Dynamic light scattering (DLS) is a non-invasive technique for measuring the size of particles, micelles, and macromolecules in solution. It analyzes the fluctuations in scattered light intensity caused by Brownian motion to determine the diffusion coefficient, which is related to hydrodynamic radius through the Stokes-Einstein equation.
In DLS, a monochromatic laser beam illuminates the sample, and the scattered light intensity is detected at a fixed angle, typically 173 degrees (backscatter). The intensity fluctuates over time as particles move in and out of phase with each other. A digital correlator generates the autocorrelation function, which decays exponentially with a decay rate proportional to the diffusion coefficient. For monodisperse samples, a single exponential fits the data. Polydisperse samples require cumulant analysis or multi-exponential fitting methods such as CONTIN or non-negative least squares.
The hydrodynamic diameter reported by DLS includes the particle core, any adsorbed surface layer, and the surrounding solvent shell. DLS is most accurate for particles in the 1 nm to 5 µm range. Larger particles sediment, and smaller particles produce insufficient scattering intensity.
Zeta potential is a measure of the electrical potential at the slipping plane of a particle in suspension. It is determined by electrophoretic light scattering, which measures the velocity of charged particles under an applied electric field using laser Doppler velocimetry. The electrophoretic mobility is converted to zeta potential using the Henry equation.
A zeta potential magnitude above 30 mV typically indicates good colloidal stability, as electrostatic repulsion prevents aggregation. Values below 5 mV indicate rapid flocculation. Zeta potential measurements are pH-dependent and are used to optimize formulation stability, predict protein aggregation, and characterize nanoparticle surface coatings.
DLS and zeta potential have broad application in nanoparticle characterization, protein formulation, paint and ink development, and pharmaceutical quality control.
