Ionically stabilized particles

In a qualitative way, colloids are stable when they are electrically charged (we will not consider here the stability of hydrophilic colloids - gelatine, starch, proteins, macromolecules, biocolloids - where stability may be enhanced by steric arrangements and the affinity of organic functional groups to water). In a physical model of colloid stability particle repulsion due to electrostatic interaction is counteracted by attraction due to van der Waal interaction. The repulsion energy depends on the surface potential and its decrease in the diffuse part of the double layer the decay of the potential with distance is a function of the ionic strength (Fig. 3.2c and Fig. 

The surfactant selection determines the emulsion properties, such as stability, particle size, viscosity, and internal phase content. A correct balance between the hydrophobic and hydrophilic character of the emulsifier is necessary for minimizing the surfactant concentration at the resin-water interface. The surfactants used in resin emulsification can be ionic (in most cases anionic), nonionic, polymeric, or a combination of these. 

Determining the amount of surface carboxyl groups as a function of the surfactant, it was shown that the dense monolayer of carboxylic groups (0.68nm2 per COOH 1.47 groups per nm2) on the particles prepared with nonionic surfactant was almost achieved with 3wt% of acrylic acid. More than 10 wt% of acrylic acid was required in the case of SDS-stabilized particles. TEM images of carboxyl-functionalized polystyrene particles stabilized with nonionic (Lutensol AT50) and ionic (SDS) surfactant are presented in Fig. 8. 

Cationic surfactants, in contrast to anionic surfactants, usually reduce both the number of particles involved in the polymerization and the rate of polymerization. The nature of the stabilizing emulsifier has a marked effect on the polymerization kinetics. For example, addition of a non-ionic stabilizer [e.g., poly(vinyl alcohol), a block copolymer of carbowax 6000 and vinyl acetate, or ethylene oxide-alkyl phenol condensates] to a seed polymer stabilized by an anionic surfactant decreased the rate of polymerization to 25% of the original rate. The effect was as if the nonionic stabilizer (or protective colloid) acted as a barrier around the seed particles to alter the over-all kinetics. It may be that the viscosity of the medium in the neighborhood of the nonionic surfactant coating of the polymer particle is sufficiently different from that of an anionic layer to interfere with the diffusion of monomer or free radicals. There may also be a change in the chain-transfer characteristics of the system [156]. 

In the chapter on Pigmentation (Chapter 8), under the heading Dispersion, we considered how to make stable colloidal dispersions of solid and found that, for stability, it was necessary to keep the particles apart. This could be done by using polymer molecules, anchored strongly to the particle, but also extending out into the solvent, in which they were soluble. These polymer molecules provide a steric barrier around the particle and this method of stabilization is called steric stabilization. We also learnt that aqueous pigment dispersions could be stabilized by adsorbed surfactant molecules, which ionized in the water to produce an electrical charge barrier around the particle ionic stabilization). Exactly the same techniques are used to stabilize emulsions. 

Both forces act into opposite directions the osmotic force tries to stretch the chain into the continuous phase, whereas the elastic force pulls the chain back to the interface. Setting Pci = Posm shows that AP a This is a much lower electrolyte dependence than in the case of low-molecular-weight ionic stabilizers where an exponential dependence of Vim is predicted (cf. equations (8.20)). Note, this scaling behavior of AR with Cl is the same as for polyelectrolyte chains in solution [2]. Regarding colloid stability, this means that polyelectrolyte-decorated droplets/particles possess an extraordinary electrolyte stability when compared to low-molecular-weight ionic stabilizers. Indeed, the Pincus brush behaviour (AP oc was experimen-... 

Since then, a number of composite particles have been developed [66-70], Lu et al. have demonstrated the effect of surfactants on the morphology of ICP-coated polystyrene (PS) or poly(styrene-co-butyl acrylate) [PST-co-BuA] particles [71]. These authors showed that when the core particles are stabilized with ionic surfactants, a raspberry-type of morphology is obtained, whereas a homogeneous coreshell morphology is achieved in the case of nonionic surfactant-stabilized particles. 

A similar mechanism can be postulated for other reductions with dtrate, and this method has been widely used for the preparation of other metal sols, such as platinum. [17, 43, 44] Citrate can also be added as an ionic stabilizer in preparations which require an additional redudng agent. The use of formate, dtrate, and acetone dicarboxylate as redudng agents at various pH s was reported to give good control over particle size in the preparation of a series of platinum hydrosds. [45]... 

Steric stabilization has been used in the case of very small particles to supplement ionic stabilization. Thus Yates stabilized sols of very small particles with a combination of an inorganic or organic base with a water-soluble nonaromatic polyhydroxy or hydroxy-ether compound, for example, polyvinyl alcohol (62). There is probably also some steric stabilization when an organic base cation such as (CHj)4N+ is present since, as Wolter (63) found, a silica sol of this type can be evaporated to a dry powder that will spontaneously redisperse in water. Such sols also can be redispersed after freezing. 

For practical applications in polymer dispersions (ie also during heterophase polymerizations) the effect of charge stabilization due to permanent charges, either by ionic stabilizers, comonomers, or initiator fragments, is much more important. The repulsive potential between two equally charged particles (yR,es) separated at a center-to-center distance of (dpp + D) is given by equation 35, where Q is the charge on each particle and a is the permittivity of the continuous phase. 

The observable properties of a latex, ie, stability, rheology, film properties, interfacial reactivity, and substrate adhesion, are determined by the colloidal and polymeric properties of the latex particles. Important colloidal properties include ionic charge, stability, particle size and morphology distribution, viscosity, solids. 

Conduchvity measurements are more specific in their application being of particular use in the case of aqueous dispersions of particles, stabilized by some type of ionic stabilizer. The information obtained with this technique is the conductivity of the continuous phase. In the case of emulsion polymerizations stabilized with ionic surfactants, this is related to the concentration of free surfactant, which, when combined with absorption isotherms, for example, or an empirical model, can be used to follow the evolution of the surface area of a latex. This is a promising method, but given its complexity and the need to develop more robust means of linking the conductivity to properties of interest, it has not found widespread use in commercial production at the current time. 

The simplest method to obtain particles is the solvent displacement developed by Fessi et al. (1987). The polymer, dissolved in a water-miscible organic solvent, is slowly added to an aqueous solution containing a surfactant or a non-ionic stabilizer such as poly(vinyl alcohol) or PEO-PPO-PEO triblock copolymers. On reaching the dispersing phase, the solvent drops diffuse in water but the water-insoluble polymer precipitates in the form of colloidal particles, on condition that the polymer concentration be low enough to prevent the formation of aggregates. 

Carboxyl and amino-functionalized latex particles were synthesized [67] by the miniemulsion polymerization of styrene and acrylic acid or 2-aminoethyl methacrylate hydrochloride, and the effect of hydrophilic comonomer and surfactant type (nonionic versus ionic) on the colloidal stability, particle size, and particle size distribution was analyzed. The reaction mechanisms of particle formation in the presence of nonionic and ionic surfactants were proposed. 

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