Determining surfactant CMC in electrolyte solutions by ultrasound velocimetry

abstract

English

High resolution ultrasound spectroscopy is a versatile tool in colloid chemistry that has started to be available also commercially several years ago. Among other, it can be used to determine the critical micellar concentration and compressibility of surfactant molecules or micelles which in turn can be used to elucidate their hydration, structure and intermolecular interactions. Concentration increment of ultrasound velocity, i.e. the change of ultrasound velocity in the solution relatively to the pure solvent per unit solute concentration, plays an important role in evaluation of ultrasonic data from such studies. We have found, however, that its use in surfactant solutions containing electrolytes can be problematic and can even lead to overlooking the point of micellization. This is due to much lower critical micellar concentration in the presence of electrolytes and, consequently, smaller changes in ultrasound velocity. Because the increment includes the reciprocal surfactant concentration the latter can manifest in a parasitic decreasing function of the increment at low concentrations leading to erroneous interpretation of micellization. Even a very small error in the measurement of ultrasound velocity at low surfactant concentrations may have a drastic effect on the increment dependence on concentration. The contribution shows that this problem can be easily overcome using linear sections of the ultrasound velocity dependence on concentration. Critical micellar concentration can be easily determined from the intersection point of linear parts and the increment can be better calculated from parameters of lines fitting these parts. Example of experimental data obtained with TTAB solution in water and in 0.15 M NaBr demonstrates differences in outcomes of ultrasound velocimetry for these two systems and in behavior of concentration increment in aqueous and electrolyte solutions as well as the efficacy of the proposed approach. Knowledge of the speed of ultrasound propagation through surfactant solutions is a necessary prerequisite for subsequent ultrasonic study of surfactant-polyelectrolyte interactions.

micellization, CMC, TTAB, ultrasound spectromety

KLUČÁKOVÁ, M.; PEKAŘ, M.

5. 9. 2010

ECIS

Antalya

P.D43.025

P.D43.025

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