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Chapter 2

hydrophobic solids, the contact angle is greater than 90^o. The surface of solids is
oil-wet and the solids tend to stay in oil phase. If the solids are amphiphilic, they
will stay at the interface of oil and water. According to the Bancroft rule ^[36], the
phase in which the emulsifiers are most soluble is the continuous phase. So the
amphiphilic solids which are somewhat hydrophobic can act as the emulsifier to
form the water-in-oil emulsions.

Ifthe particle is small enough (typically less than a micron in diameter) so that
the effect of gravity is negligible, the energy (-Δ,γ√G) required to remove a particle
of radius r from an oil-water interface of tension γ_ow is ^[351:

-Δ_intG = λt²∕_ow(1±cos^_ow)²                                          [2.21]

Here the sign inside the bracket is negative for removal into the water phase,
and positive for removal into oil. Compared with surfactant molecules, clay solids
have much larger radii, thus the energy to remove a solid particle from oil-water
interface is much larger than that to remove a typical surfactant molecule. The
clay solids will stay at the water∕oil interface to make the film more rigid acting as
the barrier of coalescence.

The pH in bitumen froth treatment process is around 8.5. During bitumen
froth treatment process, clay solids in diluted bitumen adsorb some of the oil
components and become partially oil-wet. The partially oil-wet clay solids can

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