Electrosteric stabilisation

Figure 2.17 Pair potentials calculated for 50 nm polystyrene particles in 0.1 moldm 3 electrolyte and with a -potential of —30mV. Curve a is the result for a simple polystyrene surface and curve b was calculated from the model of a 1 nm surfactant layer so that the -potential is taken as occurring at the outer edge of the adsorbed layer. A maximum in the potential of 8kBT is insufficient to provide long-term stability and the curves clearly shows how electrosteric stabilisation can achieve this...

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-... 

Stable suspensions are necessary to obtain homogeneous casts by colloidal filtration. Different kinds of stabilisation mechanisms for alumina suspensions are used in literature. Pure electrostatic stabilisation can be obtained using HNO3, while electrosteric stabilisation is obtained using polyelectrolytes like PolyMethylAcrylicAcid (PMAA) and PolyAcrylicAcid (PAA). In... 

Steinlage et al. [25] indeed used electrosteric stabilisation with APMA for a-alumina suspensions to prepare dense tube- and gear-shaped ceramic components by centrifugal slipcasting. The aqueous suspensions contained 20-33 vol-% solids and 8 vol-% APMA and were brought at pH 9.5 with NH4OH after which the slurries were treated with ultrasound. 

Another commonly used application-specific method is discrete particle encapsulation (DPE). In this method, selected chemicals are used to form a thin polymeric shell around each nanoparticle providing the characteristics a user needs. Then a second thin-shell coating is added, so the nanoparticle will disperse in the best needed format. This shell contains spacer molecules that prevent the nanoparticles from coming into contact with each other. The result is steric stabilisation for nanoparticles in non liquid solvents and polymers, and electrosteric stabilisation for those needing to disperse in a fluid. 

Surfactant molecules are amphiphilic (121), meaning that they contain both hydrophilic (water-compatible) and hydrophobic (water-incompatible) sections (151, 280). The amphiphilic nature of surfactant molecules causes them to partition between the water and monomer/polymer phases by adsorbing at the droplet/particle interface. Once adsorbed at the interface, the surfactant keeps the droplets (or particles) separated by causing them to be repelled from one another through the electrostatic and/or steric stabilisation mechanisms (296,377). The same surfactant may act as both an electrostatic and a steric stabiliser, and is known as an electrosteric stabiliser (214, 398). 

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

Other types of surfactants are the polymeric (steric) stabilisers, such as partially hydrolysed polyvinyl acetate. Also oligomeric species formed in situ, when SO radicals react with some monomer units in the aqueous phase, will have surface active properties, and can even form a colloidally stable latex Electrosteric stabilisers combine steric and electrostatic functionalities for example, inclusion of acrylic acid in a recipe results in chains with blocks comprised largely of poly(acrylic acid) which for in the aqueous phase, then pick up enough hydrophobic monomer to enter the particle and continue polymerisation in the particle interior. The hydrophilic component remains in the aqueous phase and provides colloidal stability both sterically and, imder the appropriate conditions of pH, electrostatically. This mode of stabilisation is very common in surface coatings, because it gives excellent freeze-thaw stability. 

Electrosteric stabilisation, which implies both the previously described mechanisms (e.g. Au NPs surrounded by ionic surfactants, such as tetraalky-lammonium salts the alkyl chains generate steric repulsion, while the charged head groups induce coulombic repulsion). 

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. ... 

Clearly steric, electrosteric or depletion stabilisation as described above may occur, but attraction, leading to destabilisation, can also be the result, as can be seen in Figure 4 [20],... 

Figure 11.3 Energy-distance curves for three stabilisation mechanisms (a) electrostatic (b) steric and (c) electrosteric.

In most cases, a mixture of a polymeric stabiliser such as poly(vinyl alcohol) or Pluronic (an A-B-A block copolymer of PEO, A and PPO) with an anionic surfactant, such as sodium dodecyl sulphate is used. In this case, the stabiHsing mechanism is the combination of electrostatic and steric mechanism, referred to as electrosteric. 

Three main mechanisms of stabilisation can be considered (i) electrostatic, as produced by ionic surfactants (ii) steric, as produced by nonionic polymeric surfactants of the A-B, B-A-B, A-B-A or AB graft copolymers (where A is the anchor chain and B is the stabilising chain and (iii) electrosteric, as produced by polyelectrolytes. 

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. 

---