The ISO 4311 outlines how to measure the critical micelle concentration of solutions with anionic and non-ionic surface active agents. To do so, a tensiometer is used to conduct surface tension measurements using a plate, stirrup, or ring. The measurements are repeated for different concentrations to determine the critical micelle concentration.

ISO 4311: Application and Use

Surface active molecules consist of a hydrophobic, non-polar chain and a hydrophilic, polar head. They prefer to accumulate at the interface between two phases and reduce the interfacial tension. Thus, they enable better mixing of two phases. If an interface between two phases is saturated with surface active agents, excess surface active agents molecules form micelles in the liquid phase. The concentration at which the first micelles are formed is known as the critical micelle formation concentration (CMC for short). It can be determined using a series of measurements with different surfactant concentrations.

ISO 4311: Experimentation

Apparatus

To conduct this method, a tensiometer is needed. The apparatus is the same as specified of ISO 304. The measurement is conducted using a tensiometer with different sample bodies, namely a plate, stirrup or ring.

All Force Tensiometers of the DCAT series from DataPhysics Instruments comply with this standard.

Procedure

Ten solutions with different concentrations must be prepared. The measurement itself is then conducted according to ISO 304. The results for the surface tension are plotted against the concentration. Where the curve shows a sharp change, the CMC can be found. Prepare another six solutions with concentrations around where the change occurs and repeat the measurements. Note that the Tensiometers of the DCAT series can determine the CMC automatically.

The ISO 4311 is in use since 1979 and last reviewed in 2022.

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