Microemulsions with alkyl polyglycol ethers

Stacking the isothermal Gibbs triangles on top of each other results in a phase prism (see Fig. 1.3(a)), which represents the temperature-dependent phase behaviour of ternary water-oil-non-ionic surfactant systems. As discussed above, non-ionic surfactants mainly dissolve in the aqueous phase at low temperatures (2). Increasing the temperature one observes that this surfactant-rich water phase splits into two phases (a) and (c) at the temperature T of the lower critical endpoint cepp, i.e. the three-phase body appears. Subsequently, the lower water-rich phase (a) moves towards the water corner, while the surfactant-rich middle phase (c) moves towards the oil corner of the phase prism. At the temperature Tu of the upper critical endpoint cepa a surfactant-rich oil phase is formed by the combination of the two phases (c) and (b) and the three-phase body disappears. Each point in such a phase prism is unambiguously defined by the temperature T and two composition variables. It has proved useful [6] to choose the mass fraction of the oil in the 

Considering now the variation of the phase behaviour with increasing mass fraction y of surfactant one can see that the volume of the respective microemulsion phase increases (see test tubes in Fig. 1.3(b)) until the excess phases vanish and a one-phase microemulsion is found. The optimal state of the system is the so-called X-point where the three-phase body meets the one-phase region. It defines both the minimum mass fraction y of surfactant needed to solubilise water and oil, i.e. the efficiency of the surfactant, as well as the corresponding temperature f, which is a measure of the PIT. 

C6E2 to C12E5 leads to an enormous increase in efficiency. This increase in efficiency is a result of the increasing amphiphilicity of the surfactant molecules forcing them into the microscopic water/oil interface. 

In one-phase micro emulsions the surfactant molecules partition between the microscopic water/oil interface and the microemulsion sub-phases (e.g. in swollen micelles or bicontin-uous oil- and water-rich domains) in which they are dissolved monomerically. They also dissolve monomerically in coexisting excess phases and adsorb at the macroscopic interfaces between the phases. The significance of this fact is that these parts of the surfactant are not available for the micro-emulsification of water and oil. Thus, for technical applications surfactants with high amphiphilicity but small monomeric solubilities in both solvents are desirable. 

The monomeric solubility of the surfactant in the water y cmon.a can be easily determined from surface tension measurements [37]. An interesting method to obtain ycmon.b is provided by the macroscopic phase behaviour through the determination of the mass fraction of surfactant y0 (see Fig. 1.3), i.e. the monomerically dissolved surfactant in both excess phases. Therefore, the volume fraction of the middle phase Vc/V has to be measured as a function of the mass fraction of surfactant y at a constant J = 0.5 and the mean temperature f of the three-phase body [34, 38, 39]. Plotting Vc/V versus y yields yoat Vc/V = 0andy at Vc/V = 1.Then the monomeric solubility in the oil is calculated from 

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Ryan, L.D., Schubert, K.-V. and Kaler, E.W. (1997) Phase behavior of microemulsions made with n-alkyl monoglucosides and n-alkyl polyglycol ethers. Langmuir, 13, 1510-1518. 

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