I am asking because if we change the dispersant, it will have different viscosity, refractive index, etc. Is there a standard way to do this? In the articles I found, there is no mention of this. Thank you.
Alan F Rawle
Sofia Moreira Fernandes Thank you for posting your question. There may be many others with similar questions. Few points:
- One doesn't measure zeta potential. It's inferred from the movement (mobility) in an electrical or acoustic field. In the absence of some applied field then simple diffusion coefficient is measured in the DLS technique. So, the actual measurement is a movement. This movement should be unhindered (no particle-particle interactions) so that a true velocity is measured and not an artificially low one from particle collisions. Thus, measurements are performed in fairly dilute suspensions usually. The consequences of this are described in the following bullet points.
- The continuous phase is thus fairly close to a/the pure liquid. Additions of ions (e.g. OH-, H3O+) make little difference to the fundamental properties of RI or viscosity
- The RI of the particle has no bearing on its zeta potential (ZP). It may have a bearing on the smallest particle's movement that can be detected as the actual scattering is much higher with higher relative refractive index. Thus, the lower size limit for measurement of ZP would be lower for a gold sol than for an aqueous protein. If the raw intensity is isolated in the absence of an electrical or acoustic field then we have a simple Brownian motion measurement (as in DLS) and, again, the optical properties of the particle are irrelevant to this. A knowledge of the RI of the continuous (background) phase is needed simply so that the deflection of the laser beam (if we're detecting the movement in this manner) can be known. This RI is easily obtained from literature or simple via experiment (Abbe refractometer). In the case of aqueous system this RI will effectively be that of water. It takes a huge quantity of dissolved solids content to increase the RI of water from 1.33 to 1.34, say
- The viscosity is important both for mobility (i.e. zeta potential determination) measurement and for DLS. Again, this will be close to or identical to that of the pure liquid and is usually easily measured with a viscometer or rheometer
- Verification of zeta potential derived from mobility can be effected by means of an appropriate standard either the NIST 1980 material or a secondary standard referenced back to that primary standard. The 1980 material actually is referenced as a positive electrophoretic mobility standard but most users see this as a 'zeta potential standard'. Many instrument manufacturers will provide appropriate secondary standards for verification
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Ulf Nobbmann
Hello Sofia Moreira Fernandes
in addition to the summary by Alan Rawle, in many cases the refractive index and viscosity will change only very slightly. In principle you can adjust the parameters for each measurement. You may also be able to
- use an automatic titrator for sample preparation, see for example https://www.materials-talks.com/complex-titrations-with-three-additives-and-variable-dosing/
- in general, some info about pH titrations and isoelectric points might be of interest: https://www.materials-talks.com/isoelectric-points-of-nanomaterials-qa/
- and then a little more advanced, specific and non-specific ion adsorption and zeta https://www.materials-talks.com/zeta-potential-in-salt-solution-or-any-other-ions/
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