Rapid polymeric surfactant characterization using a novel flow-injection system with dynamic surface tension detection Original Research Article

Substantial improvements in a dynamic surface tension detector (DSTD) are presented, providing the rapid measurement of the dynamic surface tension of flowing liquids. The dynamic surface tension is determined by measuring a differential pressure across the liquid–air interface of growing drops. A novel calibration procedure is described and implemented. Differential pressure signals from three drops (water, standard, and analyte) are utilized to make the dynamic surface tension measurement, thereby eliminating the need for optical imaging as required by current methods. Furthermore, corrections for drop elongation are not required, since the growing drops are precisely detached by an air burst actuation method in a regime where the surface tension forces significantly dominate gravitational forces. Drops that would fall with a volume of about 10 μl due to gravity, are precisely detached earlier at a volume as low as 1 μl. Two model surfactant systems, sodium dodecyl sulfate (SDS) and polyoxyethylene-20-cetyl ether (Brij® 58), are evaluated to demonstrate the significance of these improvements to the DSTD. Using 0.5–3.0 mM SDS solutions, the precision of the instrument is demonstrated, with a precision of 0.2 dyn/cm routinely obtained. A reproducible method to perform stress-relaxation experiments of surfactant systems, similar to those reported in the literature, is demonstrated using Brij® 58 solutions. Additional characterization of Brij® 58 solutions above the critical micelle concentration (CMC) is presented, providing insight into the impact that micelles have on dynamic surface properties. Finally, combination of a flow-injection system with the DSTD provides information-rich, multi-dimensional data. The smaller drop volume provided by automated drop detachment results in excellent data density for rapid chemical analysis. The flow-injection system with the DSTD provides rapid characterization of polymeric surfactant solutions above the CMC based on diffusional properties of the micelles and on surface active properties of the micelle/monomer system.