Methods for CMC determination of surfactants: are theyvinterchangeable?

Purpose: Surfactants are amphiphilic molecules able to decrease surface tension and to form spontaneously in water supramolecular aggregates, called micelles. This process occurs above a certain surfactant concentration termed as critical micelle concentration (CMC). CMC is a chemical-physical parameter, characteristic for each surfactant, which can be determined using different methodologies. Actually, any physical parameter that changes as a function of surfactant concentration and it is sensible to surfactant adsorption and/or aggregation can be monitored for CMC determination. However, some discrepancies have been found in CMC values reported in the literature and obtained from different techniques. In this work, CMCs of non-ionic and ionic surfactants were determined using a panel of techniques with the aim of highlighting their strengths and weaknesses in relation to the type of the analysed amphiphile. Methods: CMC of non-ionic surfactants [PEG (8)-Monolaurate e PEG (8)-Stearate] and ionic surfactants (sodium dodecyl sulfate, sodium deoxycolate and N-lauroyl sarcosinate) were determined by tensiometry, conductivity, densimetry, fluorescence spectroscopy and high-resolution ultrasound spectroscopy (sound speed and attenuation parameters) measurements. CMC values were calculated from the first or second derivative of the raw data. Results: As regard ionic surfactants (CMC > 1 mM), all analysed techniques were able to provide reliable CMC values. Among them, ultrasound spectroscopy in terms of sound speed measurement was the more straightforward technique. Conversely, non-ionic surfactants (CMC < 1 mM), only tensiometry and fluorescence spectroscopy allow a reliable CMC calculation. In fact, no CMC values can be determined by densimetry and ultrasound spectroscopy because of lack of sensitivity of the technique at a so low surfactants concentrations. Conductivity cannot be used due to the non-electrolyte behaviour of non-ionic surfactants. Conclusions: The choice of the most suitable technique for CMC determination is driven by both the nature of the surfactants and surfactant concentration at CMC. Although comparable CMC values can be obtained from different techniques, especially for ionic surfactants with higher CMC values, it is important to consider also the feasibility of an unequivocal determination of CMC from the experimental data.