Effect of composition variables and fractionation problems

Changing the composition of a surfactant-oil-water system could modify the phase behaviour as indicated in Fig. 3.12 along the paths indicated by arrows. In many cases, the dilution by water or by oil results in the appearance or disappearance of a microemulsion. In the latter case, the microemulsion can be in equilibrium with excess water, excess oil or both. The problem is easily solved whenever a good phase diagram is at hand, which is not often the case as a matter of fact [63-65]. 

Cheap commercial anionic surfactants of the petroleum sulphonate type might contain disulphonates that are likely to partition in water, thus resulting in a similar fractionation phenomenon. However, this time the candidates to partition in water are the very hydrophilic disulphonates and thus the remaining more lipophilic species are more likely to adsorb at interface. As a consequence the interfacial or microemulsion formulation is more lipophilic [33]. Since this is just the opposite of the previously discussed case of polyethoxylated nonyl phenols, the two phenomena are able to cancel out provided that 

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