The addition of CaCl2 to the emulsions caused a pronounced alteration in the -potential vs. pH curves. At
low pH, CaCl2 caused a decrease in positive charge on
the droplets, whereas at high pH it caused a decrease in
negative charge. It was interesting to note that there was
a shift in the isoelectric point of the droplets to a higher
pH when the CaCl2 concentration was increased from 0
to 1 mM, but that the isoelectric point remained constant
at higher CaCl2 concentrations. The alteration in the -
potential and isoelectric point of the droplets could have
occurred because of two dierent phenomena: electrostatic screening and/or ion binding (Hunter, 1986).
The relative importance of these two phenomena can
be assessed using the principles of colloid science. The -
potential of a charged surface is related to its surface
charge density
through the following relationship
(Hunter, 1986