Solvent composition, flocculation dispersions

Modest solvent composition changes can also affect the state of a colloidal dispersion. A charge-stabilized dispersion of polymer latex particles or gold colloids may flocculate irreversibly upon adding salt, while ion removal through dialysis may turn the dispersion into an ordered structure exhibiting Bragg reflection [23]. Obviously, the physical state of a colloidal dispersion is a function of the interactions between the colloidal particles. 

Note that reference to equation (15.60) for the case where the polymer composing the steric layer is different from that in free solution shows that, under some circumstances, AG cannot become negative. Specifically, this requires that be negative in a dispersion medium that is a better than 6-solvent for both of the polymers. A negative value for G corresponds to the free polymer and the stabilizing moieties being compatible (Flory, 1953). If equation (15.60) is correct, it shows that free polymer that differs in chemical composition from the polymer in the steric layers cannot induce flocculation if the two polymers are compatible. No experimental evidence is as yet available to ascertain the validity of this prediction. 

Much of our understanding of entropic stabihzation of pigment dispersions comes from the seminal work of Rehacek (232). A technique has been discussed to determine the thickness and composition of the adsorbed layer on the surface of a pigment dispersed in a resin solution. It has been foimd that if the adsorbed layer thickness of resin plus solvent is less than 9-10 nm, the dispersion is not stable (232-234). With monofimctional surfactants, the adsorbed layer can be thinner and still protect against flocculation. It has been shown that an adsorbed layer thickness of 4.5 nm of surfactant and associated solvent was adequate (235). In contrast to the adsorbed layer of resin, which is nommiform in thickness, the surfactant layer is comparatively uniform, and so it does not have to be as thick to provide stabilization. 

The effect of PVB additions on the dispersion behavior can be seen from Figure 4.9 to be dependent on the suspension liquid composition. In MEOH suspension, though addition of PVB reduces the viscosity of the suspension, the system is shear-thinning indicating that the suspensions remain flocculated in poorer solvents for the added... 

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