Colloidally stable, definition

Microemulsions are a convenient medium for preparing microgels in high yields and rather uniform size distribution. The name for these special emulsions was introduced by Schulman et al. [48] for transparent systems containing oil, water and surfactants, although no precise and commonly accepted definitions exist. In general a microemulsion may be considered as a thermodynamically stable colloidal solution in which the disperse phase has diameters between about 5 to lOOnm. 

The methods of disintegration rely entirely upon increasing the dispersity of a solids which process can, at least theoretically, be stopped at any instant resulting in the formation of a suspension of definite dispersity but one that is not necessarily stable. The processes of suspension formation by methods of condensation on the other hand are more complicated, owing to the fact that unless the resulting colloidal suspension possesses at least some degree of stability the process of condensation once set in operation will not cease but proceed until the transformation to the macrocrystalline structure is complete. 

From these data it is evident that a momentary suspension" stage is obtained for values of U equal to 8-25, but on attempting to increase the U values to obtain more definite persistence of the supension, a gel is formed thus to prepare a colloidal suspension of barium sulphate a medium must be employed in which U values of 8-25 may be obtained at lower concentrations, i.e. we must reduce the actual solubility of barium sulphate. By performing the double decomposition in alcohol water mixtures in which the solubility of barium sulphate is small a clear stable suspension may in fact readily be formed. 

Although most colloidal dispersions are not thermodynamically stable, a consequence of the small size and large surface area in colloids, and of the presence of an interfacial film on droplets, bubbles or particles, is that dispersions of these species, having reasonable kinetic stability, can be made. That is, suspended droplets or particles may not aggregate quickly nor settle or float out rapidly and droplets in an emulsion or bubbles in a foam may not coalesce quickly. Many food and personal care product emulsions and suspensions, for example, are formulated to remain stable for months to years. It is crucial that stability be understood in terms of a clearly defined process, and one must consider the degree of change and the time-scale in the definition of stability. 

In DLVO theory, the secondary minimum can only be created by the van der Waals force, which is essentially independent of the salt concentration across the concentration range 0.001 M < c < 0.1 M. This force has to be balanced with a force that decays exponentially as a function of k, which means that it decays by a factor exp(-10) across this range. The unhappy consequence of this prediction is that the position of the secondary minimum, and therefore the interlayer d value, varies very rapidly as a function of k, in contradiction to the experimental results. A further unhappy consequence of this balance is that it always produces a primary minimum much deeper than the secondary minimum. The full, standard DLVO thermodynamic potential energy curve, which also includes a very-short-range Bom repulsion, is shown in Figure 1.13 [23], It is therefore a definite prediction of DLVO theory that charge-stabilized colloids can only be kinetically, as opposed to thermodynamically, stable. The theory does not mean anything at all if we cannot identify the crystalline... 

Properties Light-tan to dark-brown powder no pronounced odor. Stable in dry form and relatively stable in aqueous solution. Nonhygroscopic, no definite mp, decomposes above 200C, d about 1.5. Forms colloidal solutions or dispersions in water practically insoluble in all organic solvents. 

Yellow amorphous powder, no definite mp, decomp 160 ( +216. uv max (80% methanol) 383 nm (Ej 9] 6). An amphoteric compd. Almost insol in water, benzene, chloroform, dry lower aliphatic alcohols, ether sol in basic solvents such as pyridine, collidine, and in aq lower alcohols. In coned H1S04 gives stable blue color no coloration with ferric chloride or with HO LDW i. v. in mice (using sodium carbcxymethyl cellulose as a vehicle) 6. ] 6 tng/kg, Williams et al, Antimicrob. Ag, Ckemother. 1964, 737-741. Also reported as LDjq i.v> in Swiss mice (using a colloidal suspension) 1,20 mg/kg, A. C, Parekh, C, V. Dave loc. cit. 

Definition A stable dispersion of discrete, colloid-size particles of amorphous silica in aq. sol n. 

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. 

Such designations of stable and unstable colloids are very relative and must be made in the context of the apphcation in question. It may be, for example, that a colloid that maintains its characteristics for two days would be considered stable in one application, while another would require that a minimum of two years pass without change. Obviously, then, one must be careful when discussing colloidal stability and instability. From this point on in the discussion, unless otherwise indicated, the kinetic (rather than energetic) definition of stability will be employed in its most general sense, it being assumed that all (or almost all) colloids are in reality metastable systems. Also, it must be kept in mind that stability in the present context is used in terms of... 

In Section 1.1, we defined colloidal systems as systems in which particles dispersed in a medium are snbjected to both thermal motion and motion due to external forces (e.g., gravity). This definition leads directly to the notion of stability of a colloidal dispersion. A colloidal dispersion is considered to be stable if no rapid phase separation occnrs through sedimentation (if the density of the particles is higher than that of the medium) or creaming (if the density of the particles is lower than that of the medinm). Thus, colloidal stability refers to the ability of a dispersion to resist aggregation into larger entities that then would segregate from the medium. 

The term colloidal silica here refers to stable dispersions or sols of discrete parti-cks of amorphous silica. By arbitrary definition, the term excludes solutions of poly-silicic acid in which the polymer molecules or particles are so small that they are not stable. Such solutions, which are usually obtained by acidifying sodium silicate solutions or by hydrolyzing silicon esters or halides at ordinary temperatures, have been discussed in Chapter 3 as precursors of colloidal particles. 

Micelle colloids form, for example, in aqueous solutions of soaps and dyes (see [10], pp. 8-9 [1], pp. 80-81 [19], p. 250). However, soaps dissolve normally [1, p. 81], i.e. without micelle formation, in alcohol (cf. [10], pp. 8-9). This is also true of the high-polymer material mbber if menthol is used as the solvent [1, p. 81]. The crucial role played by the solvent (cf. [15], p. 208) therefore makes it difficult to determine correctly whether a low or highmolecular substance is involved. Depending on the namre, concentration and temperature of the solvent, it is evidently the case that primary valence bonds can break too, while secondary valence bonds remain stable. Even if a colloid proves to be resistant to many different solvents, there is still some uncertainty about whether the dissolved substance can be identified definitely as macromolecular. The process is not therefore conclusive enough. Staudinger himself also felt that resistance was merely a valuable indication but not definite proof that the colloid particles are macromolecular in structure [1, p. 119]. Determination of the size [...] does not reveal the inner structure of the particles. This question is answered via chemical experiments that are carried out here at the same time, like when investigating the structure of particles of low-molecular organic compounds [...], in order to demonstrate that the atoms in a particle of a certain size are bonded by primary valences, i.e. that this particle represents a chemical molecule [10, pp. 15-16]. 

As an example, one may consider the wastewater treatment process (Cherimisinofif, 2002). The wastewater with colloidal particles is found to be a stable suspension. However, by treating it with some definite methods (such as pH control, electrolyte concentration, etc.), one can change the stability of the system. 

Microemulsion systems with their inner structure in the colloidal domain have been the subject of many theoretical and experimental studies due to their very broad applicability [1 ]. The name microemulsion was first introduced by Hoar and Schutman in 1943 as the name for a clear or transparent system obtained by titration of a milky white emulsion with a medium-chain length alcohol (e.g., 1-pentanol or 1-hexanol) [5]. A more general definition of the term microemulsion was given later by Danielsson and Lindman, who described it as a system, composed of water, oil and an amphiphilic component, being an optically isotropic and thermodynamically stable liquid solution [6],... 

Microemulsions [512, 513] are special types of emulsions that form spontaneously and have very small particles. Microemulsions are optically clear, thermodynamically stable dispersions of two immiscible liquids obtained by the use of carefully adjusted surface-active molecules (surfactants). Both liquids in a microemulsion will be present in regions of the same order of magnitude, with the dispersed phase on the order of 10-100 nm. Aggregates of surface-active molecules, or micelles, form into colloidal-sized clusters in such a way that hydrophilic groups are directed toward the water. These definitions [514] are general in nature, but they suffice for the current purpose the interested reader is directed to texts on this... 

Historically, microemulsions were discussed as a separate family of colloids that formed essentially spontaneously and were thermodynamically stable. However, microemulsions must, by definition, contain at least three components—solvent, amphiphile, and dispersed phase—and quite often contain a fourth, the so-called cosolvent. More recent experimental and theoretical work has tended to move them into the larger family of surfactant aggregates, their complex composition notwithstanding. That convention will be followed here, although there still remain a number of points of contention that need to be resolved on their classification as surfactant mesophases on a par with classical liquid crystals. 

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