I am trying to measure the particle size distribution of silica nanoparticles in mineral oil using DLS. My samples is 2 Wt% silica in oil, I have also diluted it two times and four times. But the count number I am getting even with the 2wt% is very low around 70. and the data are very noisy and I am not able to reproduce them. I was wondering should I use a higher concentration or should I use a different cell. Currently, i am using a plastic cuvette but I know that the oil does not affect the cell structure. Can someone help with this question?
It appears that your sample is sitting on the bottom of the cuvette and therefore not in free Brownian motion required for DLS. If your sample looks cloudy or faintly cloudy, there'll be enough scattering for measurement.
It is a transparent sample. However, if I add more material, the sample will turn into gel which is hard to have brownian motion. At 2 wt percent silica particles disperse very well in oil and they do not show any cloudiness. I do not know how else I can check the particle size. Do you know any other methods ?
How much scattering do you get from the oil alone? If the refractive index of the silica is very close to the refractive index of the oil, there will be very little "contrast" and thus scattering. Can you try a different oil (with a different refractive index) in your sample preparation?
Perhaps the picture is confusing – it appears to show a white solid on the bottom of the cuvette. OK, if it is transparent it’s almost certainly < 50 nm. If you dilute the original sample with IPA (this should solubilize the oil and there’s a greater RRI as Ulf hints) or similar liquid what do you see then? You could dilute with IPA, evaporate on a grid, and then examine under SEM to confirm presence of silica particles.
There are 3 things that have to work for you in order to get signal (counts). If you only have one of these then life is difficult:
- Sufficient particle size - the scattering in the Rayleigh region (typically < 60 or 100 nm) is proportional to r^6 or volume^2. In ideal situations one can get information from nm or even sub-nm sized particles
- Sufficient concentration - seems you have this
- Adequate relative refractive index (RRI). If, as Ulf says, you have an index-matched situation, then there is no optical heterogeneity and therefore no scattering. For example, if your particles have a high pore volume and absorb a lot of the oil then the effective RRI will be reduced closer to unity
Does your silica start as a 'solid'? Is it fumed silica?
Thanks so much Alan and Ulf, sorry for late reply. I did check again the refractive indices and indeed they are very close. So it might be the single factor that is causing the problem. On the other hand, the particle sizes according to the manufacturer are very variable since it is flame hydrolysis process the particle size can not be controlled. From handling them I can say definitely they are very fine nano particles but no accurate measurement has been done on them. From SEM, I did not get a reliable image, i think i saw big clumps in micron range. The particles are fused silica I believe.
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