Insight into zeta potential measurements in biopolymer film preparation

Micro- and nano-particle charge is one of the main factors determining the physical stability of both emulsions and suspensions and can be quantified by measuring their so called “zeta potential”. When all the particles have a large either negative or positive zeta potential value, they will repel each other and, as a consequence, the suspension becomes stable. By contrast, whether the zeta potential is close to 0 mV, the tendency for flocculation increases. Zeta potential is, however, a feature of the particle in its environment and not of the particle itself. In fact, its net charge in solution affects the ion distribution surrounding the particle, thus resulting in an increase in the concentration of counter-ions. The region over which this influence extends is called “electrical double layer” (EDL) and EDL splits into two regions (Figure 1). In the first, called “stern layer”, the ions are of opposite charge with respect to the particles and, being strongly bound to them, move with them. The second layer, conversely, is a “diffuse layer” where the ions are less strongly attached and, inside it, there is a boundary line between the ions moving with the particles and the not moving ones. This region, called “slipping plane”, is known as the surface of hydrodynamic shear and the potential existing in the slipping plane is called zeta potential.