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Hi Sir,
I am trying to figure out zeta potential of different polymer nano particles in Milli-Q and HEPES buffer. I formulated HMPC-LF nanoparticles with hydrodynamic particles size of around 250nm, it shows positive zeta potential both in Milli Q (pH 4) and HEPES buffer (pH 4), HPMC-LF structure is given in the attachment. It contains 15% succinoyl and 8% acetyl groups, how i can explain positive zeta potential in this case where all functional groups can only generate negative ionic specie as i understand. Moreover other modified form of HPMC (HF and MF) with same structural features but only vary in degree of succinylation and acetylation, produce negative zeta potential in Milli Q(pH 4) and HEPES buffer(pH 4), instead.
i will be grateful to you if you can guide me through.
thanks
Kind regards
Wali Inam
Masters in drug discovery and development
Abo akademi University
Finland
@Wali Inam If a particle has negative charge on the surface then it will be surrounded by a very strongly bound layer of positive counter ions. Zeta potential is measured at the slipping plane which reflects how the particle interacts with other charged particles. This slipping plane is some distance away from the particle and it's important to realize the zeta potential is not the same as surface charge. A particle moves carrying its counter ions with it and thus a positive zeta potential (particles moving to a negatively charged electrode in afield) is quite believable in this case.
@Wali Inam
Functional groups like -OH and -COOH can be protonated as well as dissociate:
R-OH2+ + -OH R-OH + H2O R-O- + H3O+ (silica is a good example of this)
R-COOH2+ + -OH R-COOH + H2O R-COO- + H3O+
In your materials, you have a variety of these functional groups. Each will have a particular pKa value for the equilibrium leading to an overall net charge at a given pH and a particular zeta potential (assuming the relationship between the net charge and zeta potential per Alan F Rawle ).
By changing the levels of substitution, this net charge at a given pH is likely to change, hence the zeta potential will likely change, too.
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