One of the first experiments (Brownian motion) encountered by high school students includes the monitoring of a pollen particle in water. The pollen particle moves through the liquid at a certain speed, which increases when a Bunsen burner heats up the liquid inside the beaker. The reason for the increase in speed is because the viscosity of the water decreases when the water is heated up. Less viscosity signifies less resistance to particle movement and can clearly be seen to increase. In the case of this experiment, there are no other parameters to be concerned with besides monitoring the temperature, the particles speed, and the calculation of the viscosity of the liquid. In reality, when particles decrease in size, more concern should be put on the chemistry within the fluid because the stability may tend to decrease and the mean size of the general mass of particles may tend to increase.
Therefore, generally when the water molecules (0.3 nm in size) collide with particles, they will make the particle to move with a certain velocity. The smaller the particle the greater the corollary and velocity, and the larger the particle the slower the velocity because of its much greater mass compared to the water molecule. Dynamic light scattering is based upon this basis. The lower and upper detection limits are determined by two factors. In the first case when the particle is so large, that its sedimentation speed is greater than the collision velocity because Brownian motion determines the upper size limit. The size of the molecules colliding with the particles determines the lower limit. When the size of a water molecule is approximately 0.3 nm, it is clear that the lower limit of detection is above this size. A depiction of the random movement to the particle in liquid is shown in Figures 1 and 2.
In the past, many nanometer sized experiments involved some kind of dilution of the sample in a “suitable” liquid. In this case, the definition of “suitable” is a liquid where the dilution liquor most closely matches the original conductivity, pH, and surfactant content of the original sample, to be measured. The effects of not taking care while diluting is agglomeration of the original particles. Generally, this agglomeration is caused by a change in the Zeta potential (either negative or positive) being reduced by a change in conductivity, pH, or surfactant content, and this is shown in the graphs below.
The dilution problems become more challenging as the zeta potential approaches the Iso Electric Point (IEP), at this point (0 mV), maximum agglomeration occurs. The pH is the most important parameter that affects the stability of a suspension. Graph 1 shows the quick decline in zeta potential as the pH approaches 7. Additionally, in Graph 2, the right-hand Y scale demonstrates how the Aluminum Oxide dramatically increases in size from just over 100 nm to over 1000 nm during the titration.
As a result, a product that is stable in low pH acidic dilutions reaches the isoelectric point and becomes agglomerates and unstable. Therefore, it is crucial to know the original pH of suspension and dilute accordingly with a solution of that pH.
The surfactant concentration is the second most important parameter that needs to be monitored and controlled. Again, if any dilution is to take place, the surfactant concentration in the dilution should remain the same. Surfactant can offer stability in a suspension and any surfactant dilution in the suspension can reduce the zeta potential and make the particles to agglomerate.
The conductivity or salt concentration is the third parameter that should be considered. In this case, an addition of salt makes the zeta potential to drop and agglomeration to take place (Graph 3). A major example of this effect is observed when river water that carries particles to the sea is met by salt water, where the particles agglomerate to form sandbanks. So again, upon dilution, the original salt concentration or conductivity of suspension must be maintained.
If accurate analysis of nanoparticles is a prerogative, it is very important that either the chemistry is not unobserved or that a method is available that enables the user to measure samples undiluted.
The potential to measure dispersions undiluted allows Microtrac MRB to bring the technology from the laboratory to the process.
Historically, the first optical measurement method by Dynamic light scattering (DLS) invented in the 1960s, employed self-beating (Homodyne) detection. This Homodyne detection (Photon Correlation Spectroscopy or PCS) involves monitoring the light scattering signals (photons) produced when the Helium Neon laser light and the particles interact in a cuvette and analyzing the data with an auto-correlator. Then, a Non-Negative Least Squares (NNLS) software fitting routines is used to determine a result. Initially, the scattering capture angles were set at 90° and later at lower and higher angles and the path length is typically 10 mm. In the 1980s, Microtrac MRB launched the controlled reference or (Heterodyne) detection method. This new method is not dumped with the “Black Box” stigma of PCS, as the heterodyne method directly measures the frequency shift power spectrum (FPS) and directly determines the particle size distribution with no curve fitting or predetermined data input to the analyzer.
Figure 3 displays a depiction of the two methods.
The optical arrangement (Figure 4) in Heterodyne FPS measures only the backscattered light (180°). The solid-state laser diode is in close contact with the particles and measures the back scattering from particles up to 300 µ distance (0.3 mm) from the probe tip into the media and directly collects the back scattered light and the reference laser light from the particles onto the detector. Since the path length is 30 times (10 mm compared to 0.3 mm) lesser on the Heterodyne FPS method than the PCS method, the company can measure very high concentrations which allow them to measure completely concentrated product in the processes without any requirement to dilute and the problems that can result from dilution (Ref 1).
Since no dilution is needed, FPS paves the way to measure lab samples by positioning the probe directly into the sample container. Furthermore, it can be incorporated into fully automatic at-line systems such as Chemspeed, which immediately measures sample parameters without supervision. On-line systems involve getting a bypass sample from a reactor vessel using the probe inserted in the bypass, but finally the best way forward is to directly measure the suspension in-line in the reactor.
The problems of not caring about the dilution chemistry are displayed in the comparison result (Graph 4) of a material which, when measured undiluted, shows a bimodal distribution with a shoulder at the fine end of the range. A large quantity of fines agglomerated when diluted 50/50 to provide a mono-modal distribution which would be confusing to many researchers.
This example perfectly finds the dangers of dilution and shows that if a sample can be measured undiluted, more trustworthy results will occur.
In the below examples (Graph 5), it can be observed that the Nanoflex FPS technology, which is not predetermined by the user or constrained by software models to expect a particular type of curve, produces accurate results unobtainable by PCS methods which model an apparently perfect bell curve (Ref 2).
In Graph 6, two populations are missed because of curve fitting routines which FPS does not use or require. It is revealed that a population from 15 to 100 nm which is entirely missed by the PCS result on the right. Actually, the post 1 µ population is also missed by the PCS analysis.
In the final example shown in Graph 7, the material has passed through a 0.2 µ filter which is shown by the maximum size of 0.2 µ in the FPS Nanoflex result on the left side but not the PCS result on the right side. Both the FPS and PCS systems accurately determined the lower limit of 20 nm, but the PCS result on the right side demonstrates an apparently perfect bell curve with a peak at 80 nm while the reality is depicted by the Nanoflex FPS result on the left side where the actual peak at 35 nm not 80 nm is located (an error in excess of 127%).
As the velocity of the particles is measured to determine size, it is essential that only the Brownian motion should be measured and not any created motion from natural flow or stirring. For this purpose, a sheath was created to cover the probe. The end of the sheath will certainly have liquid suspension moving around it; however, a particular distance up the sheath there will be place where there is no movement except for the Brownian motion. This is where the tip of the probe will be positioned and this will help measure undiluted suspensions in real-time without dilution. The best way this can be demonstrated is through the following experiment where a 125 nm Silica is dispersed in water and measured at 6 gradually increasing stir speeds in real-time. The reported size decreases from 125 nm to 55 nm because the effect of Brownian motion and the added velocity effect from the stirrer are being monitored. When the flow-guard sheath is integrated, the stirrer effects are completely nullified and the exact size of silica is attained at 8 stirrer speeds (Figure 5).
Figure 5. The Brownian motion, underlying principle of DLS measurements, is efficiently shielded from reactor stirring by a flow guard, making 180° backscatter measurements possible in-situ.
There is a technology that can measure suspensions without dilution, which is a clear advantage to researchers, QA and QC managers who have not identified the pitfalls of dilution before. Honestly, do they have the time to rectify the situation by carrying out all the suggested titration tests in order to completely understand and dilute the suspension with a diluent that is very close to the original formulation? Using the Nanoflex – No-dilution system, they can determine the suspension as it is without producing creating a new agglomerated suspension whose measured particle size has altered from the original undiluted size. Most importantly, what does exactly a no dilution system provides you, not in the future, but now? Well, as shown below (Ref 3), it is now possible to measure suspensions on-line (Figure 6) with or without dilution.
This on line closed loop process results shown in Graph 8 above reveals how the formed nanoparticles have a mean size of 100 nm after 30 minutes of ablation and reduce to 40 nm after 2 hours of circulation until the attaining the desired particle size distribution. Now, Microtrac MRB has eyes in the process for developing designer Nano-materials optimized for future application.
The knowledge to achieve excellent measurements on nanomaterials has been around for a long period of time; however, in many particle size measurements the thought that is required when diluting suspensions has not been used and is regularly ignored as a minor inconvenience. The truth is that even a change in water supply has the potential to change the outcome. By introducing the Nanoflex technology, Microtrac MRB has allowed users to measure their samples undiluted and thus achieve accurate results. When there is a change in temperature or pressure, some materials size changes. Therefore, the ability to measure on-line in a bypass or in situ in the process vessel with no dilution is really a “Holy Grail” for Research Scientists.
References and Further Reading
- M Trainer, P Freud, and M Leonardo, High-concentration submicron particle size distribution by dynamic light scattering, 1992, Amer. Lab. 24, 34
- B Freeland, R McCann, K Bagga, G Foley and D Brabazon, Nanoparticle Fabrication via Pulsed Laser Ablation in Liquid: A Step Towards Production Scale-up, Proceedings of the ESAFORM conference, 2017.
- Dr. Thomas D. Benen, Mike Trainer, Dr. Paul J. Freud Nanoparticle Sizing 2.0: Dynamic Light Scattering in the Frequency Spectrum Mode
This information has been sourced, reviewed and adapted from materials provided by Microtrac MRB.
For more information on this source, please visit Microtrac MRB.