The instrument of choice to measure automatically Critical Micelle Concentration is Sigma 700/701. Other Sigma force tensiometers and the Theta range of optical tensiometers can also be used for manual measurements.
What is Critical Micelle Concentration?
Certain molecules may be said to contain two distinct components, differing in their affinity for solutes. The part of the molecule which has an affinity for polar solutes, such as water, is said to be hydrophilic. The part of the molecule which has an affinity for non-polar solutes, such as hydrocarbons, is said to be hydrophobic. Molecules containing both types of components are said to be amphiphilic (illustration A).
Such molecules display distinct behavior when interacting with water. The polar part of the molecule seeks to interact with water while the non-polar part shuns interaction with water. There are two ways in which such a molecule achieve both these states. An amphiphilic molecule can arrange itself at the surface of the water such that the polar part interacts with the water and the non-polar part is held above the surface (either in the air or in a non-polar liquid) as shown in Figure B above. The presence of these molecules on the surface disrupts the cohesive energy at the surface and thus lowers the surface tension. Such molecules are called ‘surface active’ molecules or surfactants.
Another arrangement of these molecules can allow each component to interact with its favored environment. Molecules can form aggregates in which the hydrophobic portions are oriented within the cluster and the hydrophilic portions are exposed to the solvent. Such aggregates are called micelles. An example of a spherical micelle is diagrammed above (illustration C).
The proportion of molecules present at the surface or as micelles in the bulk of the liquid depends on the concentration of the amphiphile. At low concentrations surfactants will favor arrangement on the surface. As the surface becomes crowded with surfactant more molecules will arrange into micelles. At some concentration the surface becomes completely loaded with surfactant and any further additions must arrange as micelles. This concentration is called the Critical Micelle Concentration (CMC). It follows that measurement of surface tension may be used to find CMC. A graph of surface tension versus log of concentration of surfactant added will appear as follows:
In this graph you can see three phases:
1. At very low concentrations of surfactant only slight change in surface tension is detected.
2. Additional surfactant decreases surface tension
3. Surface becomes fully loaded, no further change in surface tension.
As shown above the technique for assaying CMC by measurement of surface tension is simple and straightforward. A graph of surface tension versus log concentration is produced. The CMC is found as the point at which two lines intersect; the baseline of minimal surface tension and the slope where surface tension shows linear decline. The graph of surface tension versus log concentration may be done by measuring a series of manually mixed solutions or automatically using an Attension Sigma 700 or 701 with optional dispenser. Manual mixing and testing surface tensions for a series of solutions is both time consuming and difficult. After solutions of surfactants are mixed time may be required for the surface tension to reach an equilibrium value. This time may vary from seconds to hours. When a researcher is trying to manually find CMC this may present real problems. If CMC values are found using the automated features of the Sigma range the stability of surface tension measurements can be easily handled. You simply enter the desired limits of stability and the maximum time allowed for measurement at any concentration and your tensiometer will continue measuring until these limits have been met. The Sigma 700 and 701 programs also automate the mixing of concentrations for even spacing across the log scale and assist in graphics analysis to find your CMC.
How is CMC Measured?
There are two possibilities to perform CMC measurements with Sigma 700/701: either by adding a concentrated solution of surfactants to an initially surfactant free solution or by diluting a concentrated surfactant solution with pure solvent. To perform a measurement of CMC with your Sigma you must first prepare your physical setup, software and chose the way to measure CMC. Hereafter, a probe is chosen (Wilhelmy plate or Du Noüy ring) and the measuring vessel is filled with the initial solution. The dispenser is filled either with concentrated surfactant or pure solvent depending on which type of CMC measurement that has been chosen. The software is programmed and the experiment initiated. This short procedure is all that is required of the researcher. The Sigma 700/701 will automatically complete the experiment.
Your experiment will proceed in stages. First a surface tension will be measured prior to any addition. Next the first addition will be made. If you wish, you have the option of programming this as a large addition to bring the system closer to a known CMC value. The instrument will then stir your solution. Next, values of surface tension will be measured. Frequently after the addition of surfactants or pure solvent the values of surface tension will take some time to stabilize. You can set the values which will decide the required limits for stability. When the system has displayed the required stability in surface tension measurements or a maximum time limit (set by you) has passed, your Sigma will make another addition and continue as above. Additions will be made such that data will be evenly spaced along a log scale of concentration. You will select the range of concentrations assayed and the number of data points desired and the Sigma will mix the appropriate solutions for measurement.
You may also select the AutoSearch option. This divides the experiment into two phases. The Search phase allows for larger additions to be made during the first portion of the experiment when data not critical to CMC calculations will occur. When a sufficient drop in ST values occurs the experiment will switch to the Measurement phase and obey the addition schedule of a normal experiment. When all requested additions have been made the experiment will terminate. The data will be automatically stored to your hard drive and can be analyzed at any later time.
Video of a fully automatic critical micelle concentration using Sigma 700/701 force tensiometer: