Why is formulation important

Formulation is important because the properties of surfactant-oil-water systems in general and the formation of microemulsions in particular, are very sensitive to it and slight deviations from a proper formulation may result in drastic changes of the properties. Consequently, formulation has to be controlled accurately, which is quite challenging because of the high number of degrees of freedom in any practical case. This is why formulation is sometimes considered as magic business . 

Formulation essentially relates to the content of the systems and generally not to the way it is attained if thermodynamically stable systems are considered. The simplest microemulsion system would contain an organic oil phase (O), an aqueous phase generally referred to as water (W), and a surfactant (S) at a given temperature (T) and pressure (p). This means that at least five variables are required to describe the system. In practice, the situation is much more complicated. Water always contains electrolytes. Moreover, oils as well as nearly all commercial surfactants are mixtures. In most cases, particularly with ionic surfactant systems, a co-surfactant (e.g. an alcohol (A)) is added, among other functions, to reduce the rigidity of the surfactant layer and thus to prevent the formation of gel-like mesophases. 

When mixtures are dealt with, some approximations allow to decrease the number of variables. For instance, if a commercial surfactant contains substances all of which behave similarly, then a so-called pseudo-component may be used to describe it. However, this is not the general case and in many instances so-called fractionation phenomena take place and the different components behave independently of each other. In this case, the actual number of components could be (much) larger than three or four. Aside the nature of each of the components of the SOW and eventually the SOWA system, T and p also influence the properties, sometimes to quite a large extent. Note that all variables describing the nature of the components as well as T and p are intensive, i.e. they do not depend on 

An accurate formulation handling is extremely useful not only to make microemulsion and to adjust their properties such as their solubilisation ability, or to attain a low interfacial tension to ease emulsification or to enhance oil recovery. Formulation has been shown to be also directly linked with emulsion properties such as their type, stability, viscosity, drop size [6] and with the efficiency of the emulsification protocol [7]. The existence and persistence of foams are dependent on formulation too [8]. Solid surface wetting is also linked with formulation as well as with many related applications. This is why an accurate numerical treatment of formulation issues is paramount in industrial research and development. 

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