Restabilisation

Our model predicts restabilisation at very high polymer concentrations. It is shown that this restabilisation is a thermodynamic effect, resulting from a decreased interparticle attraction, and is not kinetically determined, as proposed by Feigin and Napper. 

An important conclusion of this discussion is the fact that at very high <)> thermodynamic stability is re-established. Restabilisation is not a kinetic effect, as suggested by Feigin and Napper (10, 11), but is a consequence of lower free energy of the dispersion as compared to the floe. This conclusion is supported by experimental evidence for soft spheres (3, 5, 23). We should add, however, that for hard spheres is so high that experimental verification is difficult for most polymer-solvent systems due to the high viscosity of the solution. 

Our model predicts destabilization of colloidal dispersions at low polymer concentration and restabilisation in (very) concentrated polymer solutions. This restabilisation is not a kinetic effect, but is governed by equilibrium thermodynamics, the dispersed phase being the situation of lowest free energy at high polymer concentration. Restabilisation is a consequence of the fact that the depletion thickness is, in concentrated polymer solutions, (much) lower than the radius of gyration, leading to a weaker attraction. 

One may wonder to what extent our predictions for hard spheres apply to a system of soft particles in a polymer solution. A definite answer to this question cannot be given at the moment since numerical data for the depletion of free polymer chains in the neighbourhood of a surface with terminally attached chains are not yet available. Some qualitative features for such a system have been discussed using scaling arguments (24). We may expect that the depleted amount of polymer is, at least in some cases, less than near a hard surface, giving rise to weaker attraction. Both the destabilization concentration (J) and the restabilisation concentration (<(> ) could be much lower. Experimental observations support this qualitative conclusion (1-5). 

Polymers may show an optimum flocculation concentration which depends on molecular weight and concentration of solids in suspension. Overdosing with flocculant may lead to restabilisation(44), as a consequence of particle surfaces becoming saturated with polymer. Optimum flocculant concentrations may be determined by a range of techniques including sedimentation rate, sedimentation volume, filtration rate and clarity of supernatant liquid. 

Restabilisation of Mixed Plastics from Household Collection.. . . 212... 

Restabilisation of Mixed Plastics from Household Collection... 

Polyacrylamide (30% hydrolysed) is an anionic polymer which can induce flocculation in kaolinite at very low concentrations. Restabilisation occurs by overdosing, probably by the mechanism outlined in Fig. 7.32. Dosages of polymer which are sufficiently large to saturate the colloidal surfaces produce a stable colloidal system, since no sites are available for the formation of interparticle bridges. Under certain conditions, physical agitation of the system can lead to breaking of polymer-suspension bonds and to a change in the state of the system. 

Flocculation and Restabilisation of Clays Using Cationic Surfactants... 

Hunter and Nicol [5] studied the flocculation and restabilisation of kaolinite suspensions using rheology and zeta-potential measurements. Figure 21.11 shows plots of the yield value (cr ) and electrophoretic mobility (i) as a function of cetyl trimethyl ammonium bromide (CTAB) concentration at pH =9. increases in line with increases in CTAB concentration, reaching a maximum at the point where the mobility reaches zero (the isoelectric point, i.e.p., of the clay), and then decreases with further increases in CTAB concentration. This trend can be explained on the basis of flocculation and restabilisation of the clay suspension. 

TTie initial addition of CTAB caused a reduction in the negative surface charge of the clay (by the adsorption of CTA on the negative sites of the clay), and this was accompanied by reduction in the negative mobihty of the clay. When complete neutralisation of the clay particles occurred (at the i.e.p.), maximum flocculation of the clay suspension occurred and this was accompanied by a maximum in On further increases in CTAB concentration, a further adsorption of CTA occurred, and this resulted in charge reversal and restabilisation of the clay suspension. This was accompanied by a reduction in cr. ... 

At the lower dosage of 25 mgL , FeClj floes are visible. These floes were observed to break up during the filtration process. Restabilisation of the precipitate occurred at 100 mgL FeCl.v... 

In contrast to the above studies, a 58-year-old man stabilised on warfarin experienced a sharp rise in his ESTR to 8 when he started to drink half a can of light beer (5.35 g alcohol) every other day. In the previous 5 months he had an INR in the range of 1.9 to 2.5 with a stable warfarin dose, and no other explanation for the change in INR was found. He stopped taking the alcohol, and was eventually restabilised on the original dose of warfarin. ... 

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