Analytical Determination of Surface Charge

The surface charge on a solid surface can be obtained by determining the adsorption of potential-determining ions at various potentials of the interface [1]. For example, in the case of a silver iodide sol the adsorption of Ag+ and I ions is determined at various concentrations of Ag and I ions in bulk solution. Similarly, for an oxide the adsorption of H and OH ions Fand respectively) is determined as 

As an illustration of the direct surface charge determination, the results obtained on precipitated silica are shown in Eigure 19.1, where is plotted versus pH at four 

As discussed in Chapter 5, the area per surfactant ion or molecule gives information on the orientation of surfactant ions or molecules at the interface. This information is relevant for the stability of the suspension. For example, for the vertical orientation of surfactant ions (e.g., dodecyl sulphate anions), which is essential to produce a high surface charge (and hence an enhanced electrostatic 

Stability), the area per molecule is determined by the cross-sectional area of the sulphate group, which is in the region of 0.4 mn With nonionic surfactants consisting of an alkyl chain and a poly(ethylene oxide) (PEO) head group, adsorption onto a hydrophobic surface is determined by the hydrophobic interaction between the alkyl chain and the hydrophobic surface. For the vertical orientation of a monolayer of surfactant molecules, the area per molecule will depend on the size of the PEO chain, which is in turn directly related to the number of ethylene oxide (EO) units in the chain. If the area per molecule is smaller than that predicted from the size of the PEO chain, the surfactant molecules may associate on the surface to form bilayers and hemimicelles (as discussed in detail in Chapter 5). This information can be related directly to the stability of the suspension. 

The adsorption of polymers is more complex than surfactant adsorption, since the various interactions (chain-surface, chain-solvent and surface-solvent) must be considered, as well as the conformation of the polymer chain on the surface 

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