Polyelectrolyte solutions, solid surface polymer

An important factor that is not taken into account in the DLVO theory is adsorption, on the particle s surface, of long polymeric chains. The adsorption of a non-ionic polymer or a polyelectrolyte on the solid surface can cause, not only a modification of the zeta potential, but also a critical difference between the value of the zeta potential and the state of dispersion. Steric repulsion is associated with the obstmction effect of these polymers that are capable to form a sufficiently thick layer to prevent the particles from approaching one another in the distanee of influence of the Van der Waals attractive forces. Steric stabihzation will therefore depend on the adsorption of the polymeric dispersant and the thickness of the layer developed. Several interpretation models for stabilization by steric effect have been put forward. They rely either on a statistical approach, or on the thermodynamics of solutions. Steric stabilization is particularly useful in organic, fairly non-polar or non-polar environments, as in the case of tape casting (see section 5.4.3). 

Another important year for the advancement of oscillatory structural forces at mesoscopic length was 1996. Three independent studies emerged that year which reported the observation of oscillatory force interactions involving polyelectrolytes. Two of these studies dealt with polymer-surfactant mixtures [23,24], and one used only polyelectrolyte solutions [25]. Also noteworthy is that one study dealt with flexible foam Aims [23] and the other two measured forces between rigid solid surfaces [24,25]. Two different phenomena were revealed oscillatory force interactions originating from bulk solution organization and from surface-specific complexation. What follows is an overview of these different force-structure relationships. 

A very versatile approach to the formation of multilayer films has been developed by Decher, based on polyelectrolytes. If a solid substrate with ionic groups at the surface is dipped into a solution of a complementary polyelectrolyte, an ultrathin, essentially monomolecular film of the polyion is adsorbed [340]. The adsorption is based on pairing of surface bound ionic sites with oppositely charged ions, bound to the macromolecule. The polymers adsorb in an irregular flattened coil structure and only part of the polymer ions can be paired with the surface ions (Figure 29a). Ionic sites which remain with small counterions provide anchor points for a next layer formed by a complementary polyelectrolyte [342,343]. This way multilayer polyelectrolyte films can be prepared layer-by-layer just by dipping a suitable substrate alternately in an aqueous solution of polyanions and polycations. The technique can be employed with nearly all soluble charged polymers and results in films with a... 

A different effect occurs with the use of polycarboxy-lates in combination with zeolites. Small amounts of polycarboxylates or phosphonates can retard the precipitation of sparingly soluble calcium salts such as CaCOs (the threshold effect ). As they behave as anionic polyelectrolytes, they bind cations (counterion condensation), and multivalent cations are strongly preferred. Whereas the pure calcium salt of the polymer is almost insoluble in water, mixed Ca/Na salts are soluble, i.e. only overstoichiometric amounts of calcium ions can cause precipitation. Polycarboxylates are also able to disperse many solids in aqueous solutions. Both dispersion and the threshold effect result from the adsorption of the polymer on to the surfaces of soil and CaCOs particles, respectively. 

Effect of Pol3rmer. In recent years, as described previously, much attention, both experimental and theoretical, has been focused on surfactant-polymer interaction in solution. Less experimental work, however, is done on the interaction between polyelectrolyte and surfactant of similar charge at the solid-liquid interface. Static adsorption experiments from the chemical literature indicate that the polymer does not affect the adsorption of the surfactant onto solid material as long as the surfactant concentration is above the CMC, apparently owing to the availability of sufficient surface sites for adsorption of the surfactant molecules [40, 41],... 

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