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“The trouble with facts is that there are so many of them.”
—Samuel McChord Crothers
Cosmetic formulation aspires to be a true science, but compared to physics or chemistry, it has very little in the way of theoretical frameworks to guide this work. Most formulation is based on empirical knowledge or trial-and-error experimentation. There is no equivalent of F=ma or E=mc2 for making a shampoo or microemulsion.
One of the few general tools available is hydrophil/lipophil balance (HLB) theory, but it is restricted to nonionic emulsions and, even within that narrow realm, still has shortcomings. Beyond HLB, there is a vast theoretical desert where phase diagrams provide a welcome oasis, offering a systematic approach to analyzing complex systems. Personal care products are complex—let there be no doubt. The sheer number of variables and the abundance of radically different product formats underlie the unique challenge faced at the lab bench.
Phase diagrams often are seen at meetings and in literature, but it is easy to skim over them without appreciating their value. Phase diagrams allow exploration of the properties of single materials, mixtures and assemblies. They help in analyzing systems as diverse as aerosol gas blends, deodorant sticks and fragranced lotions. When analytical methods are applied to the separate phases, a clear picture emerges of the complex interactions resulting from multiple variables.
A simple case is the behavior of water. As the temperature changes under standard atmospheric conditions, water changes from a solid to a liquid to a gas (Figure 1a). When the pressure varies, the situation immediately gets more complicated (Figure 1b). New possibilities arise: to go directly from solid to gas in a process called sublimation and the triple point—where solid, liquid and gas coexist.