Adsorbing Polymers Bridging Flocculation and Steric Stabilization

2 Adsorbing Polymers Bridging Flocculation AND Steric Stabilization 

k To calculate G Qi), we have to include the Hamaker constant A3 of the adsorbed layer. In the case of a compact adsorbed layer such as the ones that are usually formed by globular protein molecules (cf. Chapter 15), A3 may significantly differ from the Hamaker constant of the supporting medium Aj. With respect to G g Qi), the electrokinetic potentials corresponding to the potentials at the peripheries of the polymer-coated particles have to be considered. Now, for compact adsorbed layers of thickness d, t G.,J]i) can still be calculated using Equation 16.18, provided that the above-mentioned modifications are taken into account and that h is replaced by (h-2d). Thus, polymers that adsorb in a compact or flat orientation may either improve or worsen colloidal stability, depending on their influences on the potentials and, usually to a lesser extent, on the Hamaker constant of the whole system. 

FIGURE 16.9 (a) Polymer bridging between two particles and (b) Gibbs energy of interac- 

Bridging flocculation of charged particles is enhanced by iuCTeasing the ionic strength (i.e., reducing k ). In the aggregates thus formed, the particles are not in direct contact with each other. Their separation compares with the polymer layer thickness because closer separation would be detrimental to the polymer conformational entropy and, hence, causes Gbr(/t) to become less negative. 

FIGURE 16.10 (a) Steric stabilization between polymer-coated surfaces and (b) Gibbs 

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