Measuring Colloidal Stability

Colloidal stability is one of the primary concerns of chemists working with mixtures that are intermediate between homogenous and heterogeneous. This requires two or more distinct phases to coexist in equilibrium, typically a mobile-phase dispersant and a dispersive (solid particles, oil droplets, etc.). The underlying problem therein is that heterogeneous mixtures tend towards separation due to the distinct physicochemical and thermodynamic properties of their constituent parts.

A homogenous suspension, usually referred to in the literature as a solution, typically exhibits fair system stability due to the comparative similarity between the mixture’s constituent parts. It could be argued that colloidal stability is a matter of engineering heterogeneous mixtures that behave homogenously; although a truly homogenous solution would be incapable of meeting the many demands placed upon colloidal systems.

In this blog post, Particulate Systems explores the colloidal stability in more depth, with a focus on how to measure colloidal stability of liquid-based systems using industry-leading Turbiscan® technology.

Colloidal Stability in Different Systems

To understand the sheer breadth of colloidal chemistry, and the difficulty associated with measuring colloidal stability, it is worth plotting out examples of the different types of colloids based on dispersant-dispersive mixtures:

  • Gas-liquid colloids refer to liquid droplets dispersed in a gas-phase dispersant, the most common examples of which are liquid aerosols (i.e. spray-based lubricants).
  • Gas-solid colloids feature solid particles dispersed in a gas-phase (i.e. dry powder aerosols).
  • Liquid-gas colloids use a liquid continuous phase medium to disperse gas particles, typically resulting in foams (i.e. whipped cream).
  • Liquid-liquid colloids refer to a mixture of two immiscible fluids or gels, which is also described as an emulsion (i.e. milk).
  • Liquid-solid colloids refer to any mixture of a liquid continuous phase and solid particles, often called a sol (i.e. paint).
  • Solid-liquid colloids involve the dispersion of liquid droplets in a solid or semi-solid continuous medium. These mixtures are more commonly known as gels (i.e. gelatin).

Solids can be used as a continuous-phase medium for gases and other solids, but they are generally interrogated for stability via different means due to their generally solid structure.

The above mixtures, meanwhile, can generally be tested for stability using a choice of similar technologies. Accelerated aging via heating and centrifugation can provide interesting insights into the destabilization phenomena affecting colloidal systems, but such measurements are typically unrepresentative of the bulk material in real-world conditions. Many measurement techniques also rely on the qualitative determination of instability such as visual control.

Measuring Colloidal Stability with Particulate Systems

Particulate Systems supplies the Turbiscan® TOWER for multi-sample colloidal stability analysis, building on the industry-tested success of the full Turbiscan® range. This flagship instrument is built to measure the stability of liquid-based colloids using the principle of static multiple light scattering (S-MLS).

S-MLS detects particle migration and size evolution using an incident light source and two distinct optical detectors; transmission and backscattering. These signals are intrinsically related to particle size and concentration. Monitoring these channels over time provides accurate insights into particle size evolutions and migrations over time, with user-defined measurement periods providing the greatest possible insight into both short- and long-term stability.

The Turbiscan® TOWER features six distinct sample positions that can each be configured for distinct temperature profiles (0 – 80°C) to simulate different use/storage conditions and acquire a faster colloidal stability assessment than ever before. These data are based on fully-quantitative methods, providing one of the most reliable methods available for colloidal stability measurements. If you would like to learn more about how to measure the colloidal stability of your system using the Turbiscan® TOWER, simply contact a member of the Particulate Systems team today.

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