Phase Diagrams of Ionic Surfactants

Normal and reversed phases are easily distinguished using conductivity measurements. For normal phases, which are water rich , the conductivity is high. 

In contrast, for reversed phases, which are water poor , the conductivity is much lower (by several orders of magnitude). 

After the hexagonal phase there is transformation into another cubic phase of the bicontinuous type. Then, we find the lamellar phase and, finally, solid hydrated surfactant. 

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Of the many factors which affect the nature of the phase diagram of ionic surfactants, the most influential is probably the nature of the solubilizate. Ekwall... 

Figure 4.20. Three>dimensional phase diagram of ionic surfactant/water system. T, Krafft point Pc critical solution pressure K—Pg, critical solubility curve.

In contrast to the small effects which temperature change has on the phase behaviour of ionic surfactants [38] there is a very pronounced change in the appearance of phase diagrams of oil-water-non-ionic surfactant systems with increase in temperature. Changes induced by temperature in the relative positions and extent of isotropic and liquid crystal phases present in the ascorbic acid-water-polysorbate 80 system have been recorded by Nixon and Chawla [39] (Fig. 2.21). Temperature increase decreases the width of the liquid crystal band the most pronounced effect occurring between temperatures of 25 and 30° C where the polysorbate concentration at which liquid crystals first appear (Li + LC) is increased from about 35 to 36% to 44% polysorbate in the presence of ascorbic acid. 

The Kraft point (T ) is the temperature at which the erne of a surfactant equals the solubility. This is an important point in a temperature-solubility phase diagram. Below the surfactant cannot fonn micelles. Above the solubility increases with increasing temperature due to micelle fonnation. has been shown to follow linear empirical relationships for ionic and nonionic surfactants. One found [25] to apply for various ionic surfactants is ... 

Solubilisation can best be illustrated by considering the phase diagrams of non-ionic surfactants containing poly(oxyethylene oxide) head groups. Such surfactants do not generally need a cosurfactant for microemulsion formation. At low temperatures, the ethoxylated surfactant is soluble in water... 

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-... 

As a practical example for the phase behaviour of surfactants, Figure 3.18 shows the phase diagram of a pure non-ionic surfactant of the alkyl polyglycol ether type C Em. n denotes the length of the hydrocarbon chain and m the degree of ethoxylation [20]. 

Figure 3.26 Schematic phase diagram of a ternary system consisting of water, oil and ethoxy-lated non-ionic surfactant.

The ideas underlying elemental structures models are to establish microstructures experimentally, to compute free energies and chemical potentials from models based on these structures, and to use the chemical potentials to construct phase diagrams. Jonsson and Wennerstrom have used this approach to predict the phase diagrams of water, hydrocarbon, and ionic surfactant mixtures [18]. In their model, they assume the surfactant resides in sheetlike structures with heads on one side and tails on the other side of the sheet. They consider five structures spheres, inverted (reversed) spheres, cylinders, inverted cylinders, and layers (lamellar). These structures are indicated in Fig. 12. Nonpolar regions (tails and oil) are cross-hatched. For these elemental structures, Jonsson and Wennerstrom include in the free energy contributions from the electrical double layer on the water... 

Fig. 10 Binary phase diagram of a a) monomeric nonionic surfactant, b) non-ionic polysurfactant in aqueous solution A heterogeneous mixed crystals B heterogeneous melt C homogeneous isotropic solution D homogeneous mesomorphous phases...

Consider the phase diagram of a three-component system of water, ionic surfactant and medium-chain alcohol, as described in Figure 15.4. At the water comer... 

The phase diagram of sodium dodecyl sulfate-water is representative of many ionic systems (Figure 3.7) [5], In Figure 3.7 Liquid is the aqueous micellar phase Ha is the hexagonal lyotropic liquid crystal, sometimes called the middle phase and La is the lamellar lyotropic liquid crystal, sometimes called the neat phase. On the surfactant-rich side, several hydrated solid phases are present. 

The development of cosmetic microemulsion cleansers with alkyl polyglycosides (APG) was described by Forster et al. [4]. This class of non-ionic surfactants has excellent environmental and skin compatibility. Cosmetic cleanser multicomponent systems are required to have good foaming and cleansing performance. Figure 8.3 shows a pseudo-ternary phase diagram of a five-component formulation. It consists of water, the oil dioctyl cyclohexane (DOCH), the non-ionic surfactant C12/14-APG, the anionic surfactant fatty alcohol ether sulphate (FAES) and the co-surfactant sorbitan monolaurate (SML). The phase diagram... 

Figure 10.5 Phase diagrams of the systems PbO/NaCI-suet-technical non-ionic surfactant at = 0.50 and e = 0.10. Although less efficient than Lutensol A07 and Lutensol A08, Eusapon OD is the most suitable alternative for Lutensol AP9 as the X-point is located near the degreasing temperature and no Lc,-phase forms.

Figure 10 Phase diagram of a simple fragrance compound system (phenethyl alcohol) with water and a commercial non-ionic surfactant (Brij 30) (see text).

Fignre 4.12 shows the effects on phase behavior of both surfactant concentration and temperatnre for an ionic surfactant. The hquid crystalline phases melt at snfficiently high temperatnres. A similar diagram for a particidar nonionic... 

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