4.1.2. Surface Charge

Surface charges, expressed as the Zeta Potential (ZP), critically influences the interaction of NCM with the dispersion media [72]. The ZP is commonly measured using Laser Doppler Electrophoresis (LDE), which evaluates the electrophoretic mobility of suspended NCM in a dispersion medium. It is a measure of the energy potential at the boundary of the outer layer. It is generally accepted that NCM that possess a ZP more positive than +30 mV or more negative than −30 mV exhibit colloidal stability that is maintained by repulsion due to electrostatics. Much like with PS measurements, heterogenous samples have misleading ZP measurements due to the ZP of larger particles dominating the scattering signal to the detriment of smaller particles [76]. The measurement of the ZP is, in part, dependent on the ionic strength and valency in an NCM dispersion. High ionic strength

and valency ions compress the electric double layer, resulting in a reduced ZP. The pH also has a significant influence on ZP and in alkaline suspensions, the NCM acquire a negative charge and vice versa. Therefore, ZP should be reported with a corresponding pH at which the measurement was taken [70]. Additionally, it is recommended that information of the suspension be precisely described when reporting the ZP, including the ionic strength, composition of the dispersion medium, and pH [77,78]. However, the ZP is of little consequence if the NCM is stored as a solid intended to be redispersed prior to use.
