Zeta potential is a physical property which is exhibited by any particle in suspension. It can be used to optimize the formulations of suspensions and emulsions. Knowledge of the zeta potential can reduce the time needed to produce trial formulations. It is also an aid in predicting long-term stability. deflocculation. Figure 1schematically represents some of these processes.
2 VA = -A/(12 π D )
where A is the Hamaker constant and D is the particle separation. The repulsive potential VR is a far more complex function. VR = 2 π ε a ζ2 exp(-κD) where a is the particle radius, π is the solvent permeability, κ is a function of the ionic composition and ζ is the zeta potential.
Three of the fundamental statesof matter are solids, liquids and gases. If one of these states is finely dispersed in another then we have a ‘colloidal system’. These materials have special properties that are of great practical importance. There are various examples of colloidal systems that include aerosols, emulsions, colloidal suspensions and association colloids. In certain circumstances, the particles in a dispersion mayadhere to one another and form aggregates of successively increasing size, which may settle out under the influence of gravity. An initially formed aggregate is called a floc and the process of its formation flocculation. The floc may or may not sediment or phase separate. If the aggregate changes to a much denser form, it is said to undergo coagulation. An aggregate usually separates out eitherby sedimentation (if it is more dense than the medium) or by creaming (if it less dense than the medium). The terms flocculation and coagulation have often been used interchangeably. Usually coagulation is irreversible whereas flocculation can be reversed by the process of Figure 1: Schematic diagram showing various mechanisms where stability may be lost in a colloidal dispersion
ColloidalStability and DVLO Theory
The scientists Derjaguin, Verwey, Landau and Overbeek developed a theory in the 1940s which dealt with the stability of colloidal systems. DVLO theory suggests that the stability of a particle in solution is dependent upon its total potential energy function VT. This theory recognizes that VT is the balance of several competing contributions: VT = VA + VR + VS VS is thepotential energy due to the solvent, it usually only makes a marginal contribution to the total potential energy over the last few nanometers of separation. Much more important is the balance between VA and VR, these are the attractive and repulsive contributions. They potentially are much larger and operate over a much larger distance Figure 2(a): Schematic diagram of the variation of free energy withparticle separation according to DVLO theory.
DVLO theory suggests that the stability of a colloidal system is determined by the sum of these van der Waals attractive (VA) and electrical double layer repulsive (VR) forces that exist between particles as they approach each other due to the Brownian motion they are undergoing. This theory proposes that an energy barrier resulting from therepulsive force prevents two particles approaching one another and adhering together (figure 2 (a)). But if the particles collide with sufficient energy to overcome that barrier, the
Zetasizer Nano series technical note
attractive force will pull them into contact where they adhere strongly and irreversibly together. Therefore if the particles have a sufficiently high repulsion,the dispersion will resist flocculation and the colloidal system will be stable. However if a repulsion mechanism does not exist then flocculation or coagulation will eventually take place.
adsorbs, the thickness of the coating is sufficient to keep particles separated by steric repulsions between the polymer layers, and at those separations the van der Waals forces are too weak to cause the...