For silica particles (size 1-5 um), with increasing the concentration of monovalent salts (NaF, NaCl, NaBr, NaI, etc.) from 1mM to 10 mM, more negative electrophoretic mobility(EM) and subsequently more negative zeta potential (ZP) values were obtained. Then with increasing the concentration to 50mM the magnitude of ZP decreased toward the positive values (attached figure). From EDL theory we know that with increasing the concentration (Ionic Strength) of ions, the Debye length is decreased and due to the accumulation of counterions around the particles, the surface potential will be screened and less negative ZP is expected to be obtained. Meantime, studies have shown that monovalent anions (Cl, Br, F, etc.) do not adsorb in the Stern layer around silica particles to make the ZP more negative. Meantime, for 10mM solutions of these salts, usually, two different sets of data were obtained., i.e, ~30% of the data are less negative than the ZP of 1mM solution while ~70% of data are more negative. Similar results have been published lately ( Article Effects of sort and concentration of salts on the electrosur...
).How can I explain these increase in the ZP with increasing the concentration from 1mM to 10 mM?
Foad Raji
I think that over the past time you have found why the potential is becoming more and more negative. You and I should be interested in the zeta potential jump from the number of dimensions. The curve shows the phase transition in the system versus the number of measurements. This can happen due to contamination of the cell. Ion adsorption on the electrodes or over time you have something wrong in the sample if you took from one sample. It would be nice if you use a timeline. If you can please be sure to answer me.
Hello Foad Raji , it is not clear to me what the x-axis in your plot represents. It is not related to time? Are you indicating that for your 10mM data, the results randomly flip between negative ~55mV and negative ~35mV with the majority being the lower value? Is the corresponding measured sample conductivity constant or does that also show a step transition? Do the data have the same (or close to the same) number of sub runs? Were these data taken of a single sample in one cuvette and repeated measurements, or new sample in different cuvettes?
Hello Ulf Nobbmann ,
Thank you for your consideration. The x-axis presents the manually rearranged zeta potential values in ascending mode. The zeta potential for 10 mM randomly flipped between -35 to -55 mM. The conductivity of the solution was approximately the same for all the measurements.
After preparing 10mM solution, silica particles were added to the solution (1 g/l) and it was stirred for 30 minutes. Then it was allowed for coarse particle to be settled down (10 min) and 5 ml from top of the solution was transferred to the cuvette. The zeta potential was measured in multi-cycle mode for 10 measurements in a raw. Then it was repeated for the same solution in another 10 cycle. After that, I changed the solution with a new one and washed both the cuvette and the electrode and repeated the measurements for 2 additional 10 cycles. The trend of changes for both of the solutions was approximately the same. The majority of the measured z-potential values are more negative compared to 1mM solution.
The attached file shows the zeta potential results for 20 cycles in a raw for 1mM and 10 mM KNO3 solution at pH 5.
Thank you in advance for your consideration.
Thanks for the clarifications. I am not sure why you see the flip between two values, it could be an artifact of your instrumentation or method. In general, if increased concentration of one species changes the value of zeta this could be an indication of one species attaching to the surface. The more ions you add, the higher the coverage of the particle surface - up to a point then Debye screening would take over.
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