I began with 15 kDa dextran (-3 mV average zeta), and hit it with oxidant to produce an 80% dialdehyde content dex-aldehyde with a zeta of -7.5 mV. After dialysis, I conjugated an amine compound to the dex-aldehyde to form a polyimine and now have a polymer with a -15 mV zeta potential. These were all done in PBS pH 7.5 with 5% DMSO at 1 mg/mL concentrations.
My question is, what chemical characteristics are causing the change in zeta potentials, as I am not adding polarizable groups (like carboxylic acids) to my polymers? Also, does the RI have a large influence on the molecules at very low concentrations?
Since you didn't mention which was the oxidizing agent and the amine compound, we can only try to guess. One possibility is that the oxidation was more intense than you expected, leading to carboxylic acid groups (explains the zeta -7.5 mV). When you added amines (basic groups), you could have promoted the deprotonation of some COOH groups, increasing zeta potential (-15mV).
Like I said, we can only try to guess without knowing the exact conditions of your procedure.
Regarding the RI: for polymer solutions, dn/dc can be very important, specially with polyelectrolytes (which seems to be your case). I strongly advise you to measure the dn/dc of your sample, otherwise all the scattering data interpretations will be wrong.
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