Stability of oil-in-water emulsion with mobile surface charge

Experimental and theoretical studies have been carried out for clarifying the effect of the migration of surface charges on the stability of electrically charged oil-in-water (O/W) emulsion droplets. Application of a 10 kHz +20 V/−20 V square wave to a dense oil-in-water emulsion does not accelerate the demulsification, because the migration of surface charges during a half cycle is compensated during the next half cycle. On the other hand, application of a 20 V/0 V square wave of the same frequency shortens the demulsification time from 1 h to only 1 min, because the migration of the charges during a 20 V half cycle is not completely compensated during the next half cycle. This experimental result confirms that the migration of surface charges induced by the approach of emulsion droplets also plays a crucial role in the coalescence of emulsion droplets under no external electric field.The effect of the migration of surface charges on the stability of oil-in-water emulsion systems under no external electric field has been theoretically estimated in the framework of the DLVO theory by calculating the height of energy barrier preventing the coalescence of two-charged oil droplets. The energy is expressed byU=2εlκς2(zd2e2η+εlκkT)e−κwzd2e2η(1+e−κw)+εlκkT(1−e−κw)−AH12πw2where ɛl is the dielectric constant of water; κ  , the Debye–Hückel reciprocal length; ww, the separation distance between two oil surfaces; ζ, the zeta potential; ±zde are electric charges of adsorbed positive and negative ions on the surfaces; kT, the thermal energy; AH, the Hamaker constant; and η, the total number density of the adsorbed positive and negative ions and is equal to the absolute value of (ɛlκ/zde)ζ if only positive or negative ions are adsorbed. The height of the energy barrier estimated from the above equation is much different from that estimated from a conventional equation that is derived under the assumption that the electrostatic potential between two-charged droplets is given by the superposition of the electrostatic potentials of the isolated droplets.