Colloidal systems steric stabilization

Controlled hydrolysis is one of the most popular methods for processing silica spheres in the range of 10-1,000 nm. The method was developed by Stober, Fink, and Bohn (SFB) [226-229] and is based on the hydrolysis of TEOS in a basic solution of water and alcohol. Particle size depends on the reactant concentration, i.e., the TEOS/alcohol ratio, water concentration, and pH (>7). This method has been extended to other metal oxide systems with similar success, particularly for Ti02 synthesis [85,230]. The hydrous oxide particles precipitated by the hydrolysis of an alkoxide compound have the same tendency to agglomerate as that described for metal colloid systems. Different stabilizers can be used to stabilize these particles and prevent coagulation (step 2). These stabilizers control coagulation by electrostatic repulsion or by steric effects [44], similarly to the metal colloid systems. 

The introduction of free polymer into the dispersion medium appears to make no significant difference to the number of phases accessible to the system. This can be discerned from the following simplistic considerations using the Phase. Rule, which will be assumed to be applicable. We consider a dispersion of colloidal particles sterically stabilized by terminally grafted polym chains. Free polymer is dissolved into the dispersion medium. 

Later, when particles are present, they will capture oligomeric radicals in solution and dN/dt = Ri-Rco As more particles are formed, flocculation will occur if the particles are caducous. Presumably only electrostatic repulsions are involved in the stabilization of these colloids, although it is recognized that in other types of systems, "steric stabilization is possible. ... 

It should also be noted that all theoretical and experimental data describe the static or equilibrium situation. The transition from statics to dynamics needs much more investigations. For particulate filled polymers it is important how the particles, covered with an adsorption layer, interact with each other. This problem was only considered to solve the problems arising from steric or adsorption stabilization of colloid systems. For stabilization, the concentration profiles are very important. From the scaling point of view, the ques-... 

Suppose we have a physical system with small rigid particles immersed in an atomic solvent. We assume that the densities of the solvent and the colloid material are roughly equal. Then the particles will not settle to the bottom of their container due to gravity. As theorists, we have to model the interactions present in the system. The obvious interaction is the excluded-volume effect caused by the finite volume of the particles. Experimental realizations are suspensions of sterically stabilized PMMA particles, (Fig. 4). Formally, the interaction potential can be written as... 

The hard sphere (HS) interaction is an excellent approximation for sterically stabilized colloids. However, there are other interactions present in colloidal systems that may replace or extend the pure HS interaction. As an example let us consider soft spheres given by an inverse power law (0 = The energy scale Vq and the length scale cr can be com-... 

In studying the stability of colloidal dispersions it is of considerable advantage if the particles concerned are monodisperse and spherical. For aqueous, charge-stabilised systems polymer latices have proved invaluable in this regard. With non-aqueous systems, steric stabilisation is usually required. In this case it... 

In Section 3.4a we examine a model for the second virial coefficient that is based on the concept of the excluded volume of the solute particles. A solute-solute interaction arising from the spatial extension of particles is the premise of this model. Therefore the potential exists for learning something about this extension (i.e., particle dimension) for systems for which the model is applicable. In Section 3.4b we consider a model that considers the second virial coefficient in terms of solute-solvent interaction. This approach offers a quantitative measure of such interactions through B. In both instances we only outline the pertinent statistical thermodynamics a somewhat fuller development of these ideas is given in Flory (1953). Finally, we should note that some of the ideas of this section are going to reappear in Chapter 13 in our discussions of polymer-induced forces in colloidal dispersions and of coagulation or steric stabilization (Sections 13.6 and 13.7). 

The principles of colloid stability, including DLVO theory, disjoining pressure, the Marangoni effect, surface viscosity, and steric stabilization, can be usefully applied to many food systems [291,293], Walstra [291] provides some examples of DLVO calculations, steric stabilization and bridging flocculation for food colloid systems. 

A colloidal species that adsorbs onto and acts to protect the stability of another colloidal system. The term refers specifically to the protecting colloid and only indirectly to the protected colloid. Example when a lyophilic colloid such as gelatin acts to protect another colloid in a dispersion by conferring steric stabilization. See also Gold Number, Protection. 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

Recent experimental studies (1-3), on systems of sterically stabilized colloidal particles that are dispersed in polymer solutions, have highlighted the role played by the free polymer molecules. These experiments are particularly relevant because the systems chosen are model dispersions in which the particles can be well approximated as monodisperse hard spheres. This simplifies the interpretation of the data and leads to a better understanding of the intcrparticle forces. DeHek and Vrij (1, 2) have added polystyrene molecules to sterically stabilized silica particles dispersed in cyclohexane and observed the separation of the mixtures into two phases—a silica-rich phase and a polystyrene-rich phase—when the concentration of the free polymer exceeds a certain limiting value. These experimental results indicate that the limiting polymer concentration decreases with increasing molecular weight of... 

Fig. 1. Interaction potential between two colloidal particles as a function of the reduced centre-to-centre separation R = r/2a, where a is the radius of the particles. Curve 1, steric repulsion due to the adsorbed layer (Vs) curve 2, attraction due to the free polymer (Vd) curve 3, van dcr Waals attraction (X7.,) curve 4, sum of the contributions given by curves 1—3. System polvisobutene-stabilized silica particles and polystyrene (free polymer) in cyclohexane at 308 K. Molecular weight of the free polymer = 82,000, volume fraction of polystyrene, 0 = 0.02, a = 48 nm, thickness of the adsorbed layer 6 = 5 nm, x = 0.5 for polystyrene—cyclohexane, x, = 0.47 and xs = 0.10 for polyisobutene— cyclohexane, AjkT 4.54 and v = 0.10.

Polymers are often used to stabilize colloidal systems by grafting them on the particle surfaces to provide steric repulsion [1,2], Polymers can also induce flocculation due to either depletion or bridging interactions [3],... 

If the grafted polymer is not adsorbed on the second plate, then the grafted polymer provides only steric interaction between the plates. This repulsive steric interaction increases the stability of the colloidal system. To calculate the steric interaction between two plates, the adsorption constant Kads in eq 11 should be taken to be zero in this case. 

The steric stabilization, which is imparted by polymer molecules grafted onto the colloidal particles, is extensively employed.3 Amphiphilic block copolymers are widely used as steric stabilizers. The solvent-incompatible moieties of the block copolymer provide anchors for the polymer molecules that are adsorbed onto the surface of the colloidal particles, and the solvent-compatible (buoy) moieties extend into the solvent phase. When two particles with block copolymers on their surface approach each other, a steric repulsion is generated bet ween the two particles as soon as the tips of the buoy moieties begin to contact, and this repulsion increases the stability of the colloidal system.4-6 Polymers can also induce aggregation due to either depletion 7-11 or bridging interactions.12 15... 

In the latter case the total interaction, which is what can be measured, is affected by the net charge of the surface and the adsorbed layer, ion-ion correlations, bridging interactions and steric confinement of the polymer chain [116]. We note that polyelectrolytes are often present as additives in colloidal dispersions and the character of the forces generated by the polyelectrolyte adsorption layers has a paramount influence on stability of these colloidal systems. With the aim to illustrate what can be learnt about polyelectrolyte adsorption layers using the SFA, we will look at the influence of the polyelectrolyte charge density on the forces acting between surfaces coated with polyelectroytes. We will consider an example where the polyelectrolyte charge density is varied by a systematic... 

The other kind of systems largely studied, consists of polymethylmethacrylate (PMMA) or silica spherical particles, suspended in organic solvents [23,24]. In these solvents Q 0 and uy(r) 0. The particles are coated by a layer of polymer adsorbed on their surface. This layer of polymer, usually of the order of 10-50 A, provides an entropic bumper that keeps the particles far from the van der Waals minimum, and therefore, from aggregating. Thus, for practical purposes uw(r) can be ignored. In this case the systems are said to be sterically stabilized and they are properly considered as suspensions of colloidal particles with hard-sphere interaction [the pair potential is of the form given by Eq. (5)]. 

Boyko and coworkers reported use of poly(vinyl alcohol) (PVA) with different molecular weights for steric stabilization of PVCL microgels [64], In this system, t he A), of temperature-sensitive PVCL microgels decreased from 280 to 180nm with increased PVA concentration in the reaction system from 2 to 10 g L 1 (Fig. 3). The colloidal stability of the PVCL microgels was improved with PVA of lower molecular weight and lower degree of hydrolysis. 

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