The effect of NaCl content on the separation behaviour of the
70/30 emulsion after microwave treatment is shown in Fig. 5.
The addition of 15 g/L NaCl appears to make little difference to
the separation of the emulsion compared with the emulsion made
with pure water. However, when 35 g/L is added the separation
time is decreased from that of the emulsion made with pure water.
Fig. 6 shows the effect of NaCl content on the separation behaviour
of the 90/10 emulsion after microwave treatment. The addition
of NaCl caused a reduction in separation time across the
whole range of heating times tested, and similar results were also
observed in the 80/20 and 95/5 emulsions.
There are several underlying physical mechanisms that combine
to give the separation profiles shown in Figs. 5 and 6. The
addition of electrolytes reduces the interfacial tension between
the oil and water phases and therefore increases the probability
that adjacent droplets will coalesce to form larger droplets. Fig. 4
shows that this effect is minimal for most of the untreated emulsions
studied. However, interfacial tension also decreases with
increasing temperature [2], so coalescence is promoted during
microwave heating, and this effect may be enhanced by the presence
of electrolytes.
The effect of salinity on the dielectric properties of the water
phase, and hence its ability to be heated, is another factor. Increasing
NaCl concentration introduces an ionic conduction component
to the loss mechanism, leading to improved heating compared
with pure water. The dielectric properties of pure and saline water
are shown in Table 2.
The dielectric property data shown in Table 2 can be used to
determine the relative electric field and power density distribution
within the water phase of the emulsions. Exact determination of
these parameters would require very sophisticated 3D electromagnetic
simulations, using sub-micron mesh sizes to account for the
size distribution of the water droplets in the emulsion. An alternative
but equally valid approach is to establish the relative power
dissipated within water droplets if the electric field strength at
the interface is constant. Fig. 7 shows the result of this approach.
The electric field decays as the distance from the interface increases,
and the relative strength of the electric field does not differ
significantly between pure water and the two salt concentrations
used in this study. However, the power density changes dramatically
due to the large change in dielectric loss factor at 35 g/L. In
this case the power density at the interface is more than three
times that of pure water, leading to higher interface temperatures
and subsequently a reduced viscosity and reduced interfacial
tension. The evidence from Figs. 4 and 7 therefore leads to the