In the previous column, we discussed some concepts and applications of static surface tension. This month we consider dynamic surface tension. A static surface tension determination provides the final score; dynamic profiling lets you track the whole ball game. Put another way, static surface tension gives you the final destination dynamic measurements tell you the course of the journey, even some good stopping off points. Dynamic surface tension is important in tracking changes during the fractions of seconds of surface formation, an important consideration when surfactants are present. Dynamic measurements track from time zero, the time of highest surface tension, when none of the surfactant molecules have aligned at a surface, to equilibrium, the lowest surface tension, when no additional surfactant molecules can be added to the surface. The shape of the curves can provide insight to optimize wetting characteristics. Further, by varying surfactant concentration, an additional variable is introduced, enabling the more specific characterization afforded with three dimensional analytical techniques.
Dynamic surface tension applications
People use dynamic surface tension characterization in diverse areas One is formulation of aerosols optimize delivery of oral pharmaceuticals. Designers of aerosol dispensing systems may perform dynamic surface tension measurements to optimize nozzle design. Dynamic measurements have even been demonstrated as a tool for characterizing lung surfactants.
Dynamic measurements allow detection and tracking of multiple surfactants. When two or more surfactants are present in a given solution, dynamic measurements may fall and rise several times as heavier surfactants, that move to the surface more slowly, dis- place lighter ones that get there first. One equipment provider detected atypical curves during a demonstration of what was stated to be a blend containing a single surfactant. After ruling out equipment problems and replicating the results, it was discovered that one of the solvents in the blend, presumed to be additive-free, actually contained surfactant.
Another application goes against the “mantra” that the lowest surface tension is the best. An ink jet manufacturer had several ink formulations. One of these exhibited a flashback into the node. After dynamic surface tension measurements, it was concluded that the surface tension of this formulation was too low; the droplet did not remain as a drop but retreated back onto the node.
Dynamic surface tension technique
Dynamic surface tension is most commonly measured by the Maximum Bubble Pressure method. If a probe consisting of a tube with an orifice is lowered into the test fluid and gas bubbles are forced through it into the liquid, the maximum gas pressure, that corresponds to the moment that the bubble breaks free, is proportional to the surface tension. Because the pressure is also affected by such parameters as the depth of immersion and viscosity of the fluid frequently two or more probes with differing orifice sizes are employed in a differential manner so that these competing effects can be cancelled in the measurement.
How does this technique work dynamically? By varying the bubble rate, the time from surface generation (when the bubble first emerges from the orifice) to bubble release (the time of the measurement) can be varied. Thus one can trace the evolution of the surface tension from shortly after surface formation (measured with a rapid bubble rate), when there is minimal surface at the surface, to a time long enough for surfactant molecules to diffuse to the surface and align themselves with hydrophobic ends pointing out of the liquid and lowering the dace tension (using slow bubble rates). In other words, the surface tension can be measured as a function of surface age, the time since formation of the surface. These times can be as short as a few milliseconds or as long as over 100 seconds (to measure equilibrium or static dm tension). Competing effects due to multiple surfactants can cause the surface tension to decrease with surface age, then increase, and then decrease again as one surfactant reaches the surface only to be displaced and replaced by another.
The authors acknowledge the helpful comments of Victor Janule, Sensadyne Instrument Division, Chem-Dyne Research Corp.
Victor P. Janule, Fingerprinting Surfachnts Using Dynamic Surface Tension Measurement (Pharma-Chem; Scheduled for publication, June 2003)