The use of zeta potential to investigate the pKa of saturated fatty acids

• Wide variety of literature values for the pKa of saturated fatty acids.
• Microelectrophoresis used to determine pKa for C10–C18 saturated fatty acids.
• Average value of surface pKa is 4.0 for C10–C18 saturated fatty acids.
• Existence of isoelectric points of fatty acids between 2 and 3 confirmed.
• Some implications of pKa values for mineral flotation have been discussed.

Reported values for the acid dissociation constant (pKa) of the saturated fatty acids show a wide range, depending on their method of measurement. Although the pKa values of the shorter chain saturated fatty acids are generally recognised to be approximately 4.5–5.0, pKa values for the longer chain fatty acids have been reported to be much higher, up to pH 10. It is the purpose of this study to investigate the values of pKa measured using microelectrophoresis for saturated fatty acids with chain lengths between C10 and C18. This returned an average pKa value of 4.0 for these saturated fatty acids. By the method of measurement, this value should be considered as the surface pKa value as opposed to bulk pKa values. The graphs of mobility vs. pH showed isoelectric points between 2 and 3, with the existence of positive surfaces on the particles of fatty acids confirmed. Some examples of the relevance of pKa values for saturated fatty acids in the flotation of non-sulphide minerals have also been given.

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