Dynamic light scattering, also known as quasi elastic light scattering (Quels) and photon correlation spectroscopy (PCS), measures the laser light that is scattered from dissolved macromolecules or suspended particles. Due to the Brownian motion of the molecules and particles in solution fluctuations of the scattering intensity can be observed.
According to the semiclassical light scattering theory when light impinges on matter, the electric field of the light induces an oscillating polarization of electrons in the molecules. The molecules then serve as secondary source of light and subsequently radiate (scatter) light. The frequency shifts, the angular distribution, the polarization, and the intensity of the scatter light are determined by the size, shape and molecular interactions in the scattering material.
DLS measures the scattered light of a sample in very short time periods and correlates the data. Due to the fact that large molecules or particles move slower than small molecules a defined correlation function results. From the correlation function the diffusion coefficient (D) of the molecules can be calculated by fitting the data.By fitting the points of autocorrelation to the function G(t), the diffusion coefficient can be measured and related to the equivalent sphere of diameter d using the Stokes – Einstein equation:
n = diluent viscosity (water = 8.94*10-4 kg/(ms)
T = Temperature (K) (room temp = 298 K)
D = diffusion coefficient (in m2/s)
kB = Boltzmann constant (1.3807*10-23 J/K)
d = sphere diameter (m)
The DLS technique is especially impressive because there is no sample information needed. Only the solvent viscosity and the refractive index of the solvent must be known. For nanoparticles the density of the particles might be important. The refractive index increment, an important parameter in static light scattering experiments, is not involved in the calculation of Rh.