The hydrodynamic radius rH is one of the main results from a dynamic light scattering experiment on Prometheus Panta and is calculated by entering the measured diffusion coefficient into the Stokes-Einstein equation.

The rH represents the size of a hypothetical hard sphere that diffuses, i.e. undergoes Brownian motion in the same way as the particle under investigation. This means that every factor influencing diffusion of the particles in solution will influence the output rH.

Factors influencing diffusion and thereby also the rH (see image below):

• Particle core size: Large particles will diffuse slower than small particles.
• Ionic strength of the solvent: Low ionic strength (<10 mM) extends the electric double layer and the particles appear larger.
• Particle surface structures: Any change to the particle surface, such as compounds attached via linkers or adsorbed polymers, will reduce diffusion speed and the particles will appear larger.
• Shape: For non-spherical particles, the output rH will be a mixture of the rH 's along all different axes of the particle. Therefore, the hydrodynamic radius is not necessarily comparable to the size determined with other methods like transmission electron microscopy (TEM).
• Weak inter-particle interactions: Attractive or repulsive forces may reduce or increase diffusion speed, respectively.
• Temperature and Viscosity: Increasing temperature will increase diffusion speed, while increasing viscosity decreases it. Viscosity itself is also temperature-dependent. Both have to be accurately known and to be entered into the Stokes-Einstein equation. 

In PR.Panta Control, the rH of particles in a sample is obtained by fitting the autocorrelation function with suitable algorithms. The result will be the average radius over all particles in the solution or a distribution of radii. 

Note that different sizing techniques will report different types of radii. The rH is, for example, not equivalent to the radius of a particle obtained through electron microscopy. When comparing sizing results from Prometheus Panta to literature values, ensure you are not comparing apples to oranges and check if the size in the reference refers to the radius or the diameter.