. The necessary volatilisation without thermal decomposition can be achieved by derivatisation of the analytes, but these manipulations are time- and manpower-consuming and can be susceptible to discrimination. Additionally, each derivatisation step in environmental analysis is normally target-directed to produce volatile derivatives of the compounds to be determined. Unknown surfactants that are simultaneously present, but differ in structure and therefore cannot react with the derivatisation reagent, are discriminated under these conditions. 

Partition Coefficients of nonvl-phenyl-poly-(ethoxy)-ethanol (NPE) Surfactants. The solubility of surfactants in water and hydrophobic solvents is well documented (11,12,22), but only a few attempts at measuring partition coefficients between immiscible liquids have been reported (2,4,9,10). Partition coefficients of surfactants are of theoretical interest because of their relation to observed surfactant properties such as emulsification, wetting and detergency. Partition coefficients (K ) may be also of considerable practical value for predicting surfactant recov and recycling in industrial processes. For example, in the cold water extraction of tar sand, an effective surfactant with a high Kp could be efficiently recycled in the process water and would not follow the bitumen into the upgrading stream. 

Similarly to the solubility of active drugs, the solubility of surfactants that were used in CFC systems has significantly changed. Surfactant solubility in HFA 134a ranges from 0.005% to 0.02% w/v, much lower than the concentration required to stabilize suspensions (0.1-2.0% w/v) (24,42). The surfactants can be solubilized with the addition of cosolvents such as ethanol. However, it is most likely that cosolvents will be incompatible with suspension formulations because drug solubility will also be promoted and crystal growth will occur. 

Values of the solubility of surfactants in equilibrium with crystal phase rarely have been measured. In detergent mixtures, other surfactants present in the mixture solubilize the 2-n-(p-sulfophenyl)-alkanes and the fabric softener cationics. A mixing effect which reduces the CMC can explain this. 

The phase diagrams of two-component surfactant-water systems are typically quite different for nonionic and ionic compounds. As exemplified in Fig. 2.22 there are at low temperatures different liquid crystalline phases while at intermediate temperatures there may be a total mutual solubility of surfactant and water98. At higher temperatures, there is, as already noted, a separation into two phases with a very large two-phase region. One of the phases contains very little surfactant, while the other contains appreciable amounts of both components. The cloud-point curve can be described as a liquid-liquid solubility curve with a lower consolute tempera-... 

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. 

A number of factors can affect the phase type that is observed. These factors generally act by changing the partitioning of the surfactant between the brine and oil phases. In general, any change in the surfactant-oil-brine system that increases the solubility of surfactant in oil relative to brine will cause the phase environment type to shift from II(-) to III to II(+) as indicated in the following scheme ... 

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. 

Consani KA, Smith RD. Observations on the solubility of surfactants and related molecules in carbon dioxide at 50°C. J Supercrit Fluids 1990 3 51-65. 

With nonionic surfactants, both types of microemulsions can be formed, depending on the conditions. With such systems, temperature is the most cracial factor as the solubility of surfactant in water or oil is temperature-dependent. Microemulsions prepared using nonionic surfactants will have a limited temperature range. 

Burauer, S., Sachert, T., Sottmann, T. and Strey, R. (1999) On microemulsion phase behavior and the monomeric solubility of surfactant. Phys. Chem. Chem. Phys., 1, 4299-4306. 

The expertise with EOR was used for finding suitable microemulsion-forming systems for LNAPL. However, the high polarity of chlorinated hydrocarbons with very low or even negative equivalent alkane carbon numbers (EACN) required novel types of surfactants [56]. The enhanced solubility of surfactants in the oil phase makes most surfactants less effective for solubilisation. DNAPL extraction by mobilisation, however, is problematic owing to the high density of the pollutants, since they may be displaced into deeper soil compartments [57]. This probably happened in at least one field test [58]. 

Dry surfactant never showed spherulites between crossed polarizers but did display interference colors in thick samples, and bright and dark regions in thin samples. Thus the dry surfactant was polycrystalline and structurally different from the spherulites (see Figure 7). Pressed vigorously between slides, dry surfactant displayed characteristic ellipses between crossed polarizers (17a). Evidently surfactant in water absorbed enough to alter markedly its structure. Evidently too the solubility of surfactant in water was less than 0.7 wt%. 

Mixtures containing in addition from 0.1 to 10 wt% NaCl were also examined for particles and scattering. It was found, qualitatively, that the solubility of surfactant in aqueous salt solution decreased drastically with increasing salt concentration. The solubility was less than 0.01 wt% surfactant in the presence of 0.3 wt% salt. Upper estimates of solubility are summarized in Table II below. 

By the spectroturbidimetry and visual observation criteria, the solubility of surfactant in decane was found to be slightly less than 0.04 wt% at 25°C and about the same at temperatures up to about 50°C. Around 50°C, biphasic systems containing 0.046, 0.12 or 7.7 wt% surfactant became clear. The white particles of surfactant-rich phase appeared to melt. A 15.5 wt% sample remained biphasic between 50 and 80°C, in which temperature range the solubility was estimated to be about 9 wt%. The behavior of surfactant in hexadecane was similar the solubility was about... 

Above 50°C, the solubility of surfactant in decane increased to about 9 wt% and Class II peaks were readily observed at 71°C and 15.5 wt% (Spectrum 14) and at 65°C and 7.7 wt% (Spectrum 16). It was noted that Class I peaks were much more intense than Class II peaks whenever the latter were observed (Spectra 14 and 16), evidently because they came from both surfactant and decane. The latter was present at 30-fold higher molar concentration than the dissolved surfactant. 

Fig. 2.21. Correlation between solubility of surfactants and alkyl chain length n 1—fatty acids (25°C), 2—amines (22°C), 3—alkyl sulfonates (25°C), 4—alcohols (25°C) (Lin and Somasundaran, 1971).

The above conformities are confirmed by trends in the solubility of surfactant homologs, c0, which decreases with increasing adsorption activity. As the surfactant chain length is extended by each CH2 group, the solubility indeed decreases by a factor of 3 - 3.5. 

Good agreement between these two series of curves can serve as convincing evidence of the fact that the properties of the adsorption layers of both soluble and insoluble surfactants are close, and are not directly related to the solubility of surfactant molecules in the supporting liquid. 

The situation is different when oil-soluble surfactants dissolved in a liquid hydrocarbon adsorb at the same interface the extension of the hydrocarbon chain length results in only a small decrease in the surface activity. This is related to a small increase in the solubility of surfactants in oil upon extending the hydrocarbon chain length. The energy of surfactant adsorption from the oil phase at the water - oil interface is controlled by the hydration of the polar groups, which takes place when surfactants move to the interface from the oil bulk. 

The amount of substance present in the micellar state, cmjc = mnmic / NA may exceed the concentration of it in the molecular solution by several orders of magnitude. The micelles thus play a role of a reservoir (a depot) which allows one to keep the surfactant concentration (and chemical potential) in solution constant, in cases when surfactant is consumed, e.g. in the processes of sol, emulsion and suspension stabilization in detergent formulations, etc. (see Chapter VIII). A combination of high surface activity with the possibility for one to prepare micellar surfactant solutions with high substance content (despite the low true solubility of surfactants) allows for a the broad use of micelle-forming surfactants in various applications. 

Due to micelle formation the total surfactant concentration undergoes an abrupt increase. Since true (molecular) solubility of surfactants, determined by the CMC, remains essentially constant, an increased surfactant concentration in solution is caused by an increase in a number of formed micelles. Micellar solubility increases with increase in temperature, and thus a continuous transition from pure solvent and true solution to micellar solution, and further to different liquid crystalline systems and swollen surfactant crystals (see below), may take place in the vicinity of the Krafft point. 

Hoefling et al. " " and Harrison et al.t" have identified a number of factors influencing the solubility of surfactants in CO2. They concentrated on the functionality of various chelates and surfactants. Even though these solutes have low vapor pressures, they found that solutes with low solubility parameters, and low dipolarity/ polarizability parameters (e.g., with fluoroalkyl functional groups) were more soluble in CO2 than those with opposite properties. Favorable C02-tail interactions aid partitioning into the CO2, bending of the interface about water, and reduced micelle-micelle interactions. They also reported that solutes that were Lewis bases tended to have higher solubilities. 

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