As technical people we want our formulations to be governed by clear technical rules and to use simple concepts to organise our world. If we organise our world according to simple definitions,1 we would observe:
• A solution is a homogeneous mixture composed of one or more substances, known as solutes, dissolved in another substance, known as a solvent.
• A suspension is a colloidal dispersion in which a finely-divided species is combined with another species, with the former being so finely divided and mixed that it does not rapidly settle out. In everyday life, the most common suspensions are those of solids in liquid water.
• An emulsion is a mixture of two immiscible substances. One substance (the discontinuous phase) is dispersed in the other (the continuous phase).
However, the world of cosmetic chemistry is rarely so easy to organise. Consider a fully dissolved 1% solution of sodium chloride in water. This simple system has sodium ion (Na+), chloride ion (Cl–) and water, roughly equally distributed over the entire mass of the system. The solution is clear and homogeneous. Now consider a 1% solution of a surfactant. Surfactant, or surface active agent, has a water soluble head and a water insoluble tail. A very well known surfactant is sodium lauryl sulfate (CAS 151-21-3). Like NaCl, sodium lauryl sulfate has two opposite ions, but sodium lauryl sulfate in water is very different. The presence of a large fatty portion makes the product surface active. The structure of sodium lauryl sulfate is shown in Figure 1. A 1% solution of sodium lauryl sulfate, like that of sodium chloride, is clear but not homogeneous. As one adds sodium lauryl sulfate to water, the orientation of the material in the water is driven by achieving the lowest overall free energy. In this case, minimise disrupting hydrogen bonding in water. The sodium lauryl sulfate organises itself at the air water interface and then begins to self assemble into micelles. Figure 2 shows this.2 The first box shows pure water, having a surface tension of around 72 dynes/cm2. As surfactant is added, demonstrated by the second box, surface tension is falling as dilute surfactant organising at the surface. As the surface reaches saturation a very significant situation develops. The surface tension no longer drops even with additional surfactant. It is at this concentration, called critical micelle concentration, that micelles become the dominant form of surfactant. This is shown in the third box in Figure 2.
Group opposites3
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