The characterization of nanoparticles in solution create since the characterization techniques use light scattering specifically while determining stability and size distribution. Bubbles are bigger than nanoparticles and will exponentially scatter more light while interfering with particle movement, giving inaccurate results. Bubbles are normally introduced when samples are pipetted into small cuvettes that are commonly used for dynamic light scattering which is the existing method for calculating the size distribution of nanoparticles in solution.
The larger issue is that during zeta potential measurements bubbles are spontaneously formed. Zeta (ζ) potential is a physical parameter which represents a particle's surface charge and can be directly related to the particle's stability in solution. For quantifying zeta potential, a voltage is applied to the solution and the electrically directed speed of the particles is determined through a light scattering technique, known commonly as Phase- Analysis Light Scattering (PALS). Bubble generation is also caused by electrolysis, an insidious reaction that occurs in all zeta potential measurements.
Electrolysis is a phenomenon based on basic principles when a direct electric field is applied to a conducting solution, an otherwise non-spontaneous decomposition reaction occurs. Low voltages can be applied to a saline solution and electrolysis will occur because of its high degree of electrical conductivity. The end result involves splitting of water into its constituent elements, hydrogen and oxygen. This surely leads to gas formation at the site of the reaction at room temperature and pressure.
This can be seen by bubbles formed at each electrode, the cathodic electrode reaction will result in hydrogen gas evolution (Reaction 1) while the anodic electrode reaction will result in oxygen gas evolution (Reaction 2) at applied voltages above 1.7 V.
Reaction (1) 2 H+ (aq) + 2 e- → H2 (g)
Reaction (2) H2O (l) → O2 (g) + 4 H+ (aq) + 4 e-
The DelsaMax PRO offers simultaneous dynamic light scattering (DLS) and zeta potential measurements with rapid speed, with 32 independent detectors working together. With a particle diameter sizing range of 0.4 nm to 10,000 nm for DLS and a particle radius sizing range of I nm to 7,500 nm for zeta potential, the DelsaMax PRO represents the latest generation of light scattering technology for measurements in the submicron range. However, even with rapid speed, a small inner-electrode spacing of 1.6 mm and platinum electrodes which minimize gas
evolution. The DelsaMax PRO may still generate bubbles during zeta potential measurements in highly conducting aqueous solutions. The DelsaMax PRO can be connected to the DelsaMax ASSSIST in order to reduce erroneous measurements due to bubble formation. The DelsaMax ASSIST pressurizes the flow cell of the DelsaMax PRO by first closing the inlet and outlet valve for sample flow, then a separate gas source pressurizes the closed system up to 500 psi approximately 34 bar.
The overall impact of the DelsaMax ASSIST is similar to the reverse of opening a pressurized soda bottle. When depressurized, the gas solubility in the soda drops and bubbling starts. This natural phenomenon is reversed by the DelsaMax ASSIST and the pressurization system eliminates bubbles from the flow cell in three highly effective ways. The following equations govern the impacts.
Note in Equation 1, c represents the saturated gas concentration in solution while kH is Henry's law constant. In Equation 3, γ is surface tension and R is the radius of the bubble.
Firstly and most importantly the gas solubility in solution increases with high partial pressure ρ commonly called Henry’s law as shown in equation 1. Thus evolved gas bubbles will dissipate into solution, eliminating bubbles and the spurious results caused by the light that bubbles scatter.
Next as dictated by the ideal gas law as shown in Equation 2, gas at the same temperature and higher pressure will occupy a lower volume; an order of magnitude increase in pressure leads to a corresponding order of magnitude decrease in bubble volume, resulting in a 100-fold decrease in light scatter intensity due to bubbles. Finally, the decreased bubble volume will result in an increased radius of curvature of the bubbles and higher surface tension. The higher surface tension leads to increased Laplace pressure leading to bubble collapse.
Utility of the DelsaMax ASSIST with the DelsaMax PRO
The utility of the DelsaMax ASSIST with the DelsaMax PRO is demonstrated in Figure 1. The steps followed are:
- 100 nm latex control beads were diluted by adding three drops of beads in 10 ml of carbonated water.
- The solution was immediately injected into the DelsaMax ASSIST which was connected to the DelsaMax PRO.
- The system is next pressurized with a nitrogen gas source
- Trials were run at 25° C, four acquisitions/ run, and five seconds/acquisition.
- The six trials ran in an unpressurized state (4.9 psi) had an average diameter of 4,306 ± 1,680 nm.
- The light scattered by CO2 bubbles led to highly skewed diameters. The six trials ran in a pressurized state (29.9 psi) had an average diameter of 104.4 ± 7.0 nm.
The result agrees well with the latex bead assay sheet value of 100.32 ± 12.313 nm. Figure 2 is a plot of the Phase-Analysis Light Scattering (PALS) forward monitor amplitude during each trial run. The forward monitor amplitude is the measure of the unscattered and unabsorbed light transmission through the flow cell. For this specific experiment, a near-zero PALS forward monitor amplitude could only be caused by bubble formation throughout the entire flow cell volume.
Figure 1. Plot of reported 100 nm bead diameter vs. flow cell pressure. At ambient pressure, bubble formation of evolving CO2 gas from the seltzer water dominates the light scattering signal, giving spurious results. In a pressurized state above 2 bar, the gas bubbles either collapse or dissolve into solution, allowing the true size distribution of the 100 nm standard latex beads to be measured. Error bars in the graph are the trial polydispersity.
Figure 2. Plot of Phase-Analysis Light Scattering (PALS) forward monitor amplitude over the course of each trial. During trials 1 through 3 and 7 through 9, when the flow cell is in an unpressurized state, bubbles scatter and obscure the light, leading to a forward monitor amplitude of 0.002 V. In a pressurized state, the flow cell is nearly optically clear, with only minimal light scattering from the dilute 100 nm latex beads, leading to a high PALS forward monitor amplitude above 2 for all pressurized trials.
This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.
For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.