Dispersion formulation

Solid/liquid dispersions are systems consisting of a liquid in which small solid particles are immersed. Typically, the continuous liquid phase is an aqueous phase or an oil; the solid particles consist of metal oxides or organic polymers. Typical examples are paints and all colored pastes used to make coatings, certain structural materials (cements, reinforced elastomers) and many pharmaceutical or cosmetic products.

Most dispersions are used as manufacturing intermediates: a dispersion is produced in a liquid state, applied by spreading, extrusion or injection, and undergoes a physical (evaporation) or chemical (reaction) transformation to convert the film or material to a solid state. Others are used as such, as vectors for active molecules (in pharmaceuticals or body care products), or as capture agents for target molecules (in medical diagnostic tests).

For the formulator, dispersions pose two types of problem. Firstly, they are never stable systems, in the thermodynamic sense. This is because the solid particles are separated from the continuous liquid phase by interfaces, which increase the free energy of the dispersion compared with a system in which all the solids would be brought together in a single homogeneous domain. There are therefore several possible evolutions, through aggregation, ripening or coalescence, which the formulator must master. Secondly, dispersions have original properties, which differ from the properties of the constituent phases. These include mechanical properties (dispersion flow when in a fluid state, mechanical strength when in a solid state), optical properties (transparency or opacity, reflectance or light scattering), and physico-chemical properties (wetting, adsorption or adhesion, release or capture of active ingredients).

The formulator must ensure, through his action on the compositions of the dispersed phase and the continuous phase, that the dispersion's properties of use justify the efforts made to manufacture it.