Phase diagrams solubilised systems

For this system the temperature of phase inversion (PIT) is between 45°C and 55°C. Variation of both the temperature and the surfactant concentration in a system with a fixed ratio of water and oil leads to a phase diagram that is called informally the Kahlweit fish due to the shape of the phase boundaries that resemble a fish. In Figure 3.24 (left), this diagram is given for the system water/tetradecane/CnEs. For small surfactant concentrations (<15%), the phases already discussed occur but, at higher emulsifier concentrations, the surfactant is able to solubilise all the water and the hydrocarbon which results in a one-phase microemulsion D or a lamellar phase La. 

The maximum additive concentration (MAC) is defined as the maximum amount of solubilisate, at a given concentration of surfactant, that produces a clear solution. Different amounts of solubilisates, in ascending order, are added to a series of vials containing the known concentration of surfactant and mixed until equilibrium is reached. The maximum concentration of solubilisate that forms a clear solution is then determined visually. This same procedure can be repeated for the different concentrations of surfactant in a known amount of solubilisate in order to determine the optimum concentration of surfactant (Figure 4.24). Based on this information, one can construct a ternary phase diagram that describes the effects of three constituents (i.e., solubilisate, surfactant, and water) on the micelle system. Note that unwanted phase transitions can be avoided by ignoring the formulation compositions near the boundary. In general, the MAC increases with an increase in temperature. This may be due to the combination of the increase of solubilisate solubility in the aqueous phase and the micellar phase rather than an increased solubilization by the micelles alone. 

Solubility data are expressed as a solubility-concentration curve or as phase diagrams. The latter are preferable since a three-component phase diagram completely describes the effect of varying all three components of the system - namely, the soluhilisate, the solubiliser and the solvent. The axes of the phase diagram form an equilateral triangle (see Fig. 6.37), each side of which is divided into 100 parts to correspond to percentage composition. 

A typical phase diagram of a solubilised system is shown in Fig. 6.38. In solutions of high water content the oil is solubilised in... 

Hgure 6.38 Partial phase diagram for the Brij 97 (CigHjj (OCH2CH2)ioOH)-water-mineral oil solubilised system. 

The propane system shown in Fig. 11.3 is clearly subcritical as the critical temperature of propane is about 96°C. An increase of the C02 fraction ((3) in the mixture of C02 and propane shifts the one-phase region (1), i.e. the bicontinuous microemulsion, to lower temperatures. For pure C02 the bicontinuous microemulsion (1) exists around 35°C, which is higher than the Tc = 31°C of C02. In other words, the C02 solubilised in the microemulsion is supercritical Knowing how to tune the phase behaviour of these systems, one can easily shift phase diagrams on the temperature scale by simply choosing an appropriate surfactant. Other tuning parameters are the oil-to-water fraction and the temperature which maybe adjusted such that, e.g. a C02-in-water droplet microemulsion forms. 

In Winsor s type 1 systems (r< 1) the affinity of the surfactant for the water phase exceeds its affinity for the oil phase. Thus, the interface will be convex towards water. Davis used a triangular diagram to show that a type 1 nSOW system can be one or two phases. A system in the two-phase region will split into an oil phase containing dissolved surfactant monomers at CMC (critical micelle concentration in the oil phase) and an aqueous microemulsion—a water phase containing solubilised oil in normal surfactant micelles. 

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