Dynamic Light Scattering Particle Size Analysis

Dynamic light scattering (DLS) is a non-invasive, well-established technique for measuring the size and size distribution of molecules and particles in the sub-micron range. Typical applications of dynamic light scattering are the characterization of particles, emulsions or droplets which have been dispersed or dissolved in a liquid. The Brownian motion of particles or molecules in suspension causes laser light to be scattered at different intensities. The larger the particle, the slower the Brownian motion will be. Analysis of these intensity fluctuations yields the velocity of the Brownian motion and hence the particle size using the Stokes-Einstein relationship. DLS provides a value that refers to how a particle diffuses within a fluid so it is referred to as a hydrodynamic diameter.

Variables that affect the DLS measurements are as follows:

• Scattering Angle: Particles of different sizes scatter with different intensities in dependence of the scattering angle. Thus the angular dependence of the scattered light must be determined to obtain an accurate particle size. A high-quality analysis should always be performed at several scattering angles (multiangle DLS).
• Ionic Strength of Solvent: The ions in the medium and the total ionic concentration can affect the particle diffusion speed by changing the thickness of the electrical double layer called the Debye length.
• Sample Structure: The nature of the surface and the particle shape can change the apparent size by several nanometers. A rod like particle or polymer projecting out into the medium will reduce the diffusion speed.