Determination of Critical Micelle Concentration Using Fluorescence Polarization

Ever wonder how soaps and detergents cleaned greasy dishes? Soaps and detergents are part of a group of compounds, known as surfactants, which form micelles in solution. A micelle is an aggregation of surfactant molecules in a colloidal suspension. A typical micelle in aqueous solution forms a micelle with the hydrophilic (water loving) head regions in contact with the water and the hydrophobic (water hating) tail regions buried in the inner portion of the micelle. Surfactants are soluble to some extent in aqueous solution, but aggregate into micelles when surfactants reach a sufficient concentration. This concentration is referred to as the critical micelles concentration (CMC) (Figure 1). Below the CMC micelles are not present, while above the CMC, the concentration of unaggregated surfactant will stay constant and the number of micelles will increase as the surfactant concentration increases.




Figure 1. Micelle formation with increasing surfactant concentration

Why is this important in getting your dishes clean? Much of what we refer to as “dirt” is actually oils and greases that normally are not soluble in water. In order to wash things with water there needs to be some sort of means to solublize these oils in order to remove them using water. That is where micelles come into play. Because the center of the micelle is a lipid hydrophobic environment, many of the same greases and oils, which are repelled by water, will partition into the center of the micelle. The hydrophilic outer layer of the micelles interacts with the aqueous environment, coating the hydrophobic center. As you can imagine the more micelles that are present the more grease and oils can effectively be removed. If there are no micelles it is difficult to wash the oils away.

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