Suspensions of particles in a liquid medium are usually encountered in a wide range of industries and find use in different types of applications, such as medicines, ceramics, foodstuffs, liquid abrasives, inks, etc. One major criterion which is essential across these applications is suspension stability. This article considers the importance of zeta potential, particle size, and rheology on sedimentation behavior and describes how these properties can be leveraged to induce stability.
Thermodynamic and Kinetic Stability
A suspension must be able to suspend the dispersed phase for the lifespan of the product or should be easily dispersed if sedimentation occurs. Several factors contribute to the dispersed phase stability and these may be kinetic or thermodynamic in origin. Kinetic stability can be induced by improving the viscosity of the suspending medium and thus slows down the aggregation and sedimentation of particles. While in thermodynamic stability, electrostatic and steric induce stability via particle repulsion.
In case of sub-micron suspensions, Brownian motion is often important to maintain the particles in a dispersed phase, but for larger particles the effect of gravity becomes important if there is a significant variation in density between continuous and dispersed phases.
Magnitude of Zeta Potential
Figure 1. Schematic diagram showing the difference of free energy with particle separation for a suspension with (a) a high zeta potential (b) low zeta potential.
In order to prevent particles from becoming aggregated, it is essential to provide some kind of barrier. This can be done by introducing a charge onto the surface of the particle by altering the pH. In case the repulsive force surpasses the attractive force, a stable system should result. For a suspension which is electrically charged, such a force balance can be illustrated by DLVO theory where the total energy (VT) is the sum of repulsive ((VR) and attractive (VA) contributions as shown in Figure 1. This theory suggests that an energy barrier, which results from the repulsive force, inhibits two particles approaching and combining together unless the particles have adequate thermal energy to surmount this barrier. The magnitude of the zeta potential indicates the size of this potential barrier.
If all the particles in suspension exhibit a large positive or negative zeta potential, they will repel each other and there will be no inclination for the particles to come together. But, if the particles exhibit low zeta potential, then there will be inadequate repulsion to prevent the particles from coming together. The common dividing line between unstable and stable suspensions is taken as +30 or -30 mV; particles having zeta potentials beyond these limits are generally considered as stable.
In this study, the sample used was a microcrystalline silicon dioxide with a density of 2.6 g/c and a Dv(50) of 3.7 µm as measured on a Malvern Panalytical Mastersizer 3000 with Hydro S dispersion unit. The sample was analyzed using steady shear rheological measurements and zeta potential measurements to evaluate stability as a virtue of pH.
For zeta potential measurements, a Malvern Panalytical Zetasizer Nano ZS together with an MPT2 autotitrator was used to prepare a dilute dispersion of the material in deionized water for analysis. Rheological measurements were then conducted on concentrated dispersions (75% w/w) of the silica sample in deionized water. Next, samples were adjusted with HCl to provide pH values, which are equivalent to those utilized in the zeta potential experiment.
Then, using serrated parallel plates, rheological tests were carried out on a Kinexus Pro and Gemini 2 rheometer. A shear stress ramp test and an equilibrium step shear rate test were performed on the samples. In the first test, the shear stress was ramped linearly from 0 to 100 Pa in 60 seconds to ascertain the yield stress. In the second test, the shear rate was increased stepwise from 0.1 to 100 s-1 to produce a flow curve i.e. shear viscosity versus shear rate. All tests were carried out at 25ºC.
Results and Discussion
Figure 2. Zeta potential and isoelectric titration data for a standard silica sample.
Figure 3. Equilibrium flow curves for silica samples prepared at different pH values.
Figure 2 demonstrates the effect of pH on a suspension of silica particles dissolved in deionized water with a standard particle size of 3.7mµ as determined using the Mastersizer 3000. Figure 3 shows the equilibrium flow curves for silica samples prepared at varied pH values.
In spite of having a negative zeta potential beyond 30 mV, this suspension has been shown to be unstable, creating a small sedimented layer upon standing. This indicates that the forces of gravity are extremely dominant and hence zeta potential would not be anticipated to considerably affect sedimentation stability.
When gravitational forces driven by size and density of particles dominate the system, the electrostatic interactions cease to give stability and hence another means of stabilization is needed. One way to achieve this is to improve the suspension’s kinetic stability by slowing down the rate of sedimentation. The rate of sedimentation can then be calculated from Stokes' law presuming a dilute suspension of round particles .
where η denotes the shear viscosity of the continuous phase and V the sedimentation velocity.
Thus, increasing the viscosity will reduce the rate of sedimentation by the same factor, whilst reducing the size of particle size will bring down the sedimentation rate by a factor of 4. However, this expression applies only for dilute suspensions where interactions between particles are minimal. Settling of concentrated suspensions is more challenging because of the interactions between adjacent particles.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
For more information on this source, please visit Malvern Panalytical.