I have prepared an emulsion using a plant extract as an emulsifier. the extract is rich in saponins (around 4%), and sugars (more than 50%). It also contains 1% of proteins. I used this crude extract (2% w/w) to preparre o/w emulsions (10% oil). The emulsions were pretty stable at all ph values except of 1 and 2 were there was a huge increase in the drople size (from 110 nm to 270 nm and 627 nm respectively). However, they were relatively stable at ph 3 (157 nm). When I measure te zea potential it gives me values of -31 mv, -18 mv and 0.21 mv for ph 3, 2 and 1 respectively. I believe the active compound is saponins butt this molecules have a pka of 3.6 which makes me wonder why the emulsions were stable at ph 3 (below the pka).
So at pH above 3.6 saponins have negative charge? And are they neutral at pH below 3.6?
Dear Omar Gonzales-Ortega thank you very much for yoir reply. Yes saponins are supposed to be neutral at ph below 3.6! That's why I wonder how the emulsions were stable at ph 3 which is below the pka. In principal this is due to the relatively high zeta potential at ph 3.0 (-31mv). So I would like to understand what's the possible source of this negative charge and therefore the stability of emulsions at ph 3 (below the pka of saponins)
Thank you in advance
In theory at pH 3.0 you have 20% of the saponins with a negative charge according to the reaction
neutral saponin = negative saponin + H+
Nevertheless when the negative saponin starts covering oil droplets it will seemed as it is disappearing from the aqueous solution. Thus the previous reaction will promote the production of more negative saponin molecules to fulfill the equilibrium requirement.
Dear Omar Gonzales-Ortega,
Thank you very much for your reply.
If you can please give me more details about this or some references to check i would be very appreciative. One more thing, this extract is not only made from saponins. It only has 4% which mean that when i make an emulsion with 1% w/w extract as surfactant it only contains (the emulsion) 0.04% w/w saponins. What do you think about this sir?
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