Stabilization particles

In practice, tliere are various ways by which ( ) can be detennined for a given sample, and tire results may be (slightly) different. In particular, for sterically stabilized particles, tire effective hard-sphere volume fraction will be different from tire value obtained from tire total solid content. 

Because model colloids tend to have a ratlier well defined chemical composition, elemental analysis can be used to obtain detailed infonnation, such as tlie grafted amount of polymer in tire case of sterically stabilized particles. More details about tire chemical stmcture can be obtained using NMR techniques (section B1.13). In addition, NMR... 

The behaviour of tliese systems is similar to tliat of suspensions in which short-range attractions are induced by changing solvent quality for sterically stabilized particles (e.g. [103]). Anotlier case in which narrow attractions arise is tliat of solutions of globular proteins. These crystallize only in a narrow range of concentrations [104]. 

Spray Drying Detailed descriptions of spray dispersion dryers, together with apphcation, design, and cost information, are given in Sec. 17. Product quality is determined by a number of properties such as particle form, size, flavor, color, and heat stability. Particle size and size distribution, of course, are of greatest interest from the point of view of size enlargement. 

Nanoparticles of the semicondnctor titanium dioxide have also been spread as mono-layers [164]. Nanoparticles of TiOi were formed by the arrested hydrolysis of titanium iso-propoxide. A very small amount of water was mixed with a chloroform/isopropanol solution of titanium isopropoxide with the surfactant hexadecyltrimethylammonium bromide (CTAB) and a catalyst. The particles produced were 1.8-2.2 nm in diameter. The stabilized particles were spread as monolayers. Successive cycles of II-A isotherms exhibited smaller areas for the initial pressnre rise, attributed to dissolution of excess surfactant into the subphase. And BAM observation showed the solid state of the films at 50 mN m was featureless and bright collapse then appeared as a series of stripes across the image. The area per particle determined from the isotherms decreased when sols were subjected to a heat treatment prior to spreading. This effect was believed to arise from a modification to the particle surface that made surfactant adsorption less favorable. 

PVP, a water soluble amine-based pol5mer, was found to be an optimum protective agent because the reduction of noble metal salts by polyols in the presence of other surfactants often resulted in non-homogenous colloidal dispersions. PVP was the first material to be used for generating silver and silver-palladium stabilized particles by the polyol method [231-233]. By reducing the precur-sor/PVP ratio, it is even possible to reduce the size of the metal particles to few nanometers. These colloidal particles are isolable but surface contaminations are easily recognized because samples washed with the solvent and dried in the air are subsquently not any more pyrophoric [231,234 236]. 

Theory of Steric Stabilization. A detailed description of the competing theories can be obtained from other publications (1-3) and only an outline will be given here. Almost all the acceptable theoretical descriptions have their origins in the Flory-Krigbaum theory (10) for a dilute polymer solution which has been adapted to the case of sterically stabilized particles. 

Comparison of Theory and Experiment. The expression for the free energy of interpenetration of sterically stabilized particles may be obtained by combining Equations 2, 3 and 6. Using these expressions can be calculated as a function of both... 

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. 

Moreover, particle size can significantly affect the material properties of the nanoparticles and is important for their interaction with the biological enviromnent (e.g., as concerns their ability to pass fine capillaries or to leave the vascular compartment via fenestrations after intravenous administration). Particle sizing results are thus crucial parameters in the development and optimization of preparation processes as well as in the evaluation of dispersion stability. Particle sizing, however, has also been employed for other purposes for example, to evaluate the size dependence of the nanoparticle matrix properties [1] or to obtain additional information on the particle shape [2,3]. 

DAR Jones. Depletion flocculation of sterically stabilized particles. PhD thesis, Bristol,... 

Figure 11.2.IB describes the propagation process of particles. When a sufficient amount of stabilized particles is formed, all the oligomeric radicals and polymer or primary particles are captured by existing stable particles before they precipitate and form new particles by themselves. This is what is called the growth process of particles by heteroaggregation.

The 1.5-nm nanoparticles readily react with thiol or amine-terminated ligands under mild conditions to yield thiol- or amine-stabilized nanoparticles. Triphenylphosphine-stabilized particles thermally decompose with the production of (PPh3)AuCl and metallic gold. 

Near monodisperse Au NPs in the size range of 1—4nm can be obtained using dodecylthioether end-functionalized PMMA as stabilizer. Particle size is controlled by varying the concentration of the stabilizing polymer, which can be readily displaced by thiol ligands to yield MPCs of the usual type [97]. 

Vincent, B., Edwards, J., Emmett, S., Jones, A. (1986). Depletion flocculation in dispersions of sterically-stabilized particles ( soft spheres ). Colloids and Surfaces, 18, 261-281. 

The specific ability of certain finely divided, insoluble solids to stabilize foam has long been known [Berkman and Egloff, op. cit., p. 133 and Bikerman, op. cit., Chap. 11]. Bartsch [Kolloidchem. Beih, 20, 1 (1925)] found that the presence of fine galena greatly extended the life of air foam in aqueous isoamyl alcohol, and the finer the solids, the greater the stability. Particles on the order of 50 pm length extended the life from 17 seconds to several hours. This behavior is consistent with theory, which indicates that a solid particle of medium contact angle with the liquid will prevent the coalescence of two bubbles with which it is in simultaneous contact. Quantitative observations of this phenomenon are scanty. 

Miniemulsion polymerization involves the use of an effective surfactant/costabi-lizer system to produce very small (0.01-0.5 micron) monomer droplets. The droplet surface area in these systems is very large, and most of the surfactant is adsorbed at the droplet surfaces. Particle nucleation is primarily via radical (primary or oligomeric) entry into monomer droplets, since little surfactant is present in the form of micelles, or as free surfactant available to stabilize particles formed in the continuous phase. The reaction then proceeds by polymerization of the monomer in these small droplets hence there may be no true Interval II. 

The final increase in particle size, shown in figure 7, is pro bably caused by limited flocculation,since particle coverage by emulsifier is very limited. Conductimetric titration of emulsifier shows that only a part of it is used for stabilizing particles Typi cal results are shown in figure 9. 

According to the aggregative and coagulative nucleation mechanisms which have been derived originally from the homogeneous nucleation theory of Fitch and Tsai [128], the most important point in the reaction is the instant at which colloidally stabilized particles form. After this point, coagulation between similar-sized particles no longer occurs, and the number of particles present in the reaction is constant. As shown in Fig. 6, the dispersion copolymerization with macromonomers is considered to proceed as follows. (1) Before polymerization, the monomer, macromonomer, and initiator dissolve completely into the... 

Production of Sterically Stabilized Particles (at Critical Point)... 

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