Colloids Applications

As particle size decreases, surface area increases as a function of total volume. In the colloidal size range there is much interest in particle-particle interactions. Most colloidal commercial products are designed to remain in a stable condition for a defined shelf life. Milk is an example where homogenization is used to reduce droplet size to delay the onset of phase separation (i.e., creaming with the fat rising to the surface). Commercial suspensions may be formulated to keep particles in suspension without sedimenting to the bottom. Examples of phase separation mechanisms are shown below.

The following forces play an important role in the interaction of colloid particles:

• van der Waals forces: This is due to interaction between two dipoles that are either permanent or induced.
• Electrostatic interaction : Colloidal particles often carry an electrical charge and therefore attract or repel each other.
• Steric repulsive forces between polymer-covered surfaces arise due to osmotic repulsion and volume restriction.

Stabilization serves to protect colloids from aggregation and/or phase separation. The two main mechanisms for colloid stabilization involve steric and electrostatic modifications. Electrostatic stabilization is based on the mutual repulsion of like electrical charges. By altering the surface chemistry to induce a charge on the surface of particles it is possible to enhance the stability of the colloidal dispersion.

Scientists working to improve colloidal stability measure particle size, zeta potential, or both. Various techniques are now capable of measuring particle size into the colloidal region including dynamic light scattering (DLS), laser diffraction and acoustic attenuation. The SZ-100 nanoPartica DLS system can measure particle size and zeta potential of colloidal dispersions and has the option of an automatic titrator for zeta potential vs. pH studies. The LA-950 laser diffraction particle size analyzer is the best choice when particles above 1 micron may also be present in the particle system. The DT-1201 acoustic spectrometer is uniquely capable of measuring both particle size and zeta potential of colloidal dispersions without dilution.

Download the application note on Colloid Stability (You need to be logged in).

Frequent system verification should be an integral component in the use of any particle characterization effort using any instrumental technique. Various particle size standards and reference materials can be used for the verification process. Ludox™ silica is a well known and characterized colloidal material that has been studied using various particle size analysis techniques including acoustic spectroscopy, laser diffraction and dynamic light scattering. The results from these studies indicate that Ludox™ silica is an appropriate sample to use as a reference material.

Download the Technical Note on using colloidal silica as a particle size and charge reference material (You need to be logged in).

Download the Application Note on sizing colloidal silica particles using DLS (You need to be logged in).