Determination of critical micelle concentration with the rotating sample system.

A novel experimental approach using the rotating sample system (RSS) is proposed here for the determination of the critical micelle concentration (CMC) of surfactants. The RSS has been conceived in our laboratory as a convection platform for physicochemical studies and analyses in microliter-sized sample drops. The scheme allows for vigorous rotation of the drop despite its small size through efficient air-liquid mechanical coupling. Thus, changes in surface properties of aqueous samples result in corresponding modulation of the hydrodynamic performance of the RSS, which can be utilized to investigate interfacial phenomena. In this work, we demonstrate that the RSS can be used to study the effects of surfactants on the surface and in the bulk of very small samples with hydrodynamic electrochemistry. Potassium ferrocyanide is employed here with cyclic voltammetry to probe the air-water interface of solutions containing Triton X-100. The CMC of this surfactant determined using this approach is 140 ppm, which agrees well with reported values obtained with conventional methods in much larger samples. The results also demonstrate that besides the CMC, variations in bulk rheological properties can also be investigated in very small specimens using the RSS with a simple method.

Hydrodynamic electrochemistry in 20 microL drops in the rotating sample system.

The Rotating Sample System (RSS) has been conceived in the authors' laboratory as a convection platform for microliter-sized solution volumes. Convection is achieved by rotating a small drop of sample on a stationary substrate by humidified gas jets directed tangentially at the drop base with the working electrode and a liquid junction embedded in it. Simplicity and portability of the device, and substrates complete with microfabricated electrode and junction made potentially disposable, are further competitive advantages with respect to competing, conventional analytical systems. In this work the RSS' performance with variation of system parameters such as the position and size of gas jets used for sample rotation, and position of the working electrode in the substrate are studied. Trace levels of Pb could be detected with this system and is reported here.