D2 Dynamic Light Scattering (DLS) and Zeta Potential

Topic

The Brownian motion of the particles in this colloidal suspension—the first of the two phenomena measured by this model—produces random fluctuations in the intensity of the light scattered by those particles when illuminated by a laser. Dynamic Light Scattering (DLS) records these fluctuations in scattered light and summarizes them in an autocorrelation function, which compares the scattered intensity at a given moment with the intensity at subsequent moments spaced increasingly further apart in time. Faster Brownian motion causes this autocorrelation function to decay more rapidly, as the scattered intensity loses its resemblance to the intensity from earlier moments more quickly.

According to the Stokes-Einstein relationship, the rate at which this autocorrelation function decays translates into the hydrodynamic size of the colloidal particles; this relationship directly links the speed of Brownian motion within a medium of known viscosity to the particle's effective size, including the solvent layer moving along with it. A faster decay of the autocorrelation function results—via the Stokes-Einstein relationship—in a smaller calculated hydrodynamic size, whereas a slower decay results in a larger hydrodynamic size.

The electrophoretic mobility of these particles—the second phenomenon measured by this model, representing the speed at which they move under an applied electric field—determines their zeta potential according to Henry's equation. In practice, this zeta potential quantifies the effective magnitude of the electrostatic repulsion between the particles, a phenomenon already described by another model in this set. A higher electrophoretic mobility results—via Henry's equation—in a higher zeta potential (in absolute value). Therefore, this model—utilizing the hydrodynamic size and zeta potential with which it was initiated—directly measures the colloidal stability described by another model within this set: the fact that the hydrodynamic size (measured via dynamic light scattering) increases over time serves as a direct experimental indicator that the colloidal suspension is aggregating, according to that other model; meanwhile, the fact that the zeta potential (derived from electrophoretic mobility) remains far from zero serves as a direct experimental indicator that the electrostatic repulsion required by that same model is still sufficient to counteract the underlying van der Waals attraction. Monitoring both measurements—hydrodynamic size and zeta potential—over time using this model makes it possible to detect, before it is too late, the early signs that the colloidal suspension is approaching a state where it violates its colloidal stability criterion.

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