When Matter Organizes Itself
In a solution of surfactants, a new form can arise from the behavior of its parts.
A clear liquid can hide arrangements that are surprisingly intricate. Dissolve some surfactants in water and, under suitable conditions, some of the molecules gather into small aggregates called micelles. The solution may still look uniform to the eye. Its components now spend part of their time in organized groups.
The term self-assembly describes systems where interactions between components can bring about organization without a tool placing each piece. A chemist still chooses the ingredients and the conditions. Those choices determine the interactions available to the parts and the forms that can result.
Two ends of the same molecule
One useful model of a surfactant is a molecule with a water-facing head and a tail whose contact with water is less favorable. Gather several molecules into a small sphere and the heads can face the surrounding water while the tails occupy the interior. Figure 02 makes that arrangement visible as a ring of heads and a center of inward-pointing tails.
The ring is what a flat section through a sphere suggests. A real aggregate is three-dimensional, and the molecules have room to move. The drawing fixes their positions so we can identify the two regions. In solution, aggregates coexist with dissolved molecules or ions. Individual components can leave or join them.
When there is enough company
Adding a small amount of surfactant to water does not guarantee that a large fraction of it forms micelles. For many surfactants there is a relatively narrow range of concentrations where the amount of material in aggregates starts to rise markedly. The critical micelle concentration describes this transition. It can be inferred from a change in a measured property of the solution as concentration varies.
A concentration is one part of a set of conditions. Molecular shape, temperature and the surrounding solution can change which aggregates are favored. The convenient spherical picture is a starting point for asking about those conditions. Treating it as the only possible shape would hide the very sensitivity that makes self-assembly interesting.
Reading the result
A repeating crystal is another familiar example of order that can emerge through interactions between constituents. The comparison helps locate two different meanings of order. In a periodic crystal, a structural motif repeats through space. A micelle has an inside and outside arranged around a center and exchanges components with its surroundings. Its order is local and dynamic.
When we encounter another self-assembling system, the most useful question is often what the interactions make easy. In the micelle model, they make it favorable for the tails to gather away from water while the heads remain exposed. That small piece of chemistry accounts for the broad shape of the diagram and gives us a way to reason about how the arrangement might change.
