|Figure 1. Micelle formation with increasing surfactant concentration|
How do you measure CMC? One means to determine the CMC of surfactants is to take advantage of their size with fluorescence polarization measurements. Fluorescence polarization (FP) uses the differences in rotation speed between small molecules and large molecules. Large molecules, such as micelles, rotate slowly and have large polarization values, while small molecules, such as fluorescein, rotate rapidly and have a low polarization value. If a fluorescein derivative was to partition into a micelle then the polarization would increase. Thus the surfactant concentration at which the polarization values begin to increase would identify the concentration at which micelles begin to form (Figure 2).
|Figure 2. Critical Micelle Concentration (CMC) Determination using Fluorescence Polarization|
There are a number of factors that play a role in determining the CMC for a specific surfactants or a mixture of surfactants. The foremost determinate is probably the chemical structure of the molecule, but other factors such as ion concentration and temperature also play a role. Another factor that can influence the CMC is the amount of interface present in the system. Air-liquid interfaces such as the surface of a solution or that present with bubbles will have a coating of surfactant that actually sequesters some of the surfactant. The bottom line is that in order to get your dishes clean you will need to put enough dish detergent in the sink to produce micelles and be sure to avoid making a lot of foamy suds, as it actually diminishes the ability to remove oils and grease. Be sure to point these facts out to your children as they do their chores or you could just put the dishes in the dishwasher and come back an hour later. But don’t forget the soap ….
By: BioTek Instruments, Paul Held, Laboratory Manager