Dear Muhammad,

Intensity value is weighted with d^6, volume with d^3, and number with d. So one can imagine if there is a portion of large particles in the sample, the difference between intensity and number values is high. If these values are close to each others, this means that you have a homogeneous sample. If you are using Malvern instrument, you get another value namely Z-average. Z-average comes out of the second cumulant analysis.

Principally one has to always report intensity weighted value (this is the real measured value by the machine before performing additional math to convert it to other values) which is sensitive to the presence of a small fraction of aggregates and clusters. But at the same time, this value is also sensitive to dusts and air bubbles which may exit in the sample. So, if there is a huge difference between intensity and volume values, I would suggest to plot volume values and see if the peak related to the intensity is still visible. If it is still clear in the volume plot, it means you have quite a portion of large particles.

Dear Muhammad,

Intensity value is weighted with d^6, volume with d^3, and number with d. So one can imagine if there is a portion of large particles in the sample, the difference between intensity and number values is high. If these values are close to each others, this means that you have a homogeneous sample. If you are using Malvern instrument, you get another value namely Z-average. Z-average comes out of the second cumulant analysis.

Principally one has to always report intensity weighted value (this is the real measured value by the machine before performing additional math to convert it to other values) which is sensitive to the presence of a small fraction of aggregates and clusters. But at the same time, this value is also sensitive to dusts and air bubbles which may exit in the sample. So, if there is a huge difference between intensity and volume values, I would suggest to plot volume values and see if the peak related to the intensity is still visible. If it is still clear in the volume plot, it means you have quite a portion of large particles.

Hi,

In answer to the second part of your question, the choice on which representation of the data to refer to will depend on what you are interested in about the sample, e.g. intensity weighted will clearly show the presence of even low quantities of contaminant, where as a volume weighted solution will show a what size is more abundant.

This is discussed further in the following article:

http://www.materials-talks.com/blog/2017/01/23/intensity-volume-number-which-size-is-correct/

http://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/

Returning to the first part of your question, hydrodynamic size includes a double layer around the particle, and the size of this is dependent on the ionic conditions of the suspension. This can be seen by preparing aliquots of sample with different concentrations of salt. The effect again will vary for different surface charges on the particle, so in general, more information is needed to be able to estimate the contribution of the double layer in the hydrodynamic size.

For molecular samples, if we know the Mw, and hydrodynamic size, we can calculate the size of the double layer.

The question has been answered very well above. I would add though a question of my own - why do you think that the hydrodynamic diameter is not the 'actual size'? The hydrodynamic diameter reflects how the particle behaves in colloidal suspension - with protective or surfactant layers. In the case of say a gold colloid the electron dense center of the particle can be shown with techniques that probe this such as electron microscopy or SAXS but the protective layer cannot be seen. Have a look at the certificates for the NIST standards RM 801x and there you'll find different values depending on how the particle is probed by different techniques. For example (see page 2 in):

https://www-s.nist.gov/srmors/certificates/8011.pdf

Thanks a lot all for valuable discussion. With reference to first part of the question, I was curious to estimate the real solid size of nanoparticles.

Hi,

DLS is not able of eatimating the physical solid size of nanoparticlea as it ia based on the diffusion of the whole particle. You should try some other techniques like TEM or SAXS.

'Real solid size of nanoparticles' is an oxymoron - in a powder there are no free independent discrete particles < 100 nm. Solid-solid diffusion means that you have a collection of fused sub- and post-micron particulate system. As Rudy Rucker said ' Dry nanotechnology is a dead-end'. See, for example (open in IE):

Dispersion and nanotechnology http://tinyurl.com/hpywsge

Dear Abbas,

For real solid size of nanoparticles, you should consider the TEM or SEM analysis of your samples. The hydrodynamic diameter of nanoparticles determined using zeta sizer is might be important to know especially for their applications involving absorption. The average hydrodynamic diameter determined using option i.e. intensity by number may give bit precise depiction.

@ Syed There is no 'real solid size' of a nanoparticle. I think you are under an illusion.

Dear @Alan why do you think there is no solid size? This is really what separates these materials from liquids and gells. By your last statement you are rejecting all others opinions mentioned above.

@ Muhammad Perhaps I am rejecting all the opinions above....

OK, let me explain my comments considering 4 systems. I’ll not even get into the concept of property equivalents that all particle sizing techniques to deal with when considering ‘size’. (See attached Basic Principles):

• Powder. Because of solid-solid diffusion any primary particles of < 100 nm will be fused by solid bonds (‘solid-bridging’) to adjacent neighbors. As an example, see an early electron micrograph from 1941 (ASTM STP 51 page 93) of TiO2 particles. This behaves as a collection of fused sub- and post-micron particles. There are no free, independent, discrete particles < 100 nm and the transport properties are those of the fused mass. For example, this system will block a 1 um filter. It is disingenuous to say that the particle size is 300 nm by some visual or software ‘dispersion’ even though the fundamental building block was (more on this later) particles maybe 10 - 20 nm in size forming 300 nm aggregates which agglomerate to larger sizes. We cannot say that there is a true solid size here

• Colloidal/sol dispersion. One of the issues of the original question. Here the system is stabilized either sterically (with a growth of polymer) or with charge (electrostatically) repulsion via a surrounding absorbed layer of ions. In both cases the surrounding layer (be it ions or polymer) is fundamental to the stability of the system. This surrounding layer is a part of the particle just as much as the core metal (for example). Differences can be seen with different techniques which examine the electron-dense core of the system (e.g. electron microscopy) as opposed to those techniques which measure the hydrodynamic diameter (e.g. DLS) and consider the surrounding layer as well. In suspension we can’t consider a solid size at all without considering the preceding and it’s obvious therefore that there can’t be a ‘true’ solid size

• Atomized particles e.g. soot from diesel engines or metals such as Ni produced by decomposition of Ni(CO)4. Here on production we can have separate discrete independent particles < 100 nm. These can be observed by techniques such as light scattering. Thus, we could consider a size here independent of any absorbed layers and we’re then into the realm of equivalent diameters for non-spherical particles. Again no ‘true’ size here but rather an equivalent size of a spherical particle with the same property of our particle. Bring the particles together or next to a substrate and we’re into the first case above. Alter our technique of observation (complementary information) and we will get another 'true size'

• A solid nanoparticle supported on an inert (?) substrate such as found in supported metal catalysts. Here we have another example of steric stabilization, but again we must define equivalents. Under electron microscopy or image analysis this may be a circular equivalent (the diameter of the circle having the same number of pixels as our irregular particle. Again we have solid-solid diffusion between the metal and the support making any metal - support boundary impossible to define. In air, or in the presence of oxygen, of course, the surface will be fully oxidized, so we should define or assume what actually is the boundary of the ‘particle’. Again we should not software deconvolute particles from the whole system. We can see again there is no true solid size in this system and that the size is both open to interpretation and definition and also related to an equivalent size dependent on the technique used to observe ('measure') the particles