Phase behaviour nonionic surfactants

Thuresson, K. and Lindman, B. (1997) Effect of hydrophobic modification of a nonionic cellulose derivative on the interaction with surfactants. Phase behaviour and association. J. Phys. Chem. B, 101, 6460-6468. 

Regarding the phase behaviour of ternary mixtures of water, oil and a nonionic surfactant. Fig. 3 shows schematically the so-called one-phase channel. This sec-... 

The choice of surfactant, which is mostly constrained by the choice of the oil and the resulting phase behaviour of the microemulsion, can have different effects on the enzyme stability and activity. In general we have to differentiate between ionic and nonionic surfactant types ... 

In contrast to nonionic surfactants, ionic surfactants build up a high zeta-po-tential at the water-oil interface which can also can influence the enzyme activity. Most investigated systems used AOT as the surfactant because its phase behaviour is well understood. However, AOT is often not very suitable, because it can totally inhibit enzymes (e.g. the formate dehydrogenase from Candida bodinii). The usage of lipases in AOT-based microemulsions is generally unfavourable as AOT is an ester that is hydrolysed itself. 

Crystallization-induced phase separation can occur for concentrated solutions (gels) of diblocks [58,59]. SAXS/WAXS experiments on short PM-PEO [PM=poly(methylene) i.e. alkyl chain] diblocks revealed that crystallization of PEO occurs at low temperature in sufficiently concentrated gels (>ca. 50% copolymer). This led to a semicrystalline lamellar structure coexisting with the cubic micellar phase which can be supercooled from high temperatures where PEO is molten. These experiments on oligomeric amphiphilic diblocks establish a connection to the crystallization behaviour of related nonionic surfactants. 

Frank, C., Frielinghaus, H., AUgaier, J. and Prast, H. (2007) Nonionic surfactants with linear and branched hydrocarbon tails Compositional analysis, phase behaviour, and film properties in bicontinuous microemulsions. Langmuir, 23, 6526-6535. 

Kahlweit, M. (1982) The phase behaviour of the type H20-oil-nonionic surfactant-electrolyte. /. Colloid Interface Sci., 90,197-202. 

Kunieda, H., Hanno, K., Yamaguchi, S., and Shinoda, K. (1985) The three-phase behaviour of a brine/ionic surfactant/ nonionic surfactant/oil system evaluation of the hydrophile-lipophile balance (HLB) of ionic surfactant. 

As an example of the different phases of surfactants. Figure 3.27 shows the phase diagram of a pure nonionic surfactant of the alkyl polyglycol ether type (20). In particular, the phase behaviour of nonionic surfactants with a low degree of ethoxylation is very complex. As the lower consolute boundary is shifted to lower temperatures with a decreasing EO (ethylene oxide) number of the molecule, an overlapping of this boundary... 

The phase-inversion temperature (PIT) is the temperature at which the continuous and dispersed phases of an emulsion system are inverted (e.g. an o/w emulsion becomes a w/o emulsion, and vice versa). This phenomenon, introduced by Shinoda (16), occurs for emulsion systems containing non ionic surfactants, and can be a valuable tool for predicting the emulsion behaviour of such systems. The phase inversion occurs when the temperature is raised to a point where the interaction between water and the nonionic surfactant molecules decreases and the surfactant partitioning in water decreases. Hence, surfactant molecules... 

The phase behaviour of PS systems is also affected by specific interactions between the two cosolutes, similar to hydrophobic interactions in the case of HM-polymers. This may enhance phase separation for nonionic systems but decrease it for ionics. For a mixture of oppositely charged polymer and surfactant,... 

Source From Determination of retention behaviour of some nonionic surfactants on reversed-phase high-performance liquid chromatography supports by spectral mapping in combination with cluster analysis or non-linear mapping, in J. Chromatogr. 

The phase behaviour of surfactants is best illustrated using nonionic surfactants of the poly(ethylene oxide) type. Figure 3.18 illustrates this with the phase diagram for the binary system, dodecyl hexaoxyethylene glycol monoether-water [14]. 

Figure 4.3. Schematic phase diagram (prism) of mixtures of water (A), oil (B) and nonionic surfactant (C) as a function of temperature tie-lines are shown within the two-phase regions. In addition, the three phase triangles, critical points (cps), critical lines (els), and critical end-points (ceps) are shown. Also shown is a slice through the prism at equal masses of oil and water (o = 0.5 dark shaded region). Next to the prism are test tubes illustrating the phase behaviour (o = 0.5) at low, intermediate and high temperatures. Reproduced by permission of Academic Press (redrawn from Kahlweit et al. (86))...

Rather than fix the water/salt ratio as a brine pseudocomponent, another useful possibility is to fix the alcohol/ionic surfactant ratio as the pseudo-component (fix 8) and vary the salt concentration. An increase of the lyotropic salt concentration in ionic surfacant plus alcohol cosurfactant systems has the same effect as increasing temperature or salt in nonionic surfactant mixtures - a lipophilic shift is observed, and the phase behaviour progresses from 2 to 3 to 2 (15). If salt is placed in the position occupied previously by the cosurfactant in Figure 4.8, and the fixed ratio of alcohol/ionic surfactant placed as a combined pseudocomponent (fixed 5) at the surfactant position, at equal amounts of oil and water (a = 0.5) a plot of salt concentration (e) versus overall cosurfactant/surfactant concentration (y) also yields a fish -shaped phase diagram (45). Therefore, in either the case of fixed salt concentration (fixed ) or the case of fixed alcohol/ionic surfactant ratio (fixed 8), the optimally formulated microemulsions for the chosen fixed ratio of 8 or s, are found at X, where the tail and body of the fish meet (see Figure 4.8). Consequently, the phase behaviour of simple monodisperse ethoxylated alcohol surfactants in oil and water qualitatively mimics that of much more complicated mixtures containing ionic surfactants, cosurfactants and salt. Alcohol... 

One particular advantage of using mixtures of nonionic and ionic surfactants as microemulsifiers is the formation of temperature-insensitive microemulsions (17). Recall that the temperature-dependence of the phase behaviour of balanced microemulsion mixtures with ionic surfactants such as Aerosol OT (see Figure 4.10) is opposite to that found for ethoxylated alcohols (see Figure 4.5). Upon raising the temperature of Aerosol OT mixtures, a hydrophilic shift occurs (2-3-2), although with hoxylated alcohols, a lipophilic shift occurs (2-3-2). Intuitively, upon mixing ionic and nonionic surfactants, the temperature dependence should cancel at a particular ratio (5) of the two surfactants. 

Measurements of the optimal temperature for microemulsion formation (T) are plotted as a function of AOT surfactant concentration 0) and salinity ( ) in Figure 4.11 (46). Rather than T simply averaging upon mixing the two surfactants, due to the opposing temperature-dependence of the nonionic and ionic systems, the curves of constant salinity (e) diverge at a value of 8 70 wt%. A pole is found in the phase behaviour (a flip in the curvature of the lines of constant e) for e between 0.8 and 1.0 wt%. Thus, at <5 ... 

Figure 4.11. Phase behaviour of mixtures of ionic and nonionic surfactants in microemulsion systems of C12E5/AOT/ decane/water/NaCl. Optimal temperatures for microemulsion formation (T) are plotted as a function of wt% AOT in the surfactant mixture (5), and wt% NaCl in water (e). Reproduced by permission of the American Chemical Society (from Kahlweit and Strey (46))...

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