Particles colloids, definition

Bavykin, Dmitry V. is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998) Luminescent and photocatalytic properties of CdS nanocolloids . Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. 

With all polarization phenomena, the flux at a definite time is always less than the original value. When steady-state conditions have been attained a further decrease in flux will not be observed, i.e., the flux will become constant as a function of time. Polarization phenomena are reversible processes, but in practice, a continuous decline in flux can be observed. Such continuous decline is a result of membrane fouling, which may be defined as the irreversible deposition of retained particles, colloids, emulsions, suspensions, macro molecules, salts, etc. on or in the membrane. This includes adsorption, pore blocking, precipitation, and cake formation. 

It is well known that colloidal suspensions can share many features with simple molecular systems such as gas, liquid, and solid crystalline and amorphous glass phases. This is particularly true when the colloid is nearly monodisperse for then the interparticle interactions, which are usually size dependent, are nearly all the same and hence the phase boundaries, which depend on the interactions, are distinct. Indeed, as the size distribution of a colloid narrows, one could claim that the colloidal suspension transforms into a solution, just as the different particles, by becoming alike or even identical, are transforming to molecules. Unlike simple molecular systems, which by their definition have no variety and are not dissolved in a medium, particle colloids and solutions can vary the interactions via changing size, surface groups, solvent, etc., and thereby change the phase diagram. 

Siiica Bead SB-145] Siiica Bead SB-150] Siiica Bead SB-300] Siiica Bead SB-700. See Silica Silic acid (polyortho). See Silica, hydrated Silica, colloidal EINECS/ELINCS 231-545-4 Synonyms Colloidal silica Colloidal silicon dioxide Silica sol Silicon dioxide colloidal Definition A stable dispersion of discrete, colloid-size particles of amorphous silica in aq. sol n. 

After colloidal silica particles of definite and uniform size became available, it was possible to esterify the surface and show that the product indeed consists of a monomolecular layer of oriented butoxy replacing some of the hydroxyl groups on the silica surface so that the outer surface of the particle is essentially a layer of hydrocarbon groups. This hydrocarbon surface may be aliphatic in nature, as when an alcohol such as butyl is employed, or aromatic, as when benzyl is used. The nature of the hydrocarbon surface affects the solubility and dispersibility of the particles. Thus when silica particles 17 nm in diameter are esterified with benzyl alcohol, a dry powder product is obtained that dissolves to a clear solution in benzene, but is not soluble in an aliphatic solvent such as kerosene. On the other hand, when the same silica is esterified with an aliphatic branched-chain octadecyl alcohol, the powder dissolves readily in kerosene (442). 

The universal van der Waals attraction which occurs in all disperse systems is described in Vol. 1. The dipole-dipole, dipole-induced dipole and London dispersion forces for atoms and molecules are described. This is followed by the microscopic theory of Hamaker for colloidal particles and definition of the Hamaker constant. This microscopic theory is based on the assumption of additivity of all atom or molecular attractions in each particle or droplet. The variation of van der Waals attraction with separation distance h between the particles is schematically represented. This shows a sharp increase in attraction at small separation distances (of the order of a few nanometers). In the absence of any repulsion, this strong attraction causes particle or droplet coagulation which is irreversible. The effect of the medium on the overall van der Waals attraction is described in terms of the effective Hamaker constant which is now determined by the difference in Hamaker constant between the particles and the medium. The macroscopic theory of van der Waals attraction is briefly described, with reference to the retardation effect at long sepeiration distances. The methods that can be applied for determination of the van der Waals attraction between macroscopic bodies are briefly described. 

A nanoparticle is widely considered to be a particle with at least one dimension that is less than 100 nm as reflected in the lUPAC definition that a nanoparticle is a microscopic particle, often restricted to the so-called nanosized particles. The definition partially overlaps that of colloids which are dispersions in a fluid medium of particles between 1 nm and 1 pm in size. More usefully the term nanoparticle often implies a particle of sufficiently small size in contrast to bulk media of the same chemical composition such that one or more of its properties—optical, electronic, mechanical,. ..—is size dependent. 

For tire purjDoses of tliis review, a nanocrystal is defined as a crystalline solid, witli feature sizes less tlian 50 nm, recovered as a purified powder from a chemical syntliesis and subsequently dissolved as isolated particles in an appropriate solvent. In many ways, tliis definition shares many features witli tliat of colloids , defined broadly as a particle tliat has some linear dimension between 1 and 1000 nm [1] tire study of nanocrystals may be drought of as a new kind of colloid science [2]. Much of die early work on colloidal metal and semiconductor particles stemmed from die photophysics and applications to electrochemistry. (See, for example, die excellent review by Henglein [3].) However, the definition of a colloid does not include any specification of die internal stmcture of die particle. Therein lies die cmcial distinction in nanocrystals, die interior crystalline stmcture is of overwhelming importance. Nanocrystals must tmly be little solids (figure C2.17.1), widi internal stmctures equivalent (or nearly equivalent) to drat of bulk materials. This is a necessary condition if size-dependent studies of nanometre-sized objects are to offer any insight into die behaviour of bulk solids. 

Colloidal suspensions are, per definition, mixtures of mesoscopic particles and atomic liquids. What happens if there are several different species of particles mixed in the solvent One can invent several different sorts of mixtures small and large particles, differently charged ones, short and long rods, spheres and rods, and many more. Let us look into the literature. One important question when dealing with systems with several components is whether the species can be mixed or whether there exists a miscibility gap where the components macroscopically phase-separate. 

The term particle and particle size are so highly ambiguous as to require precise definition. As used in this article particles will.be limited by size to those distinct entities which have physically detectable boundaries in any direction within the limits of 0.05 and 10 microns (1 micron, p -0.001mm). This size range covers those particles which can be directly measured without magnification down to those which exhibit colloidal behavior... 

The colloidal particles can be crystalline or constitnte an amorphons agglomeration of individual molecnles. The definition also includes nonaggregated large macromolecules such as proteins. An arbitrary distinction is made between hydrophobic colloids (sols) and hydrophilic colloids (gels), which depends on the degree and type of interaction with the aqneons solvent. 

An aqueous colloidal polymeric dispersion by definition is a two-phase system comprised of a disperse phase and a dispersion medium. The disperse phase consists of spherical polymer particles, usually with an average diameter of 200-300 nm. According to their method of preparation, aqueous colloidal polymer dispersions can be divided into two categories (true) latices and pseudolatices. True latices are prepared by controlled polymerization of emulsified monomer droplets in aqueous solutions, whereas pseudolatices are prepared starting from already polymerized macromolecules using different emulsification techniques. 

Whether there is currently a nanotechnology is a question of definition. If one asks whether there are (or are soon likely to be) commercial electronic fluidic, photonic, or mechanical devices with critical lateral dimensions less than 20 nm, the answer is no, although there may be in 10 to 20 years. There is, however, a range of important technologies—especially involving colloids, emulsions, polymers, ceramic and semiconductor particles, and metallic alloys—that currently exist. But there is no question that the field of nanoscience already exists. 

The following mechanism emerges form the observation reported so far on the colloidal stability of multichain PNIPAM particles at elevated temperature. Let us note first that such PNIPAM particles are examples of mesoglob-ules, according to the definition of Timoshenko and Kuznetsov [215] ... 

The classification of wastewater in terms of size distribution is normally done from a practical point of view. Typically, a distinction is made between soluble, colloidal and suspended components (Figure 3.6). While this definition for determining what solids are is rational as far as physical transport processes in sewers are concerned, when dealing with the microbial processes for sewer conditions, an extension of the solids definition is required. Particles larger than about 10-4 pm cannot be transported through the cell wall and are, therefore, from a microbial point of view, considered particles. 

Graham s definitions were expanded, and the concept of a colloidal state of matter evolved. According to this view, a substance could occur in a colloidal state just as it could occur under various conditions as a gas, liquid, or solid. If a colloidal solution was, at that time, defined as a solution in which the dispersed particles were comprised of large molecules, the ascertion would have been more acceptable. 

Additive, exhibiting surface activity, that is added to a suspending medium to promote uniform and maximum separation of extremely fine solid particles, often of colloidal size (see Note 2 in Definition 1.39)... 

Modified from [2], within which particles with equivalent diameters usually between 0.01 pm and 100 pm are specified. This extends beyond the size range specified for a colloidal system. To avoid confusion the definition proposed here is reeommended. 

There are some very special characteristics that must be considered as regards colloidal particle behavior size and shape, surface area, and surface charge density. The Brownian motion of particles is a much-studied field. The fractal nature of surface roughness has recently been shown to be of importance (Birdi, 1993). Recent applications have been reported where nanocolloids have been employed. Therefore, some terms are needed to be defined at this stage. The definitions generally employed are as follows. Surface is a term used when one considers the dividing phase between... 

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