Experiment 6.1 Zeta potential measurements at the silica water interface

Experiment 6.1 Zeta potential measurements at the silica/water interface 

The stability of most colloidal solutions depends critically on the magnitude of the electrostatic potential ( /o) at the surface of the colloidal particle. One of the most important tasks in colloid science is therefore to obtain an estimate of /o under a wide range of electrolyte conditions. In practice, the most convenient method of obtaining /q uses the fact that a charged particle will move at some constant, limiting velocity under the influence of an applied electric field. Even quite small particles (i.e. 1 xm) can be observed using a dark-field illumination microscope and their (average) velocity directly measured. The technique is called microelectrophoresis . 

At low electric fields [0(lV/cm)] the speed (U) of the particles is directly proportional to the applied field ( ) and hence we can define a parameter called the electromobility ( x) of the particles, given by U/E. Using the Poisson-Boltzmann theory of the diffuse electrical layer next to a charged surface, a simple relationship between p and V /o can be 

The magnitude of the electrostatic potential at the silica surface is, as expected from the law of mass action, pH-dependent. The variation in surface (or zeta) potential with pH must therefore be dependent on the dissociation constant of the surface silanol (Si-OH) group. 

In this experiment a zeta-meter is used to determine the variation in the zeta potential of silica at constant pH (5.7) over a range of concentrations of a cationic surfactant CTAB, which should adsorb on the ionized silanol groups on the silica particle surface. 

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