Surfactant phase diagram

The surfactant phase diagrams for several surfactants have been developed in order to understand the phase structure of surfactants in solution at high concentration. With these... 

Figure 3. Schematic oil-water-surfactant phase diagram with microstructures depicted.

This region appears in the 10 and 20% surfactant phase diagrams at all temperatures when the amount of water is increased beyond the solubilisation limit. The region is formed up to a concentration of tetradecane of 80% in all cases. 

Generally, water/oil/surfactant phase diagrams are not worked out in so much detail [47, 105] before particle synthesis and often, only the area important for synthesis is delineated for obvious practical reasons (partial phase diagrams, as in Fig. 3.3). We shall describe at the end of this Section, for the uninitiated, the basics of the phase diagrams and the procedures of setting them up. Preparation of microemulsions will be taken up in Section 3.3. 

The saturated solution is sometimes difficult to determine, especially when a pure liquid solute is in equilibrium with a surfactant phase. In collections of thermodynamic equilibrium surfactant phase diagrams (17), solubilization is represented by tie-lines between an isotropic or liquid crystal phase and a nearly pure solute. Experimental determinations of these defines require two separate experiments the first of these is the analysis of the total solute content in a well-separated surfactant solution, while the second one is to control that any initial point taken on a given tie-line produces identical compositions of the phase containing the solute. When micro-phase separation occurs in one part of the phase diagram, the surfactant solution appears to be isotrope, although two phases are co-existing. In such a case, the two experiments are the only way to determine this microphase separation. 

Figure 4 Hypothetical typical water-surfactant phase diagram. (After Ref. 2.)...

Although the study of surfactant solution properties throughout the complete concentration range is of obvious theoretical and occasional practical importance, no attempt is made to cover in detail those phases more stmctured than the simplest aggregates of surfactants in dilute solution. For more information on surfactant phase diagrams, the reader is referred to the excellent works of Laughlin cited in the Bibliography for this chapter. 

Fig. XII-8. A schematic friction phase diagram showing the trends found in the friction forces of surfactant monolayers. (From Ref. 53.)...

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 ... 

Figure C2.3.10. Ternary phase diagram of surfactant, oil and water illustrating tire (regular) and (reverse) L2 microemulsion domains.

Of all the characteristic points in the phase diagram, the composition of the middle phase is most sensitive to temperature. Point M moves in an arc between the composition of the bottom phase (point B) at and the composition of the top phase (point T) at reaching its maximum surfactant concentration near T = - -T )/2. (Points B and Tmove by much smaller amounts, also.) The complete nonionic-amphiphile—oh—water—temperature... 

The example illustrates how Monte Carlo studies of lattice models can deal with questions which reach far beyond the sheer calculation of phase diagrams. The reason why our particular problem could be studied with such success Hes of course in the fact that it touches a rather fundamental aspect of the physics of amphiphilic systems—the interplay between structure and wetting behavior. In fact, the results should be universal and apply to all systems where structured, disordered phases coexist with non-struc-tured phases. It is this universal character of many issues in surfactant physics which makes these systems so attractive for theoretical physicists. 

FIG. 13 Phase diagram of a vector lattice model for a balanced ternary amphiphilic system in the temperature vs surfactant concentration plane. W -I- O denotes a region of coexistence between oil- and water-rich phases, D a disordered phase, Lj an ordered phase which consists of alternating oil, amphiphile, water, and again amphi-phile sheets, and L/r an incommensurate lamellar phase (not present in mean field calculations). The data points are based on simulations at various system sizes on an fee lattice. (From Matsen and Sullivan [182]. Copyright 1994 APS.)... 

The lowest value of Qeff corresponds to different structures for different along the bifurcation line. The sequence of phases is always the same for various strengths of surfactant (with 7 > 27/4) and for increasing p it is L—>G—>D—>P—>C. For 7 = 50 (strong surfactant, like C10E5) the portion of the phase diagram corresponding to the stable cubic phases is shown in Fig. 14(b). For surfactants weaker than in the case shown in Fig. 14 the cubic phases occur for a lower surfactant volume fraction for example, for 7=16 cubic phases appear for p 0.45. 

A. Ciach, J. S. Hoye, G. Stell. Microscopic model for microemulsion. II. Behavior at low temperatures and critical point. J Chem Phys 90 1222-1228, 1989. A. Ciach. Phase diagram and structure of the bicontinuous phase in a three dimensional lattice model for oil-water-surfactant mixtures. J Chem Phys 95 1399-1408, 1992. 

With increasing water content the reversed micelles change via swollen micelles 62) into a lamellar crystalline phase, because only a limited number of water molecules may be entrapped in a reversed micelle at a distinct surfactant concentration. Tama-mushi and Watanabe 62) have studied the formation of reversed micelles and the transition into liquid crystalline structures under thermodynamic and kinetic aspects for AOT/isooctane/water at 25 °C. According to the phase-diagram, liquid crystalline phases occur above 50—60% H20. The temperature dependence of these phase transitions have been studied by Kunieda and Shinoda 63). 

An example for a partially known ternary phase diagram is the sodium octane 1 -sulfonate/ 1-decanol/water system [61]. Figure 34 shows the isotropic areas L, and L2 for the water-rich surfactant phase with solubilized alcohol and for the solvent-rich surfactant phase with solubilized water, respectively. Furthermore, the lamellar neat phase D and the anisotropic hexagonal middle phase E are indicated (for systematics, cf. Ref. 62). For the quaternary sodium octane 1-sulfonate (A)/l-butanol (B)/n-tetradecane (0)/water (W) system, the tricritical point which characterizes the transition of three coexisting phases into one liquid phase is at 40.1°C A, 0.042 (mass parts) B, 0.958 (A + B = 56 wt %) O, 0.54 W, 0.46 [63]. For both the binary phase equilibrium dodecane... 

Figure 2a shows a schematic phase diagram for lyotropic liquid crystals. This figure shows the formation of micelles, cubic phases, bicontinuous cubic phases, and lamellar phases as the concentration of surfactant increases. Also shown in this figure is a schematic diagram of an ordered bicontinuous cubic phase (Fig. 2b). Another interesting example in...

FIG. 2 Example media (a) Surfactant-water phase diagram. (Reprinted from Ref. 206, Copyright 1991, with permission from Elsevier Science.) (b) Ordered periodic and bicontinuous structures. (Reprinted from Ref. 178 with permission from Academic Press, Ltd.) (c) Nonordered membrane structures from ternary microemulsions. (Reprinted with permission from Ref. 177, Copyright 1989, American Chemical Society.)... 

Figure 6.4. Schematic phase diagram for a three-component (oil, water, surfactant) system showing some of the self-assembled structures which form in the various regions.

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