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What Is Critical Micelle Concentration?

Andrew Kirmayer
Andrew Kirmayer

Cleaning products usually have surfactants, or particles that serve as active agents at the surface of water. The molecules are structured with a head that is attracted to water, while the tail is repelled from it. This tail attaches to oil and grease and can suspend dirt in a watery solution. When these particles reach a density at the surface where no more can fit there, this is typically called the critical micelle concentration. A micelle is a spherical or elliptical structure formed when the surfactant molecules come together, and the heads gather at the surface while the tails aim inward.

The critical micelle concentration is a measurement that can help determine when surfactants will form these structures. Factors that can be used to predict micelle formation include surface tension. The tension at the surface of water typically decreases with a greater concentration of surfactants. Particle concentration and surface tension variables can be graphed to provide a visual clue as to the mathematical onset of the critical micelle concentration. Computer controlled instruments and laboratory software are often used to control solutions and take these measurements.

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Woman holding a book

Other factors that can affect the critical micelle concentration are temperature, atmospheric pressure, as well as other surface active substances in the solution. Micelles generally form only above a certain temperature. The micelle concentration of sodium dodecyl sulfate in water, for example, is typically 77°F (25°C) if no other compounds are added. Fewer parts of the surfactants repelled from water are exposed, usually reducing the energy and surface tension of the fluid.

Once the critical micelle concentration is reached, any surfactants added generally form micelles immediately. Conductance as well as photochemical properties of a solution can also affect the point of micelle formation, but how the solution is measured can affect these, and therefore the critical micelle concentration. It is usually important to know how many surfactant molecules are at the surface interface and how many remain in bulk. Sometimes air bubbles rising up to the surface can disrupt the interface and lower the overall concentration of surfactants.

The properties of surfactants can be plotted to determine the rates of change. Within a certain range, the concentration can be found; this is often accomplished through experimentation and mathematical calculation. It is often possible to perform the reverse measurement, which can be useful at low surfactant concentrations and when there are large quantities of solvent.

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