General Classification of Dispersing Agents

This method is suitable for small particles that undergo Brownian motion. The light scattered by small particles will show intensity fluctuations as a result of the Brownian diffusion (Doppler shift). By application of an electric field as the particles undergo Brownian motion and measuring the fluctuation in intensity of the scattered light (using a correlator) one can measure the particle mobility. 

Two laser beams of equal intensity are allowed to cross at a particular point within the cell containing the suspension of particles. At the intersection of the beam, which is focused at the stationary level, interferences of known spacing are formed. The particles moving through the fringes under the influence of the electric field scatter light whose intensity fluctuates with a frequency that is related to the mobility of the particles. 

The photons are detected by a photomultiplier and the signal is fed to the correlator. The resulting correlation function is analysed to determine the frequency (Doppler) spectrum and this is converted into the particle velocity V, 

The velocity spectrum is then converted into a mobility spectrum (allowing one to obtain the mobility distribution) and the mobility is converted into zeta potential using Hiickel s equation. 

Several commercial instruments are available Malvern Zeta Sizer, Coulter Delsa Sizer. 

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In this section we describe the general classification of dispersing agents. The adsorption of surfactants and polymers at the solid/liquid interface was treated in Chapter 5. The various classes can be summarised as follows. 

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