The effect of ph and applied electrical potential on oil removal from a solid surface in the presence of four types of surfactant solutions

Effective aqueous-based cleaning depends on the appropriate selection of surfactant(s). and pH conditions. Experiments involving the detachment of oil droplets from a metal surface m the presence of surfactant solutions are undertaken to observe the variation of droplet shape, particularly contact angle, and the time required for droplet removal. In parallel, tests of oil removal from the same metal surface in an industrial ultrasonic bath have been conducted under similar conditions. Similar trends are found for both types of tests, that is, conditions for which droplets detach more quickly also correspond to conditions of greater oil removal in an ultrasonic bath. Experiments of drop removal time and cleaning effectiveness in surfactant solutions of altered pH are conducted to better understand the role surfactants adsorbed at the surface play in the detachment process. Negatively charged oil/aqueous interfaces exhibit more efficient cleaning as well as drop removal kinetics at high surfactant solution pH, while positively charged oil/water interfaces exhibit faster detachment at low pH. Experiments are conducted, in parallel, in which the surface to be cleaned is connected directly to a low voltage power supply. A similar cleaning procedure is performed in which the dependent variable is applied electrical potential. Applied electrical potential ranges from 0 to ±4 volts (current is limited to near zero) with respect to the surfactant solution. Trends indicate that surfactant solutions containing nonionic (Triton X-100) surfactants exhibit better cleaning and faster oil drop detachment as the applied electrical potential increases in the positive direction. Cationic (CTAB) surfactant solutions perform better cleaning as voltage increases in the negative direction. Amonic (SDS) surfactant solutions perform better as voltage increases regardless of the polanty. Tests involving a zwitterionic (Chaps) surfactant support the three previous case trends. The pH and applied electrical potential is interpreted by a mechanistic model focusing on the adsorption of surfactant ions at the aqueous/solid interface and the electrostatic repulsion/attraction to the oil drop.