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Spreading and detachment of organic droplets at an electrified interface

We investigate adhesion and spreading of organic particles at aqueous/mercury electrode interface where the attractive surface forces are controlled by applied potential. Effect of potential on spreading and detachment was investigated by observing the shape of n-hexadecane droplet at the mercury pool/aqueous electrolyte interface. Mercury electrode surface is atomically smooth, fluid and chemically inert, surface charge densities and interfacial tensions are well known; the critical interfacial tension of wetting can be determined experimentally (1). Attachment of droplets depends on applied potential and reflects wetting equilibrium as a results of interfacial forces at the three boundaries and buoyancy. Microdroplets, after attachment spread instantaneously to a film at the mercury/aqueous electrolyte interface, while a larger droplet (d=5 mm) assumes a planar convex lense shape. By sweeping potentials beyond the critical value of wetting, the droplet shape changes from lense through ideal sphere to an elongated shape with decreasing contact area. The new shape established instantaneously upon changing potential. The change of droplet shape is reversible and could be repeated many times. At a constant potential, the shape of droplet remain stable over infinite period of time. Detachment of droplet (by buoyancy) takes place only when the applied potential exceeded the critical wetting potential (1) by one volt. The corresponding critical interfacial tensions of wetting and detachment of hexadecane differ by 68 dyn/cm2. The experimental difference could be interpreted as an effect of modification of the mercury/aqueous electrolyte interface by a monolayer of n-hexadecane molecules similar to recently reported behaviour of gold modified electrodes (2,3). References: 1. N. Ivošević, J. Tomaić, V. Žutić, Langmuir 10 (1994) 24125-2418. 2. D. Bizzotto, J. Lipkowski, J. Electroanal. Chem., 409 (1996) 33-43. 3. C. B. Gorman, H.A. Biebuyck, G. M. Whitesides, Langmuir 11 (1995) 2242-2246.

organic droplets; interfacial properties; attachment; charged interface

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