Metastable dispersions, formation

Metastable forms. Formation of metastable dispersions with reduced lattice energy would result in faster dissolution rates. It was found that the activation energies for dissolution for furosemide was IVkcalpermol, whereas that for 1 2 furosemide PVP coprecipitate was only 7.3 kcal per mol. ... 

Similar considerations apply to chemical or physicochemical equilibria such as encountered in phase transitions. A chilled salt solution may be stable (at or below saturation), metastable (supercooled to an extent not allowing nucle-ation), or unstable (cooled sufficiently to nucleate spontaneously). In the case of a solid, S, dispersed in a binary liquid, Li + L2, instability at the instant of formation gives way to a neutral or metastable condition wherein three types of contacts are established ... 

Some guidelines have been provided for defining the metastable region. If the seed crystals dissolve when added to the metastable solution, this implies that saturation conditions have not been reached. If the addition of the seed leads to the formation of an oil dispersion, it may be concluded that supersaturation has been realized (Anderson, 2000). 

Aggregation of the atoms or microclusters may give metal nuclei. The micro-cluster itself may work as the nucleus. Although the size of microcluster or nucleus is not clear, the nucleus may consist of 13 atoms, which is the smallest magic number, This idea may be supported by the structural analysis of PVP-stabilized Pt nanoparticles (64) and other systems. In fact, a particle of 13 atoms is considered an elemental duster. In the case of preparation of PVP-stabilized Rh nanoparticle dispersions by alcohol reduction, formation of very tiny particles, the average diameter of which is estimated to be 0.8 nm, was observed (66). These tiny particles in the metastable state may consist of 13 atoms each and easily increase in size to the rather nanoparticles with average diameter of 1.4 nm, i.e., the particles composed of 55 atoms. This observation again supports the idea that the elemental cluster of 13 atoms is the nucleus. 

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

We have worked out a sufficiently precise and reliable system of consistent thermodynamic constants (Mel nik, 1972) especially for analysis of the conditions of formation of iron ores. In this work, in addition to the constants of crystalline minerals, data on the stability and thermodynamic properties of the original finely dispersed amorphous or cryptocrystalline iron, magnesium, and silica sediments were systematized for the first time. Such sediments are metastable solid phases which in nature are converted into stable crystalline minerals during diagenesis and low-rank metamor-... 

No major ehallenges for immediate release dosage forms Controlled-release dosage forms may be needed to limit rapid absorption profile Formulations designed to overeome solubility or dissolution rate problems Salt formation Precipitation inhibitors Metastable forms Solid dispersion Complexation Lipid technologies Particle size reduetion... 

The formation of a disperse system as a result of the generation and successive growth of primary particles (nuclei) of a new stable phase may take place in any metastable system. The metastability, arising as conditions shift away from normal equilibrium conditions, may arise from deviations in the chemical composition of the phases (supersaturation) as well as from physicochemical action (changes in the temperature and pressure). 

Physico-chemical ways of achieving metastability of the initial system are usually related to changes in temperature, pressure (less often), and composition of solvent [10]. The supersaturation (supercooling) of water vapor is the reason for certain meteorological phenomena (cloud formation). The formation of disperse systems upon changes in temperature is the key for the preparation of all polycrystalline materials in metallurgy. Here control of... 

The formation of disperse structures with phase contacts takes place under a great variety of physical-chemical conditions, for instance during sintering and pressing of powders. Disperse systems with phase contacts that form in the course of condensation of a new phase from metastable solutions or melts, are commonly referred to as condensation. If the particles that form... 

In summary, mixed films of solute and solvent can be prepared as retracted films however, these are always transient or metastable systems, the probability of whose formation becomes greater the more fully does molecular adlineation of solute and solvent occur. If the concentration of polar solute is high enough, the film eventually becomes free of solvent as adsorption equilibrium is approached, and the retracted film is then free of solvent molecules. Where the solute and solvent molecules are so different in shape or size that the intermolec-ular cohesion between them through London dispersion forces becomes a minor factor, mixed films are never formed. Therefore, mixed films occur by the retraction process imder special conditions nonetheless, when they can be produced, a useful technique is available for studying the intermolecular interaction of solute and solvent molecules in the adsorbed state. 

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