Just today Tony Harrison on LinkedIn pointed to a very helpfull risk evaluation course given by Peter Sandman that can directly also be applied for nano particle risk evaluation. It can be found easily via my personal SICO blog. Or as an excerpt also on YOUTUBE directly.

I have just issued a list of recent and upcoming NANO events on SICO's NEWS blog and I hope that this survay can help a little to keep trac of what is going on. 

An attempt to screen the actual available particle sizing methods and instruments.

Details you will find in my latest blog:

please click here for the full report

You only get what you pay for!  Are low budget instruments worth what you pay for it?

“Nano” is one of the most attractive and modern terms in research, industry and marketing.
Due to the excellent definition work of the ISO committee TC229 since August 2008 there is a precise determination what “Nano” really means (ISO/TS27687). Also development in this field, as well as increasing knowledge is growing daily at an accelerating speed.

Only the size measurement technologies and evaluation methods don’t keep pace with the changing trends. Today most sizing instruments are based on a more than 30 year old PCS (photon correlation spectroscopy) technique applying micrometer liquid layers for detection of high concentrated samples in order to minimize the influence of multiple scatter. Even with today incomparably increased  computing  power they still stick to simplest 2^nd Cumulant evaluation methods or in some cases to highly damped evaluation modes of Contin style or NNLS (non negative least square). All these instruments only present result graphs or data without checking the relevance. This means without proving, that the issued result represents the measured data correctly. Issued simplifying values as e.g. “error of fit” cannot replace the direct comparison of the measured correlation function to that one representing the issued result. Also the popular heading for “Olympic targets, faster, wider, higher” has lead to an increasing number of easy to handle but inaccurate nano size measuring instruments.

In most cases the increased speed is gained by a pre selection of measured data to skip signals that disturb the smooth curve progression of correlation curves.  All this old fashioned evaluation procedures were due to the fact that bad conditioned sets of equations could not be evaluated properly with old days computing capacity. Nowadays this is possible if one takes a little effort in adjusting the evaluation range correctly, which is assisted by the software itself in a close to perfect way already.  Not to mention the incorrectness of using an intensity matrix instead of a volume matrix for evaluation.

In general terms applying PCCS technology together with not damped  NNLS evaluation based on volume matrix, today can lead to highest sensitivity and most correct results.

Nevertheless, the market is flushed with less sensitive and incorrect instruments at lowest cost. Many of those pretend additional user value by also measuring Zeta-potential in the same instrument without extra cost. But is this kind of zeta-potential measurement worthwhile?

According to Smoluchowski’s formula applied for Zeta-potential determination (pictures see the attached PDF file please) The speed and thus the Zeta-potential is depending on the particle size. Commonly the particle size used for Zeta-potential calculation is the so called Z-average diameter or cumulative mean diameter. Already from this point of view the achieved results are highly doubtable as many samples are not mono modal and thus using a mean diameter is incorrect.    

Even though  the Smoluchowski theory is very powerful because it works for dispersed particles of any shape at any concentration. Unfortunately, it has limitations on its validity. As in real systems in addition also other properties take effect like e.g. hydrodynamic hull, ionic atmosphere and degree of dissociation of the electrolyte. It does for instance not  include the Debye length 1/k, the ratio between core size and ionic layer diameters as well as the contribution of surface conductivity, expressed as condition of small Dukhin number. Approximations try to overcome this:

The Huckle approximation for the Henry function takes care of this for
small particles in low dielectric constant media (eg non-aqueous media) in this case
f(κa) becomes 1.0 .  While the Smoluchowski approximation for the Henry function takes care of particles larger than about 200 nm in electrolytes containing more that 10³ mol of salt,
in this case  f(κa) is set to 1.5 (pictures see the attached PDF file please).

This means that using a simple PCS instrument (mostly leading to a mono modal 2^nd Cumulant result only or calculating for a mean diameter) applying the described rough estimating evaluation will lead to a Zeta-potential value of lowest precision. If this is combined with a simple dilution of the sample as necessary for PCS detection, the potential may even be changed before measurement due to non iso-ionic dilution.

This is why the use of a separate acoustic Zeta-meter instead of PCS based ones is highly recommended.

Again, with respect to size determination, only PCCS instruments with most modern evaluation algorithms can supply highest resolution results and can prove the relevance of those directly. 

For more details you may refer to http://www.sympatec.com/EN/PCCS/PCCS.html

Full text as PDF: Size- & Zeta-pot.-measurement you only get what you pay for!

According to the valid definitions in ISO/TS 27687: August 2008 and the standardl particle definitions in accordance with  ISO TC 24/SC 4,  TC 146 and TC 209 there is a clear definition of nano particles respectively nano objects. Only for these nano objects, where one, two or three external dimensions are in the nanometer range below 100nm, the term nano particles should be used.
All coarser particles are submicron particles and should not be called nano particles anymore. The same is valid for so called nano structured material, which is aggregated from nano objects.

The theme of this blog is especially the proper dispersion of nano objects as preparation for particle size measurement using PCS/PCCS. All other aspects as general handling safety etc. is not part of this blog.

To find the most suitable way of sample preparation it is necessary to understand the special    character of nano particles first. Today it is common knowledge that the physical property of a specific material changes when its particle size gets as small as below 100nm. What is the reason for this change of character? A detailed explanation would lead us quite far into atomic structures and probability areas for electrons to stay in like e.g. Niels Bohr has described them for single atoms. But this would be too complex and also a model only.  Let us try to explain it from a more global sight but even this needs to employ at least some physic rules:
The second law of thermodynamics is an expression of the finding that over time, differences in temperature, pressure, and chemical potential tend to equilibrate in an isolated physical system.
This means that always the status of lowest energy differences is headed for. Nano particles are characterized by extremely high surface areas and because of that, by extremely high surface loadings. This is due to the fact, that the probability areas for electrons to stay in is quite limited in tiny particles.                                     
In a much bigger aggregates the areas are much more flexible by coordinating the areas of many small components to a combined one.
According to the mentioned law, agglomeration will appear instantly enabled mainly by Van der Waals attraction as long as the electrons/loading is not compensated by surrounding ions.
This describes the main principle of stable dispersing nano particles. The key point is to offer enough ions to compensate the surface loading of the nano particles completely. In such a case the particles will be surrounded by an ionic layer that is rejecting the coated particles from each other securely.
                                                                   

This coating by an ionic layer can only be achieved while single particles are available. Therefore it is necessary to generate single particles for a perfect dispersion. But how to?
The safest and most successful way is to do it by growing particles to the desired size. This is the bottom up way.  You will gain easy to measure samples, that can even be diluted in a save way.
Most complicated is the way to generate nano particles by top down technologies like e.g. grinding. Also this way it is possible to gain ion coated nano particles as long as wet grinding in an ionic liquid is applied, but the difficulty is to separate these from coarse remainder. As long as this coarse remainder is present, agglomeration is possible as well as outshining the low energy signals of the nano particle in PCS/PCCS measurement. The reason for this outshining is that with the growth of particles by times 10, the scattered light intensity arises by 10⁶ in the Reyleigh regime and still by 10² in the Mie regime. In such cases the only way to gain a measurable sample with respect to the nano particles is by filtering off the coarse remainder.
Many so called nano mills are able to generate some nano particles but all of them end up with a bigger amount of µm sized remainder too.
Not to talk about the difficulties of dispersing dry nano particle powder into a liquid. The wetting of such material depends on the surface tension of the liquid.  For µm particles the use of sonication is most popular and successful. In case of nano particles this success is by far less because the particles are much smaller than the wavelength of the ultra sound. This means that ordinary ultra sonic baths cannot be used. The use of high energy ultrasonic fingers generates different additional effects that can lead to a certain state of dispersion. In the high energy sonication area cavitation as well as punctual high temperature is generating evaporation of dispersing liquid and similar effects that can overcome surface tension .

In general until now only real nano particles/objects were mentioned. In many cases common terminology also calls nano structured material nano particles. This is not only wrong by definition, but is a completely different case. In nano structured material the nano particles are aggregated not agglomerated. This means that they have to be crushed to become single particles, dispersing them to single particles is by principle impossible. For sure light bound aggregates can sometimes also be diminished by hard local sonication, but this is different from dispersion and will never reach a complete dispersion.

Wrapping up:

1. Only nano objects can be measured by PCS/PCCS to gain primary nano particle diameter.
2. Stable dispersion is a question of the proper ionic layer.
3. Bottom up designed nano particles are easiest to disperse and measure.
4. Top down designed nano particles always suffer from coarse remainders and in most
   cases need to be filtered.
5. Dispersing by means of ultra sonication should be tried only if a high energy sonication finger is available. The dispersion is enabled by side effects only.
6. Dispersing dry nano powders is the most complicated task dependent on the surface tension of the liquid. Sonication support means the same as given under 5..
7. Dispersion of nano structured material is only possible to a certain extent and in special cases only. In general it is diminishing but no real dispersing.     

Sample-preparation.pdf

In relation with production and stabilization of  Nano-objects there is very often the talk about observing/ setting the right Zeta-potential. Also a lot of actual analytical instruments  promote Zeta-potential determination together with e.g. size determination.

But let us have a closer look to the use of Zeta-potential today and its value.

In research of specific structure analysis Zeta-potential measurement is irreplaceable for:

• Finding reasons for surface loading resp. The structure of “double layers”.
• Detecting the adsorption equilibrium (and adsorption kinetic) of emulsifiers, dispersants, etc.
• Tracing dependency of interface characteristics on material properties of the disperse phase.

In contrary to this for industrial purposes the Zeta-potential is merely used for:

•  Research on flocculation mechanisms.
•  Predicting stability of emulsions and suspensions.
•  Behavior forecasting of multiple component dispersions resp. of adsorption characteristic of fine particles passing through porous media.
• Determination of optimum amounts of stabilizing additives resp. flocculants.
• Tracing the change of interface characteristics during milling additives/ dispersing / emulsifying aiming for a standardized addition of additives.

These two different tasks also need different accuracy in Zeta-potential measurement.
Research needs as accurate as possible measurement of the Zeta-potential. Most industrial tasks are just interested in stability/instability determination.

This determines the specific needs of instruments for research or stability control.

Zeta-potential is, as we have seen in my last blog: “Sample preparation for particle size determination.”  , highly affected by the concentration of surrounding ions that are forming the electrical double layers. This means that correct Zeta-potential measurement needs to be done always in original concentration or at least iso-ionic dilution. As concentration of industrial nano products are mostly very high, only instruments based on acoustic principle can be used.  All optical instruments need dilution to make the light beam pass and to avoid inhibiting amounts of multiple scattering.

This is the extremely limiting background for measuring Zeta-potential in very high diluted systems by means of extended light scattering instruments. Only if the dilution is done in accurate iso-ionic manner such a measurement would be acceptably correct but dilution means reduction of statistics which is critical as I had pointed out in my blog: “Particle size measuring in the nanometer range.”

In general can be stated, that for reasonable Zeta-potential measurement in research, only acoustic measuring principles should be applied because only those can work in original concentration with high accuracy. Light scattering instruments cannot accurately enough determine Zeta-potential as high dilution has to be applied.
The mere determination of stability does not need Zeta-potential measurement and in no case a non accurate one. The observation of changes of the amplitude of correlation function within a sequence of repeated PCS/PCCS measurements is able to indicate stability/instability extremely sensitively because slightest growth of particle size is resulting in an amplified increase of scattered light intensity. The reason for this is, that with a growth of particle size by times 10 the scattered light arises by 10⁶ in the Reyleigh area and at least still 10² in the MIE regime.  This means that the value of instrument combination of PCS/PCCS size determination and Zeta-potential is mainly promotional only. Accurate PCS/PCCS measurement can do as well and if Zeta-potential is really important acoustic principle instruments should be preferred.

Zeta-potential.pdf

In my recent blog on this theme I stated:

These early instruments needed a very robust and simple calculation mode, the ‘2nd Cumulant’ method, for evaluation because the calculation power of the then available computers were still very limited. Still today this simplified calculation method is positioned in the related ISO 13321 framework. It generates as results a mean particle diameter and a value for the width of an assumed Gaussian distribution only.

Regarding this rather abreviated definition of the results given by 2nd Cumulant method I was asked to correct it to a precise one.

As 2nd Cumulant is a typical series expansion it is not resulting in any, whatever shaped curve but in a series of moments. As the name indicates only the first and second moment of this series are evaluted. The first moment gives the value for the mean diameter and the second moment gives the width . These two values are the only result of the 2nd Cumulant method. Today nearly all instruments also do graphical reports and only for this they show an assumed Gaussian distribution based on the two values given by the first two moments of the series expansion.

This means that scientifically and mathematically correct the result of a 2nd Cumulant evaluation is only the mean diameter and the width. The presented graph as a Gaussian distribution is correctly not the result but only one possibility to ease understanding to visual skilled creatures as we are.