4. Conclusions
This study has shown that droplet sizes for water in oil emulsions
as small as 50 nm can be produced using either a HPH or a
Microfluidizer at pressures of 50 MPa and greater. However multiple
passes are needed to reduce the droplet size from 100 nm
down to 50 nm for both devices, showing that for the Microfluidizer
the process is less efficient than when the emulsion is water
continuous. Our results show that for W/O emulsions the high continuous
phase viscosity leads to droplet break-up dominating over
coalescence and no evidence of coalescence was observed in either
high pressure device. The Microfluidizer usually has higher homogenising
efficiency, however, it has been shown here that both devices
produce similar droplet sizes after each pass. This is
probably a consequence of the majority of the energy dissipating
before the jets impinge as a result of the higher viscosity continuous
phase.
Salt addition reduced the interfacial tension and slightly reduced
the droplet size produced from both devices. It is proposed
that salt has two functions in high pressure homogenisation: it reduces
the interfacial activity of free fatty acids in the oil thereby
decreasing the adsorption time of PGPR and it also reduces the
electrical double layer inside the aqueous dispersed phase and
reduces interfacial tension.
Adjusting the viscosity ratio showed no change in droplet size,
suggesting that in these high pressure devices droplet break-up
occurs during elongational flow.