Microscopic Stability

The stability of colloidal suspensions is important for their processing and the final product quality (shelf life). It is related to size and concentration of the particles and is essentially affected by the particle-particle interactions. 

Very frequently, stability is referred to as the absence of particle aggregation microscopic or colloidal stability), which means that the particles keep their individuality. Sometimes, stability refers to a constant (and homogeneous) distribution of the dispersed phase in the liquid macroscopic stability). The two terms are not congruent. For instance, aggregation may produce a macroscopically stable gel, while non-aggregating particles may settle due to gravitation. 

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The calculation procedures ascribed above have practical importance. They can be employed to predict the distribution ratio of a metal ion, DJJ, between the bulk solution phase and the ion-exchanger phase. The distribution proRles at various pH s and salt concentrations can be estimated just by use of the unique microscopic stability constant, (Pi)q, if the monodentate complex formation reaction is predominant. With dehned as the ratio of the amount of the bound metal ions to the total capacity of the carboxydate ion exchanger is related to the intrinsic binding constant, K, as shown... 

Linear charge separation on a polyion backbone Microscopic stability constant of MA,-type complex Apparent stability constant of MA-type complex Total concentration of A + ions (mol dm" )... 

The total constant K is obtained as the limit of kt for Cm —> 0, taking into account the intrinsic constant of complexation for each site of the chain k) and the probability distribution of the microscopic stability constants p( ) (Eq. 3-7). 

Thermal fixation techniques are preferred in the study of microstructured liquids, hydrated biological specimens, fluid-filled porous media, etc. Because thermal fixation is used for such diverse applications, there are a wide variety of fixation techniques and associated equipment that impart microscopic stability to the specimen. 

As defined above, microscopic stability means the absence of particle aggregation (coagulation). A sufliciently high degree of such stability is, therefore, achieved when repulsive forces between particles prevent them from collision. Of particular importance in this context is the repulsive double layer interaction that is related to the ion clouds surrounding charged particle surfaces. 

Aggregation means coarsening of the size distribution. The rate of size changes can, therefore, be considered as a measure of the microscopic stability. In principle, one can use any particle sizing technique for the purpose of stability characterisation— provided that the sample preparation does not affect aggregation and that both preparation and measurement time are well below the characteristic time scale(s) of the aggregation process. For dilute suspensions, one usually employs optical sizing techniques. 

Usually one assumes absolute microscopic stability of the binary system when the zeta-potentials of the two particle components are equal in sign and large in absolute value. He-teroaggregation is expected when both components are oppositely charged, while homoaggregation occurs when the attractive interaction forces between similar particles dominate due to low surface potentials (Ra a et al. 2004). 

Finally, the book deals with the dispersion of colloidal suspensions and considers their microscopic stability (Chap. 5). Apart from a brief review on dispersion mechanisms and dispersion techniques, the chapter examines empirical scaling laws for the dispersion of p3Togenic powders and discusses aspects of monitoring dispersion processes. The dispersion of colloidal suspensions is usually accompanied by measures to stabilise the colloidal particles. The suspension stabUily depends on the interaction forces between the particles and can even be predicted under certain conditions. Nonetheless, the suspension stabUily has to be evaluated on a basis of measurements. Aspects of the comparability and extrapolation of such measurements... 

The microscopic stabilization of a Z = 114 nucleus results in a spherical nucleus that is more strongly bound than predicted by the macroscopic model. This effect produces a barrier to deformations leading to fission where there would otherwise be none [10, 12, 13, 23, 25-27]. At the time of these model calculations, the Periodic Table ended at the extreme limit of the actinides (Z = 103), with some experimental evidence for observation of the first transactinide elements. The overall trend with increasing atomic number was shorter half-lives and decreasing resistance to decay by spontaneous fission. The shell-model calculations indicated that well beyond the limits of the known elements the trends might reverse, allowing an extension of the Periodic Table [9]. This led to the concept of an Island of Stability . The term superheavy elements was coined to describe the nuclides occupying the Island. 

A beautiful and elegant example of the intricacies of surface science is the formation of transparent, thermodynamically stable microemulsions. Discovered about 50 years ago by Winsor [76] and characterized by Schulman [77, 78], microemulsions display a variety of useful and interesting properties that have generated much interest in the past decade. Early formulations, still under study today, involve the use of a long-chain alcohol as a cosurfactant to stabilize oil droplets 10-50 nm in diameter. Although transparent to the naked eye, microemulsions are readily characterized by a variety of scattering, microscopic, and spectroscopic techniques, described below. 

The numerical values of and a, for a particular sample, which will depend on the kind of linear dimension chosen, cannot be calculated a priori except in the very simplest of cases. In practice one nearly always has to be satisfied with an approximate estimate of their values. For this purpose X is best taken as the mean projected diameter d, i.e. the diameter of a circle having the same area as the projected image of the particle, when viewed in a direction normal to the plane of greatest stability is determined microscopically, and it includes no contributions from the thickness of the particle, i.e. from the dimension normal to the plane of greatest stability. For perfect cubes and spheres, the value of the ratio x,/a ( = K, say) is of course equal to 6. For sand. Fair and Hatch found, with rounded particles 6T, with worn particles 6-4, and with sharp particles 7-7. For crushed quartz, Cartwright reports values of K ranging from 14 to 18, but since the specific surface was determined by nitrogen adsorption (p. 61) some internal surface was probably included. f... 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

Syntactic Cellular Polymers. Syntactic cellular polymer is produced by dispersing rigid, foamed, microscopic particles in a fluid polymer and then stabilizing the system. The particles are generally spheres or microhalloons of phenoHc resin, urea—formaldehyde resin, glass, or siUca, ranging 30—120 lm dia. Commercial microhalloons have densities of approximately 144 kg/m (9 lbs/fT). The fluid polymers used are the usual coating resins, eg, epoxy resin, polyesters, and urea—formaldehyde resin. 

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

Continuum models of solvation treat the solute microscopically, and the surrounding solvent macroscopically, according to the above principles. The simplest treatment is the Onsager (1936) model, where aspirin in solution would be modelled according to Figure 15.4. The solute is embedded in a spherical cavity, whose radius can be estimated by calculating the molecular volume. A dipole in the solute molecule induces polarization in the solvent continuum, which in turn interacts with the solute dipole, leading to stabilization. 

Pyrocollodion and Pyrocellulose powders belong to the single-base type because they contain only NC and about 1% of a stabilizer, diphenylamine. Both are nearly completely sol in eth-alc mixts. According to microscopic observations conducted by Sapojnikoff (Ref 5), Pyrocollodion powder is much more uniform than the Fr Poudre B , both CPL and CP2 types... 

Structured laundry liquids are currently available in Europe and were recently introduced in the United States [50,51]. These products typically contain high levels of surfactants and builder salts, as well as enzymes and other additives. In the presence of high ionic strength, the combination of certain anionic and nonionic surfactants form lamellar liquid crystals. Under the microscope (electron microscope, freeze fracturing) these appear as round droplets with an onion-like, multilayered structure. Formation of these droplets or sperulites permits the incorporation of high levels of surfactants and builders in a pourable liquid form. Stability of the dispersion is enhanced by the addition of polymers that absorb onto the droplet surface to reduce aggregation. 

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