I.3.2 Particle Z-potential measurements

Zeta potential is a physical property which is exhibited by any particle in suspension and is related to the surface charge of the particle itself. This charge at the particle surface affects the

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distribution of ions in the surrounding interfacial region, resulting in an increased concentration of counter ions (ions of opposite charge to that of the particle) close to the surface. Thus an electrical double layer exists around each particle (Figure A.I.11).

Figure A.I.11. Schematic illustration of Z-potential.

The liquid layer surrounding the particle exists as two parts; an inner region, called the Stern layer, where the ions are strongly bound and an outer, diffuse, layer where they are less firmly attached. Within the diffuse layer there is a notional boundary inside which the ions and particles form a stable entity. When a particle moves (e.g. due to gravity), ions within the boundary move with it, but any ions beyond the boundary do not travel with the particle. This boundary is called slipping plane and is the distance at which it is determined the z-potential.

It has long been recognized that the zeta potential is a very good index of the magnitude of the interaction between colloidal particles and measurements of zeta potential are sometimes used to assess the stability of colloidal systems. If all the particles in suspension have a large negative or positive zeta potential then they will tend to repel each other and there will be no tendency for the particles to come together. However, if the particles have low zeta potential values then there will be no force to prevent the particles coming together and flocculating. The dividing line between stable and unstable suspensions is generally taken at either +30 or -30 mV. Particles

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with zeta potentials more positive than +30 mV or more negative than -30 mV are normally considered stable. However, if the particles have a density different from the dispersant, they will eventually sediment forming a close packed bed (i.e. a hard cake).

One of the most important factors that affect the z-potential is the pH of the sample. A zeta potential value on its own without defining the solution conditions is a virtually meaningless number. For instance, if we consider a particle in suspension with a negative zeta potential, by adding alkali to this suspension then the particles tend to acquire more negative charge whereas, by adding acid to this suspension then a point will be reached where the charge will be neutralized. Further addition of acid can cause a building up of positive charge. There may be a point where the zeta potential passes through zero. This point is called isoelectric point and is practically very important being normally the point where the colloidal system is least stable.

Also ions in solution can affect the zeta potential. They can interact with charged particle surfaces in one of two distinct ways:

1) non-specific ion adsorption where they have no effect on the isoelectric point;

2) specific ion adsorption, which will lead to a change in the value of the isoelectric point. The specific adsorption of ions onto a particle surface, even at low concentrations, can have a dramatic effect on the zeta potential of the particle dispersion. In some cases, specific ion adsorption can lead to charge reversal of the surface.

The Zetasizer Nano series calculates the zeta potential by determining the electrophoretic mobility and then applying the Henry equation. The electrophoretic mobility is obtained by performing an electrophoresis experiment on the sample and measuring the velocity of the particles using Laser Doppler Velocimetry (LDV).

An important consequence of the existence of electrical charges on the surface of particles is that they will exhibit certain effects under the influence of an applied electric field. When an electric field is applied across an electrolyte, charged particles suspended in the electrolyte are attracted towards the electrode of opposite charge (Figure A.I.12).

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Figure A.I.12. Zetasizer cell with electrodes at either end to wich a potential is applied.

Viscous forces acting on the particles tend to oppose this movement. When equilibrium is reached between these two opposing forces, the particles move with constant velocity. The velocity of the particle is dependent on strength of electric field or voltage gradient, the dielectric constant of the medium, the viscosity of the medium and the Zeta potential. The velocity of a particle in an electric field is commonly referred to as its electrophoretic mobility. With this knowledge it is possible to obtain the zeta potential of the particle by application of the Henry equation:

where z is the zeta potential, UE the electrophoretic mobility, ε the dielectric constant, η the

viscosity and f(Ka) the Henry’s function. Two values are generally used as approximations for f(Ka): 1.5 when electrophoretic determinations of zeta potential are made in aqueous media with a moderate electrolyte concentration and 1.0 for small particles in low dielectric constant media.

Finally, to evaluate the electrophoretic mobility it is used the LDV. This is a well established technique to measure the velocity of particles moving through a fluid in an electrophoresis experiment. The light scattered at an angle of 17° respect to the incident beam is collected by the detector (Figure A.I.13). This produces a fluctuating intensity signal where the rate of fluctuation is proportional to the speed of the particles. A digital signal processor is used to extract the characteristic frequencies in the scattered light.

η

)

f(K

z

ε

U

3

2

=

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Figure A.I.13. Laser Doppler Velocimetry technique.