A colloidal dispersion is composed of solid, liquid or gas particles dispersed in a continuous phase (solid, liquid or gas). Strictly speaking, the term colloidal refers to particles with at least one dimension ranging from 1nm to 1µm. Most commonly encountered colloidal dispersions are solid-liquid (suspensions), liquid-liquid (emulsions), gas-liquid (foams), and solid-gas (aerosols) dispersions.
Colloidal dispersion is an inherently thermodynamically unstable system because it tends to minimize surface energy. Hence, the stability of a colloidal system is inevitably linked to a notion of time, defined by process, use and/or application involved.
STABILITY OF COLLOIDAL DISPERSIONS
Two stability categories can be distinguished: colloidal stability and gravitational stability.
1. Colloidal stability relates to particle size change (e.g. aggregation or agglomeration). If particles are not subject to size variation, the dispersion is considered colloidally stable. Hence, colloidal stability depends on several types of interactions such as :
- Van der Walls and electrostatic interactions (classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory)
- Steric interactions (e.g. polymer adsorption)
- Hydrophobic effect
Therefore, it is tremendously difficult to theoretically predict the colloidal stability of a dispersion.
2. Gravitational stability refers to the ability of particles to resist particle migration
(e.g. sedimentation or creaming) and mainly depends on the rheological properties of the colloidal dispersion such as viscosity and density of the continuous phase, size, and density of the particles. For diluted colloidal particles in a Newtonian fluid, this migration phenomenon can be described by Stokes law.
Sedimentation is sometimes confusingly considered as colloidal instability.
For example, a particle dispersion in a solvent can be colloidally stable (there is no change in particle size) while it is gravitationally unstable (particles settle due to unmatched density with the solvent).
It is worth noting that destabilizing colloidal dispersion can lead to gravitational instability (larger particles start to settle quickly).
HOW TO MEASURE COLLOIDAL DISPERSION STABILITY?
Electrophoretic light scattering (ELS) is a common technique used to evaluate the potential of dispersion to remain stable. ELS allows measuring zeta-potential of a dispersion, which provides information about electrostatic interactions and, by extrapolation, to their tendency to agglomerate. The zeta-potential is considered to be a reliable indicator of dispersion stability, but several parameters such as steric effects, sedimentation, or hydrophobic effects, will also have a strong influence. In consequence, relying on zeta-potential values only can lead to false stability interpretations like, for example, with metal nanoparticles in complex media, aqueous silica sol, and oil in water emulsions.
Dynamic light scattering (DLS) and laser diffraction (LD) are generally used to measure size distribution, which is a key parameter to assess colloidal stability. However, these techniques may be unreliable e.g. for very large size distributions (the signal is biased towards larger size particles) or when gravitational destabilization (like sedimentation) is involved. Additionally, these techniques operate at low concentrations and thus, often require dilution of the dispersion, which may alter its original physicochemical state.
SMLS technique offers strong advantages for the characterization of destabilizing phenomena. Both gravitational and colloidal stability of dispersions can be assessed with minimal sample handling. More importantly, results are obtained analyzing formulations in their native states, thus ensuring the representativity of the results.
At Micron Scientific, we offer a range of SMLS based devices, Turbiscan, that provide quantitative stability analysis up to 200 times faster than conventional tests. If you would like any more information, please do not hesitate to contact us.