Zeta Potential

Zeta Potential and pH
• The most important factor that affects zeta potential is pH • A zeta potential value quoted without a pH is a virtually meaningless number
Electrokinetic Effects
• An important consequence of the existence of electrical charges on surfaces is that they will exhibit certain effects under the influence of an applied electric field • These effects are collectively defined as ELECTROKINETIC EFFECTS • There are four distinct effewhich the motion is induced
Henrys Function F(ka)
• The units of k are reciprocal length and 1/k = “thickness” of the electrical double layer (the Debye length) • a = the particle radius • ka = the ratio of particle radius to double layer thickness
Measuring Zeta Potential: Electrophoresis
• If an electric field is applied across a sample in a capillary cell, charged particles suspended in the medium are attracted towards the electrode of opposite charge • The particles move with a characteristic velocity which is dependent on: – Field strength – Dielectric constant of medium – Viscosity of the medium – Zeta potential
Zeta Potential and pH
60
Zeta Potential (mV)
40 20 0 -20 -40 -60 2 4 6 8 10 12
STABLE
Isoelectric Point
UNSTABLE
STABLE
pH
Zeta Potential and Conductivity
• The thickness of the double layer (κ-1) depends upon the concentration of ions in solution and can be calculated from the ionic strength of the medium • The higher the ionic strength, the more compressed the double layer becomes • The valency of the ion will also influence double layer thickness • A trivalent ion such as Al3+ will compress the double layer to a greater extent compared to a monovalent ion such as Na+
Zeta Potential
• A dividing line between stable and unstable aqueous dispersions is generally taken at either +30 or -30mV • Particles with zeta potentials more positive than +30mV are normally considered stable • Particles with zeta potentials more negative than -30mV are normally considered stable
Zeta Potential and Conductivity
• Inorganic ions can interact with charged surfaces in one two distinct ways – non-specific ion adsorption where they have no effect on the position of the isoelectric point – specific ion adsorption which will lead to a change in the position 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 leads to charge reversal of the sample
Electrophoresis
ZETA POTENTIAL is related to the ELECTROPHORETIC MOBILITY by the HENRY EQUATION
UE = 2 ε z f(k a) 3η
UE = electrophoretic mobility z = zeta potential ε = dielectric constant η = viscosity f(k a) = Henrys function
Zeta Potential and Additive Concentration
• The effect of the concentration of an additive on the zeta potential of a sample can give information in formulating a product to give maximum stability • The influence of known contaminants on the zeta potential of a sample can be a powerful tool in formulating the product to resist flocculation
Electrokinetic Effects
• Electrophoresis: the movement of a charged particle relative to the liquid it is suspended in under the influence of an applied electric field • Electroosmosis: the movement of a liquid relative to a stationary charged surface under the influence of an electric field • Streaming potential: the electric field generated when a liquid is forced to flow past a stationary charged surface • Sedimentation potential: the electric field generated when charged particles move relative to a stationary liquid
Stern layer {
Distance from particle surface
Zeta Potential
• The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system • If all the particles have a large negative or positive zeta potential they will repel each other and there is DISPERSION STABILITY • If the particles have low zeta potential values then there is no force to prevent the particles coming together and there is DISPERSION INSTABILITY
• Measurement of the zeta potential of a particle dispersion as a function of any of the above can lead to information in formulating the product to give maximum stability or in determining the optimum conditions for flocculation of the system
Zeta Potential
Slipping plane Particle with negative surface charge
Diffuse layer
Surface potential Stern potential
-100 mV 0
Zeta potential
• Electrical double layer exists around each particle which consists of two parts; an inner region (Stern layer) where the ions are strongly bound and an outer (diffuse) region where they are less firmly associated • Within this diffuse layer is a notional boundary within which the particle acts as a single entity • The potential at this boundary is the ZETA POTENTIAL
Factors Affecting Zeta Potential
• Zeta potential can be affected by
– changes in pH, – conductivity (concentration and/or type of salt) – changes in the concentration of an additive (e.g. ionic surfactant, polymer)