I have observed this in literature and in the results of our research group that polydispersivity/polydispersibility index (PDI) increases as the nanoparticle size decreases, when size is reduced by changing the composition and not the stirring rate. What is the phenomenon behind it? I am concerned specifically with solid lipid nanoparticles and niosomes, both of which use surfactants/emulsifiers as one of the major components.
Hi Mubashar,
This is probably due to the fact that the mean hydrodynamic diameter is biased toward larger sizes. In other words, one big particle is more important than 100 small particles in defining the average diameter. The same artifact might happen to the PDI.
That is why both the diameter and PDI should be interpreted with caution. They might not have an accurate physical meaning.
Cheers
Hi Mubashar,
This is probably due to the fact that the mean hydrodynamic diameter is biased toward larger sizes. In other words, one big particle is more important than 100 small particles in defining the average diameter. The same artifact might happen to the PDI.
That is why both the diameter and PDI should be interpreted with caution. They might not have an accurate physical meaning.
Cheers
Dear Mubashar
I agree with Julio.
In addition PDI is based on the size distribution . The more the homogeneous population (size based) the lesser will be your PDI. If you are using Malvern,I guess, you may get effective diameter which is d50 value....!. So it is nothing do with the size on lower side or higher size but the size distribution. So most of the time it is difficult to control the size distribution of the polymeric nanoparticles as compared to microemulsions and metalic NPs.
So try to check D10, d50 and d90 for your system along with PDI. One can also refer to the lognormal or mean square displacement (MSD) plots to understand the size distribution.
Also note the size you will get from DLS, is hydrodyanamic [i.e. it consider your particle as spherical (regardless of its shape)] and approximate size. This size most of the time is slightly higher than the actual one (as that of with SEM/TEM).
Hope this has clear your doubt.
best luck
Thank you Sagar Bachhav, I am using Malvern ZS90. Please tell me how to select D10, D50 and D90 option.
Hi,
Another explanation could be:
When we measure size of large particles, presence of similar sized artefacts (and possibly gas bubbles in case of liquid samples) does not significantly affect their size distribution and PDI.
However, for a population of particles in nanosize ranges, presence of any micrometric artefact would cause their size distribution curve to extent to the average size of large impurities in the sample, hence resulting in artificially bigger PDI.
Although manufacturers of particle-sizing equipment are improving the mathematical calculations and softwares of their techniques to ignore particles in the extreme two ends of size curve, this has been proven to solve the problem we are talking about here partially.
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