270 Emulsions and foams
With yOw close to zero, microemulsions will form spontaneously
and are thermodynamically stable. The droplets of microemulsions
tend to be monodispersed. A microemulsion may form as a separate
phase in equilibrium with excess oil (O/W) or water (W/O) (i.e. it is
saturated with respect to droplets). Microemulsions are usually of
low viscosity.
Microemulsions represent an intermediate state between micelles
and ordinary emulsions, and it is a debatable issue whether or not
they should be considered as swollen micelles rather than as small-
droplet emulsions. Droplet size, though small, is nonetheless large
enough to justify classification as emulsions. On the other hand, the
observed thermodynamic stability and reproducibility is uncharacter-
istic of ordinary emulsions.
Microemulsions are potentially exploitable in any situation where
the mixing of oil and water is desired. The possibility of using them to
enhance tertiary oil recovery has recently attracted a great deal of
attention.
The recovery of oil from natural reservoirs involves three stages. In
the primary stage, oil is forced out of the reservoir by the pressure of
natural gases. When this pressure is no longer adequate, the
secondary stage is effected in which water is pumped into the
reservoir to force out further oil. This still usually leaves well over
half of the total oil unrecovered, most of it being trapped in the pore
structure of the reservoir by capillary and viscous forces. In the
tertiary stage, some of this residual oil can be recovered by injection
of oil-miscible fluids and/or forced out by steam under high pressure.
The injection of appropriate microemulsifying surfactant mixtures
offers the possibility of significantly enhancing this final stage of oil
recovery.
Foams129-130
A foam is a coarse dispersion of gas in liquid, and two extreme
structural situations can be recognised. The first type (dilute foams)
consist of nearly spherical bubbles separated by rather thick films of
somewhat viscous liquid. The other type (concentrated foams) are
mostly gas phase, and consist of polyhedral gas cells separated by thin
liquid films (which may develop from more dilute foams as a result of