I am working on flow of cationic (Methylene blue Cat. no.: M9140 and Rhodamine B: R6626) and anionic (Tartrazine: T0388 and Amaranth: A1016) dyes on paper substrates. All the dyes are from Sigma Aldrich. I have obtained certain results which are hard to interpret without knowing the value of charge on the dyes. I tried measuring the charge by DLS and have obtained the zeta potential values of ~ 3mV but the zeta deviation is about +95 mV or more, for Rhodamine B. And similar data for all the other dyes as well. I want to know if DLS is the right way to quantify the charge. If it is, then how do I make sense of the large deviation that I am getting. If not, which other methods can I try to quantify the charge on dyes? I will be very grateful if anyone can help. Thank you!
Rhodamine B (RhB) is a cationic dye while tartrazine is an anionic dye. So you don't need to measure the charge on the dye rather you can estimate the surface charge/point of zero charge/isoelectric point (pHIEP) of your paper substrates. This can give you a lot of helpful information which will enable you to explain the dye adsorption mechanism. Good luck!
I agree with the previous response. Dye pKa values are available which will help you establish their charge. DLS is only possible on colloids, your dyes are small dissolved molecules and not colloids, so they do not scatter light to give meaningful results. What you observe in DLS is random noise caused by dust in your sample solution.
Priyanka Naik
Paper substrates have positive and negative adsorption centers. Dyes are adsorbed on them. No need to mix dyes before planting on the substrate, as they neutralize each other. After adsorption by spectra, determine the remaining concentration of dyes.
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