Repulsion forces, electrostatic polymeric

A major advance in force measurement was the development by Tabor, Win-terton and Israelachvili of a surface force apparatus (SFA) involving crossed cylinders coated with molecularly smooth cleaved mica sheets [11, 28]. A current version of an apparatus is shown in Fig. VI-4 from Ref. 29. The separation between surfaces is measured interferometrically to a precision of 0.1 nm the surfaces are driven together with piezoelectric transducers. The combination of a stiff double-cantilever spring with one of a number of measuring leaf springs provides force resolution down to 10 dyn (10 N). Since its development, several groups have used the SFA to measure the retarded and unretarded dispersion forces, electrostatic repulsions in a variety of electrolytes, structural and solvation forces (see below), and numerous studies of polymeric and biological systems. 

The dispersion polymerization system is composed of monomer, solvent, initiator, and stabilizer. The combination of monomer, solvent, and stabilizer is essential for particle preparation. That is to say, the stabilizer is chosen to meet the demand of the monomer and solvent. In any system, the stabilizer has affinity or cohesive strength for both the medium and the polymer particles. In a dispersion polymerization, the medium and polymer particles both are organic compounds. Therefore, it is not rational to rely on dispersion stabilization, which comes from the electrostatic repulsion force between particles. The stabilizer for dispersion polymerization that makes interfacial energy low must have affinity for particles due to the same quality and solvation at the surface of particles. It is desired that the stabilizer be a polymer that indicates a steric stabilization effect on the surface (5). 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

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. 

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... 

In this paper we review principles relevant to colloids in supercritical fluids colloids in liquids are discussed elsewhere [24]. Thermodynamically unstable emulsions and latexes in CO2 require some form of stabilization to maintain particle dispersion and prevent flocculation. Flocculation may be caused by interparticle van der Waals dispersion forces (Hamaker forces). In many of the applications mentioned above, flocculation of the dispersed phase is prevented via steric stabilization with surfactants, in many cases polymeric surfactants. When stabilized particles collide, polymers attached to the surface impart a repulsive force, due to the entropy lost when the polymer tails overlap. The solvent in the interface between the particles also affects the sign and range of the interaction force, and the effect of solvent is particularly important for highly compressible supercritical solvents. Since the dielectric constant of supercritical CO2 and alkanes is low, electrostatic stabilization is not feasible [24] and is not discussed here. For lyophobic emulsion and latex particles (-1 xm), the repulsive... 

The electrostatic repulsive force working between the surface of a polymeric material without electric charge and an electrolyte, and the van der Waals interaction force working between the polymer surface and a nonionized organic solute, were expressed by potential function Equations 6.16,6.17,6.29, and 6.30. There are also cases where both interaction forces are working. In... 

The attractive forces between suspension particles are considered to be exclusively London-van der Waals interactions (except where interparticle bridging by long polymeric chains occurs). The repulsive forces, as discussed in Chapter 8, comprise both electrostatic repulsion and entropic and enthalpic forces. In aqueous systems the hydrophobic dispersed phase is coated with hydrophilic surfactant or polymer. As adsorption of surfactant or polymer (or, of course, both) at the solid-liquid interface alters the negative charge on the suspension particles, the adsorbed layer may not necessarily confer a repulsive effect. Ionic surfactants may neutralize the charge of the particles and result in their flocculation. The addition of electrolyte such as aluminium chloride can further complicate interpretation of results electrolyte can alter the charge on the suspension particles by specific adsorption, and can affect the solution properties of the surfactants and polymers in the formulation. Some aspects of the application of DLVO theory to pharmaceutical suspensions and the use of computer programmes to calculate interaction curves are discussed by Schneider et al. [4]. 

After application, the liquid coating must be converted into a solid polymeric film (viscosity > 10 cps) in order to build up satisfactory performance properties (termed the film formation process) [74]. As water evaporates from a film of emulsion polymer, the distance of separation between the submicron particles continues to decrease and, ultimately, capillary tubes form. In a capillary tube, surface tension results in a force that tends to collapse the tube. Moreover, the smaller the diameter of the tube, the greater the destruction force. When the particles are so close to one another, the destruction force is strong enough to overcome the repulsion forces originating from either the electrostatic or steric interaction mechanism striving to push the neighboring particles apart. Coalescence of the particles to form a continuous film is thus possible. 

In Section 10.6 we consider strongly stable colloidal suspensions, which occur when electrostatic or polymeric repulsion forces become larger than dispersion or Brownian forces. Finally, in Section 10.7, we treat the case of dominating attraction forces that result in aggregating particles and a flocculated suspension. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

---