Electrosteric interactions

The robustness occurs due to the charge being displaced from the particle surface by about the length of the chain in a densely packed layer. Although the displacement is only around a nanometre or two, this can make a very large difference to the shape of the pair potential around 

Macosko, Rheology Principles, Measurements and Applications, Wiley-VCH, New York, 1994. 

See for example, P.W.Atkins, Physical Chemistry, 4th edn, Oxford University Press, Oxford, 1990. 

Croxton, An Introduction to Liquid State Physics, Wiley, London, 1975. 

Liquids and Solids and Other States of Matter, 3rd edn, Cambridge University Press, Cambridge, 1991. 

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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... 

Kamiya H, Fukuda Y, Suzuki Y, Tsukada M, Kakui T, Naito M. Effect of polymer dispersant structure on electrosteric interaction and dense alumina suspension behavior. J Am Ceram Soc 1999 82 3407-3412. 

Recognition and description of new interparticle interaction forces such as those owing to magnetic dipoles, steric and electrosteric repulsion,... 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

With polymers that have ionizable groups, adsorption of a polymer will alter the charge of the surface altering the electrostatic interaction energy and also provide steric protection for the colloid, because the ionized groups will give better than theta conditions for the poisoner in an aqueous solution. This type of polymer stabilization is called electrosteric stabilization because both the electrostatic and the steric play a role in stabilization. The equations for this total interaction are simply the sum of electrostatic and steric terms as well as the van der Waals attraction. 

Since the beginning of colloids science, however it is also known that the agglomeration of colloids and dispersed particles can be prevented or controlled by stabilization [8]. The attractive interactions between the colloidal particles, caused by van-der-Waals forces, need to be compensated by repulsive interactions. The latter can be based either on electrostatic repulsion due to same-sign surface charges (electrostatic stabilization), or on repulsion via a polymer shell formed through adsorption of polymers to the particle surface (steric stabilization, in presence of polyelectrolytes termed electrosteric stabilization due to additional charged-induced repulsion) [9, 10]. The stabilization by control of the interaction forces between colloidal particles has been in the focus of extensive research efforts. Already... 

The repulsive forces between the polymer particles are electrostatic, steric, or electrosteric in nature, and can be controlled deliberately by modifying the polymerization recipe. The attractive forces are the general van der Waals forces caused by dipole and induced-dipole interactions. 

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. 

Fig. 7.34. Total energy of interaction for three different stabilisation mechanisms (a) electrostatic (b) steric (c) electrosteric.

Carambassis et al. (77) and Zauscher et al. (78) reported different regimes, e.g. electrostatic and electrosteric, seen in the approach curves in the interaction between cellulose surfaces. Such behaviour is not as clearly seen in our results. This could be explained, again, by the swelling behaviour of cellulose. It is likely that the fully swollen and stabilised surfaces are softer than surfaces that are still swelling, and hence, the distance at which a electrostatically dominated interaction changes into a sterically dominated is not as distinct. 

The third class of dispersing agents which is commonly used in SC formulations is that of polyelectrolytes. Of these, sulfonated naphthalene-formaldehyde condensates and lignosulfonates are the most commonly used is agrochemical formulations. These systems show a combined electrostatic and steric repulsion and the energy-distance curve is schematically illustrated in Fig. 3.40 (c). It shows a shallow minimum and maximum at intermediate distances (characteristic of electrostatic repulsion) as well as strong repulsion at relatively short distances (characteristic of steric repulsion). The stabilization mechanism of polyelectrolytes is sometimes referred to as electrosteric. These polyelectrolytes offer some versatility in SC formulations. Since the interaction... 

Figure 7 Schematic drawing of the interaction potential energy curve expected for electrosterically dispersed particles, indicating the energetic contribution of the steric layer (Us) to the total potential energy curve (Us - -Ur- - Ua). (Adapted from Ref. 1.)...

In polar media such as water, often polyelectrolytes are used. Since polyelectrolytes are charged, we have a steric and an electrostatic effect. For this reason, the effective interaction is sometimes called electrosteric force. The electrosteric force has an important advantage over simple electrostatic repulsion it is to a large degree independent of the concentration of added salt. While electrostatically stabilized dispersion often coagulates when salt is added, electrosterically stabilized dispersions are robust and remain stable. 

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