Electrosteric effects

It is noteworthy that a basic assumption made in the derivation of the free radical desorption rate constant is that the adsorbed layer of surfactant or stabilizer surrounding the particle does not act as a barrier against the molecular diffusion of free radicals out of the particle. Nevertheless, a significant reduction (one order of magnitude) in the free radical desorption rate constant can happen in the emulsion polymerization of styrene stabilized by a polymeric surfactant [42]. This can be attributed to the steric barrier established by the adsorbed polymeric surfactant molecules on the particle surface, which retards the desorption of free radicals out of the particle. Coen et al. [70] studied the reaction kinetics of the seeded emulsion polymerization of styrene. The polystyrene seed latex particles were stabilized by the anionic random copolymer of styrene and acrylic acid. For reference, the polystyrene seed latex particles stabilized by a conventional anionic surfactant were also included in this study. The electrosteric effect of the latex particle surface layer containing the polyelectrolyte is the greatly reduced rate of desorption of free radicals out of the particle as compared to the counterpart associated with a simple... 

Electrostatics, effect on weighing, 26 243 Electrostatic self-assembly (ESA), of thin-films, 1 724-725 Electrostatic separators, 16 642 Electrostatic separation, 16 642-644 Electrostatic spray coating, 7 56-58, 74-75 Electrostatic stabilization, 10 119-121 of latex, 14 708-709 Electrosteric stabilization, 10 122 Electrostream (capillary drilling), 9 600 Electrostrictive coefficient tensor, 11 93 Electrostrictive devices, applications of, 11 103-104... 

Salt effects in polyelectrolyte block copolymer micelles are particularly pronounced because the polyelectrolyte chains are closely assembled in the micellar shell [217]. The situation is quite reminiscent of tethered polymer brushes, to which polyelectrolyte block copolymer micelles have been compared, as summarized in the review of Forster [15]. The analogy to polyelectrolyte brushes was investigated by Guenoun in the study of the behavior of a free-standing film drawn from a PtBS-PSSNa-solution [218] and by Hari-haran et al., who studied the absorbed layer thickness of PtBS-PSSNa block copolymers onto latex particles [219,220]. When the salt concentration exceeded a certain limit, a weak decrease in the layer thickness with increasing salt concentration was observed. Similar results have been obtained by Tauer et al. on electrosterically stabilized latex particles [221]. 

Fundamental studies of the effects of sodium polyacrylate have shown it to be an electrosteric dispersant, for which the steric component of interparticle repulsion is the dominant one at the typical dose rates used commercially. In such dispersions the main contributor to solution ionic strength is unadsorbed polyacrylate. As well as representing a waste of valuable dispersant polymer, this unadsorbed material also reduces the effectiveness of the adsorbed material as a steric dispersant by compressing the conformation of the adsorbed layer. 

The role of polymers on colloid stability is considerably more complicated than electrostatic stability due to low molecular weight electrolytes considered in Chapter 11. First, if the added polymer moieties are polyelectrolytes, then we clearly have a combination of electrostatic effects as well as effects that arise solely from the polymeric nature of the additive this combined effect is referred to as electrosteric stabilization. Even in the case of nonionic... 

Pettersson, A. et al., Electrosteric stabilization of Aljti, ZrOj, and SY-ZrOj suspensions Effect of dissociation and type of polyelectrolyte, 7. Colloid Interf. Sci., 228, 73, 2000. 

The major route to colloidal (effectively water soluble) PAn has been through the chemical oxidation (S2082-) of the monomer in the presence of polymeric steric stabilizers and electrosteric stabilizers (polyelectrolytes), such as poly(vinyl alcohol), polyGV-vinyl pyrrolidone), polyethylene oxide), polystyrene sulfonate), dodecylben-zene sulfonate, and dextran sulfonate. It has been found that the stabilizer can act simultaneously as a dopant, imparting new functionality to the polymer or additional compatibility for the final application. 

Figure 6. Total energy-distance curves for silica sols initially at pH 11 and 0.15 M 1 1 electrolyte, together with an illustration of the electrosteric barrier effect at pH values of 8.0 and 6.0 and electrolyte concentrations in excess of 0.15 M.

Viewed in terms of the electrosteric mechanisms outlined in Figures 6 and 7, the 1 1 electrolyte cation effects indicate that relative to Na+, Li+ enhances any stability imposed by the barrier of polysilicate plus bound cation, whereas Cs+ diminishes such effects. This observation is consistent with a hydration stabilization (6) mechanism. Thus, the more strongly hydrated the cations in the polysilicate exchange layer,... 

In a high-volume-fraction dispersion with electrosteric stabilization of the latex and an increasing dispersed-phase surface area, the high viscosity observed at low shear rates with decreasing latex size relates to electroviscous and hydration effects. Lower surface acid concentrations on some of the smaller latices may also result in partial flocculation of the latex and a higher effective volume fraction in the presence of coalescing aids (22, 26). 

Similar effects will operate at pH 6.0, initially at 0.15 M salt, where again the sol is electrosterically stabilized. As salt is added at pH 6, again the thickness and range of the electrosteric coating will eventually decrease to yield a >2kT secondary minimum at the critical coagulation concentration, which is very much >0.15 M. These effects are summarized schematically in energy-distance curves in Figure 20.6. 

Electrosteric stabilization causes the colloidal particles to become resistant against both electrolytes (based on steric repulsion) and temperature (based on the electrostatic interaction). Such stabilization is frequently used industrially by the application of mixed stabilizers (one electrostatic and one steric). Likewise, polyelectrolyte chains - either adsorbed or covalently bound - impart an extraordinary stability against electrolytes [63, 64]. This effect may contribute to stability in emulsion polymerizations containing a small percentage of ionic monomers in the monomer mixture. 

In effect, we have mainly covered attractive interactions introduced when adding polymer to a system. This is because our interest here was mainly focused on retention aids which function by enhancing the attractive interactions in the papermaking system. Polymeric systems tuned to increase the repulsive force between surfaces by way of steric interactions (caused by chain overlap and loss of conformational entropy on compression), electrosteric interactions (between charged polymer brushes), and/or electrostatic repulsions (due to charge over-compensation), are equally important in practical applications. However, repulsive polymeric forces should not by any means be viewed as purely... 

No.15, 15th July 1996, p.5128-35 EFFECTS OF POLY(ACRYLIC ACID) ELECTROSTERIC STABILISER ON ENTRY AND EXIT IN EMULSION POLYMERISATION... 

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