Charge Stabilised Dispersions

Diffuse layer potential = —50 mV and background electrolyte concentration 

Three electroviscous effects have been noted in the literature.27 The primary electroviscous effect refers to the enhanced energy dissipation due to the distortion of the diffuse layer from spherical symmetry during flow. The analysis for low diffuse layer potentials has been clearly reviewed by van de Ven28 and the result for the intrinsic viscosity with Ka— oo is  

The secondary electroviscous effect is the enhancement of the viscosity due to particle-particle interactions, and this of course will control the excluded volume of the particles. The most complete analysis is that due to Russel30 and we may take this analysis for pair interactions as the starting point. Russel s result gives the viscosity as 

Fhr = bnr 0yr0 sin20 = 3nr 0yr0 and the electrostatic repulsive force is 

The effective hard sphere diameter may be used to estimate the excluded volume of the particles, and hence the low shear limiting viscosity by modifying Equation (3.56). The liquid/solid transition of these charged particles will occur at 

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In the case of charge-stabilised dispersions above a certain concentration, addition of electrolyte often leads to gelation rather than the formation of distinct floes. Thus gelation may be regarded as the formation of an extended, continuous floe filling... 

In studying the stability of colloidal dispersions it is of considerable advantage if the particles concerned are monodisperse and spherical. For aqueous, charge-stabilised systems polymer latices have proved invaluable in this regard. With non-aqueous systems, steric stabilisation is usually required. In this case it... 

AMP-95 is a non-polymeric charge stabiliser because of its organic structure and low molecular weight of 89 it is often referred to as a micro dispersant (Figure 1). 

The stability of electrostatically charged sols has been studied extensively and is now reasonably well understood. More recently the stabilising action of adsorbed or chemically anchored non-ionic polymers has received much attention. There has been however little systematic work on polyelectrolyte stabilisers apart from a number of investigations of the flocculation of particles bearing adsorbed biopolymers, usually proteins, by simple salts ( 2). These have shown that polyelectrolyte covered particles can be more stable with respect to the addition of salt than simple charged systems, and the extra stability has been ascribed to the polymeric nature of the surface layer. The precise mechanism by which polyelectrolytes stabilise dispersions in the presence of high concentrations of salt has however remained unclear. 

Journal of Rheology 44, No.6, Nov./Dec.2000, p. 1279-92 VISCOSITY OF BIMODAL CHARGE-STABILISED POLYMER DISPERSIONS... 

In oil-containing aqueous effluents the oil is present as finely dispersed droplets in a relatively clean water phase, for example oilfield produced water, refinery process water, ballast water from cargo tankers [32]. Factors preventing coalescence include the droplet s negative electrostatic charge, stabilisation by surface active components and steric stability caused by fine solids. 

The use of surface charge to provide colloid stability to particles dispersed in dilute electrolytes in aqueous solution, or even in media of intermediate polarity, is an effective means of stabilising particles against van der Waals forces of attraction. Figure 3.16 shows typical potential... 

Dispersions of fine mineral particles can be stabilised by direct electrical charging of the particles or by steric/electrosteric protection from adsorbed polymers. Stabilisation by direct charging is well described by the classical DLVO theory. ... 

Direct addition of a hydrogen halide to an alkene gives rise to an alkyl halide, the order of reactivity being HI>HBr>HCl. In the case of an unsymmetrical alkene, the regioselectivity of the reaction may be predicted from the mechanism of the reaction. Thus, the carbocation which is the most stabilised by charge dispersal will be the one which is formed preferentially. Classically the mode of addition is described as proceeding in the Markownikoff manner. 

Ottewill and Ranee (29) using well-dialysed PTFE latices obtained the results shown in Figure 6. The change in c.c.c. with pH is clearly defined but the c.c.c. above pH 5 is at a fairly high concentration of electrolyte and insufficient positive charge is built up to stabilise the dispersion. It is also possible with this system that the very hydrophobic polytetrafluoroethylene parts of the surface do not adsorb the hydrolysed species and in the well-dialysed system adsorption can only occur on the sparse charged sites. 

Polyacrylic acid stabilised latices have been prepared by aqueous dispersion polymerisation. The method used is analogous to the non-aqueous dispersion (NAD) polymerisation methods originally used to prepare polymethyl methacrylate particles in aliphatic hydrocarbons (1. In effect the components of a NAD polymerisation have been replaced as follows aliphatic hydrocarbon by aqueous alcohol, and degraded rubber, the stabiliser, by polyacrylic acid (PAA). The effect of various parameters on the particle size and surface charge density of the latices is described together with details of their colloidal stability in the presence of added electrolyte. 

Polyelectrolyte brushes are macromolecular monolayers where the chains are attached by one end on the surface and, at the same time, the chains carry a considerable amount of charged groups. Such poly electrolyte structures have received thorough theoretical treatment, and experimental interest has been vast due to the potential of brushes for stabilising colloidal particle dispersions or for... 

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