Surface tension
Critical micelle concentration (CMC)
An important measure for the characterization of surfactants is the critical micelle concentration (CMC). Surfactants consist of a hydrophilic "head" and a hydrophobic "tail". If a surfactant is added to water then it will initially enrich itself at the surface; the hydrophobic tail projects from the surface. Only when the surface has no more room for further surfactant molecules will the surfactant molecules start to form agglomerates inside the liquid; these agglomerates are known as micelles. The surfactant concentration at which micelle formation begins is known as the critical micelle formation concentration (CMC).
Micelles are spherical or ellipsoid structures on whose surface the hydrophilic heads of the surfactant molecules are gathered together whereas the hydrophobic tails project inwards. The washing effect of surfactants is based on the fact that hydrophobic substances such as fats or soot can be stored within the micelles.
Standard procedure
The critical micelle formation concentration (CMC) can be determined by carrying out surface tension measurements on a series of different surfactant concentrations. Surfactants exhibit a specific surface tension curve as a function of the concentration. Initially the surfactant molecules increasingly enrich themselves at the water surface. During this phase the surface tension decreases linearly with the logarithm of the surfactant concentration. When the CMC is reached, i.e. when the surface is saturated with surfactant molecules, a further increase in surfactant concentration no longer has any appreciable influence on the surface tension.
- Determination of the critical micelle formation concentration
This means that in order to determine the CMC the two linear sections formed by the measuring points obtained from the series of different concentrations must be determined. The CMC is obtained from the intersection of the straight lines for the linear concentration-dependent section and the concentration-independent section.
In the K100 and K12 the CMC is determined by using the CMC Add-In of the LabDesk software. The concentration series is generated automatically with a computer-controlled Dosimat, so that only a surfactant stock solution needs to be made up. The measurements and their evaluation are carried out automatically.
Reverse CMC measurement
For reverse CMC measurements not the solvent but the parent solution is first put into the sample vessel and then diluted with the solvent step by step.
One case in which the reverse CMC measurement should be chosen is when the concentration of the sample is above the clouding point. Such a solution can’t be dosed homogeneously, but it can be diluted homogeneously by adding the solvent.
Another application for reverse CMC measurements is the case that the CMC is expected at a low concentration of the surfactant. With standard CMC measurements the region of interest is also the region where only small amounts of the sample solution are dosed and where therefore the largest error by means of dosing inaccuracy would occur. With reverse CMC measurement low concentrations are reached with large amounts of the solvent and so this error is reduced to a minimum.
Another advantage is that the dosimat only gets in contact with the pure solvent and can run in continuous operation. This makes the reverse CMC measurement an ideal method for routine measurements.
Since the end volume exceeds the initial volume by many times a cone shaped sample vessel is used.
Extended CMC-Method
For the K100 the"Extended CMC" method is available. The software LabDeskTM does not control one but two dosing units. The second unit substracts the amount of liquid previously added by the first one. Thus the accessible concentration range is increased many times over.
- Range of conventional CMC method
- Range of extended CMC method