Oil Detachment from Solid Surfaces in Aqueous Surfactant Solutions as a Function of pH

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Abstract

Effective removal of oil from metal surfaces utilizing aqueous solutions depends on the appropriate selection of surfactant(s) and pH conditions. In the experiments described in this paper, the detachment of oil droplets from a solid surface in aqueous surfactant solutions was studied and the variation in droplet shape, particularly contact angle, and the time required for droplet detachment were measured. In parallel, tests of oil removal from solid surfaces in an industrial ultrasonic bath were conducted under conditions similar to drop-detachment tests. Consistent trends were found in both types of tests; that is, conditions for which droplets detach more quickly correspond to conditions of greater oil removal in an ultrasonic bath. At high pH, faster drop-detachment (and oil removal) kinetics were observed when the oil−water interface was negatively charged (per ζ-potential measurements). In contrast, at low-pH conditions, faster kinetics were observed when the solid−liquid interface was positively charged. The pH effect is consistent with a mechanistic model in which electrostatic interactions and solid-surface hydrophobicity changes due to surfactant adsorption onto solid surfaces under various pH conditions are considered. This study also demonstrated that single-drop-detachment tests and/or ζ-potential measurements are useful as simple, timesaving screening methods for selecting promising conditions for specific cleaning applications.

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