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Zeta Potential

An Introduction in 30 Minutes

Introduction

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.

Colloid Science

Three of the fundamental states of 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 may adhere 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 either by 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

deflocculation. Figure 1 schematically represents some of these processes.

Figure 1: Schematic diagram showing various mechanisms where stability may be lost in a colloidal dispersion

Colloidal Stability 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 the potential 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 contribut-ions. They potentially are much larger and operate over a much larger distance

VA = -A/(12 π D2)

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.

Figure 2(a): Schematic diagram of the variation of free energy with particle separation according to DVLO theory.

DVLO theory suggests that the stability of a colloidal system is determined by the sum of these vander 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 the repulsive 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

        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

Figure 2(b): Schematic diagram of the

variation of free energy with particle

separation at higher salt concentrations

showing the possibility of a secondary

minimum.

If the zeta potential is reduced (e.g. in

high salt concentrations), there is a

possibility of a “secondary minimum”

being created, where a much weaker

and potentially reversible adhesion

between particles exists (figure 2 (b)).

These weak flocs are sufficiently

stable not to be broken up by

Brownian motion, but may disperse

under an externally applied force such

as vigorous agitation.

Therefore to maintain the stability of

the colloidal system, the repulsive

forces must be dominant. How can

colloidal stability be achieved? There

are two fundamental mechanisms that

affect dispersion stability (figure 3):

• Steric repulsion - this involves

polymers added to the system

adsorbing onto the particle

surface and preventing the

particle surfaces coming into

close contact. If enough polymer

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 particles to

adhere.

• Electrostatic or charge

stabilization - this is the effect on

particle interaction due to the

distribution of charged species in

the system.

Each mechanism has its benefits for

particular systems. Steric stabilization

is simple, requiring just the addition of

a suitable polymer. However it can be

difficult to subsequently flocculate the

system if this is required, the polymer

can be expensive and in some cases

Figure 3: Steric and electrostatic

stabilization mechanisms of

colloidal dispersions

a ceramic slip is cast and sintered, the

polymer has to be ‘burnt out’. This

causes shrinkage and can lead to

defects.

Electrostatic or charge stabilization

has the benefits of stabilizing or

flocculating a system by simply

altering the concentration of ions in

the system. This is a reversible

process and is potentially

inexpensive.

It has long been recognised that the

zeta potential is a very good index of

the magnitude of the interaction

between colloidal particles and

measurements of zeta potential are

commonly used to assess the stability

of colloidal systems.

Origins of Surface Charge

Most colloidal dispersions in aqueous

media carry an electric charge. There

are many origins of this surface

charge depending upon the nature of

the particle and it’s surrounding

medium but we will consider the more

important mechanisms.

Ionisation of Surface Groups

Dissociation of acidic groups on the

surface of a particle will give rise to a

negatively charged surface.

Conversely, a basic surface will take

on a positive charge (figure 4). In both

cases, the magnitude of the surface

charge depends on the acidic or basic

strengths of the surface groups and

on the pH of the solution. The surface

charge can be reduced to zero by

suppressing the surface ionisation by

decreasing the pH in case of

negatively charged particles (figure

4(a)) or by increasing the pH in the

case of positively charged particles

(figure 4(b)).

Figure 4(a): Origin of surface

charge by ionisation of acidic

groups to give a negatively

charged surface

Figure 4(b): Origin of surface

charge by ionisation of basic

groups to give a positively charged

surface