Mar 11, 2026
Dynamic Light Scattering (DLS) measures the hydrodynamic size of particles in solution by analyzing their Brownian motion. When analyzing polystyrene latex particles (or other particles with a surface charge,) adding salt is crucial to ensure accurate and reproducible results.
Why Salt is Added:
- Suppresses the electrical double layer: Polystyrene latex particles are typically negatively charged, creating a diffuse and long-extending layer of counterions around them in DI water. Adding salt compresses this charged ionic layer surrounding the particles.
- Reduces hydration and structural water effects: At very low ionic strength (e.g., deionized water), the extended double layer and associated structural water increase the measured hydrodynamic radius due to loose charge interactions between particles. Adding salt compresses the double layer, reducing this effect and yielding a size closer to the true primary particle size. This charged layer without salt effectively increases the measured hydrodynamic size measured by DLS because it moves with the particle during Brownian motion and interacts with the charge from other particles, thus resulting in lower reported diffusion coefficients thus higher reported sizes.
- Improves measurement accuracy and consistency: Using salt ensures the reported hydrodynamic size matches certified values. For example, dispersing 88nm latex in deionized water can result in a 15% higher size compared to dispersion in 10 mM NaCl. When measuring polystyrene latex standards using Dynamic Light Scattering (DLS,) always refer to the certificate that comes with the latex sample and ensure you are comparing experimental results to those on the certificate measured by Photon Correlation Spectroscopy (PCS,) otherwise known as Dynamic Light Scattering (DLS.)
Brookhaven Instruments, with a legacy of pioneering in light scattering and zeta potential analysis, is committed to advancing its technology to provide reliable and innovative solutions that support modern scientific breakthroughs in nanotechnology, pharmaceuticals, and materials science.
Contact us today to learn more about our light scattering solutions for particle size and zeta potential analysis.