Instability or metastability

No dispersion is truly stable; claims that some dispersions are "stable" or "very stable" are misleading. In fact, dispersions are either unstable or metastable. Controlling their instability is the formulator's main problem. Anyone (or almost anyone) can produce a formula, applying known principles; on the other hand, it is often very difficult to guarantee the stability of these formulas, except in the case of model systems.

Controlling the long-term stability of a dispersion is a particularly difficult problem: how can we know now whether, in a year's time, the state of the dispersion will not be partially degraded? It is illusory to hope to determine optimal stability conditions by systematically varying formulation parameters. Indeed, this approach would require the variation of far too many parameters, and, what's more, these variations have non-regular effects...