As environmental regulations limit the use of organic solvents, the use of more environmentally benign aqueous-based cleaning solutions has become the main focus for improving industrial cleaning and surface finishing processes. In order to maximize the treatment efficacy while minimizing the chemical additives, interest has turned toward the combined use of aqueous-based treatments in the presence of applied electrical potentials. Due to this interest a series of studies have been performed to determine the techniques and methods that can be utilized to improve the environmental performance of aqueous cleaning.[1-6]

Recently, Morton et al. reported a series of experiments in which they observed droplet detachment of a sessile oil droplet from a stainless steel surface submersed in a very dilute, less than 10 mM, aqueous solutions containing surfactant in the presence of very weak applied voltage differences, in the range of -3 to +3 V.[6]

In our work, we have designed a set of experiments to quantify the findings of Morton et al. Moreover, we complement the experimental work with a continuum-level theoretical model, which we use to explain the phenomena observed in the laboratory.

The experimental system is composed of an oil droplet on a metal coupon immersed in a surfactant solution. Voltage of -3 to +3V was applied to the system via the metal coupon and a counter electrode placed at a separation distance of 18mm to 30mm from the coupon. The surfactant sodium dodecyl sulfate used was in the critical micelle concentration, 8 millimolar. This work can be contrasted with other recent work on drop shape behavior, where the applied potential is generally three orders of magnitude larger.

We performed a theoretical study to model the effect of surfactant concentration and field strength on droplet shape. The model required the solution of the Laplace equation for our experimental geometry. From this solution, the field at each interface is obtained. We expanded the three surface tensions (organic droplet-aqueous solution (oa), organic droplet-metal surface (os), and aqueous solution-metal surface (as)) in Taylor series with respect to surfactant concentration and local field strength. We use these three surface tensions in Young's equation to obtain the predicted contact angle of the organic droplet. We used a non linear least squares regression technique to fit the parameters in the Taylor series in order to match the predicted and experimental contact angles.

The resulting model qualitatively captured the combined effect of (i) surfactant, (ii) applied voltage, and (iii) separation between electrodes on the change in contact angle of an oil droplet on a metal surface submersed in aqueous solution. The model shows that the surfactant affects the oa and as interfaces, potentially to a similar degree. We also show that the surfactant and local field effect are both important.

References:

1. Davis, A.N., et al., Ionic strength effects on hexadecane contact angles on a gold-coated glass surface in ionic surfactant solutions. Colloids and Surfaces a-Physicochemical and Engineering Aspects, 2003. 221(1-3): p. 69-80.

2. Morton, S.A., et al., Thermodynamic model for the prediction of contact angles of oil droplets on solid surfaces in SDS solutions. Separation Science and Technology, 2003. 38(12-13): p. 2815-2835.

3. Morton, S.A., et al., Thermodynamic method for prediction of surfactant-modified oil droplet contact angle. Journal of Colloid and Interface Science, 2004. 270(1): p. 229-241.

4. Rowe, A.W., et al., Oil detachment from solid surfaces in aqueous surfactant solutions as a function of pH. Industrial & Engineering Chemistry Research, 2002. 41(7): p. 1787-1795.

5. Rowe, A.W., et al., Oil droplet detachment from metal surfaces as affected by an applied potential. Separation Science and Technology, 2003. 38(12-13): p. 2793-2813.

6. Morton, S.A., et al., Behavior of oil droplets on an electrified solid metal surface immersed in ionic surfactant solutions. Langmuir, 2005. 21(5): p. 1758-1765.