Spontaneous dispersion

Figure B3.3.12. Sulphur atoms in liquid iron at the Earth s core conditions, simnlated by first-principle Car-Parrinello molecular dynamics, (a) Initial conditions, showing a mannally-prepared initial cluster of snlphur atoms, (b) A short tune later, indicating spontaneous dispersal of the snlphur atoms, which mingle with the surroundmg iron atoms. Thanks are dne to D Alfe and M J Gillan for this figure. For fiirtlier details see [210. 211].

FIGURE 11.3 One-way ANOVA (analysis of variance). One-way analysis of variance of basal rates of metabolism in melanophores (as measured by spontaneous dispersion of pigment due to G,.-protein activation) for four experiments. Cells were transiently transfected with cDNA for human calcitonin receptor (8 j-ig/ml) on four separate occasions to induce constitutive receptor activity. The means of the four basal readings for the cells for each experiment (see Table 11.4) are shown in the histogram (with standard errors). The one-way analysis of variance is used to determine whether there is a significant effect of test occasion (any one of the four experiments is different with respect to level of constitutive activity). 

Ti02 nanotubes were used to support M0O3 observing a spontaneous dispersion of molybdenum-oxide on the surface of nanotubes, which was different from that observed on titania particles.Supporting tungsten oxides a preferential orientation of the (002) planes was observed. Vanadium-oxide in the form of nanorods could be prepared using the titania nanotube as structure-directing template under hydrothermal... 

Niobium Products Co., 50 m /g). Many different synthesis methods have been used to prepare supported metal oxide catalysts. In the case of supported vanadium oxide catalysts, the catalysts were prepared by vapor phase grafting with VOCI3, nonaqueous impregnation (vanadium alkoxides), aqueous impregnation (vanadium oxalate), as well as spontaneous dispersion with crystalline V2O5 [4]. No drastic reduction of surface area of the catalysts was observed. 

The influence of different preparation methods upon the structure and reactivity of supported vanadia catalysts has recently been investigated by several laboratories.55 58 These studies concluded that the same surface vanadia species forms on a specific oxide support independent of the preparation method, even when starting with V205 crystals. The reason for this phenomenon is that vanadium oxide spontaneously disperses on oxide supports to form surface vanadia species, which is schematically depicted in Figure 7. 

Figure 7 Metal oxides spontaneously disperse on oxide supports (self assembly). 10...

So far we have elaborated on the phenomenon of spontaneous dispersion of oxide or salt as monolayer or submonolayer onto the surface of supports with highly specific surfaces. At this point one may well inquire into the nature of this interesting phenomenon. It is only natural for us to think about, first of all, the questions concerning the origin of spontaneity, the dispersion of oxide or salt as monolayer or submonolayer instead of multilayer, and the ubiquity of the phenomenon. 

Spontaneous Dispersion of Oxides and Salts onto Internal Surface of Zeolites... 

Water-free, spontaneously dispersing on contact with water. They are intended for immediate use. 

Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO... 

Okumura, K., Yoshimoto, R., Uruga, T., Tanida, H., Kato, K., Yokota, S. and Niwa, M. (2004) Energy-dispersive XAFS studies on the spontaneous dispersion of PdO and the formation of stable Pd clusters in zeolites. J. Phys. Chem. B, 108, 6250-6255. 

Wetting and dispersion depends on the liquid surface tension and 0, the contact angle between the Hquid and solid. W, W and Wg are spontaneous when 0 < 90°, and Wd is spontaneous when 0 = 0. Since surfactants are added in sufficient amounts ( dynamic lowered sufficiently), spontaneous dispersion is the rule rather than the exception. 

For smaller particles, smaller stresses are exerted. Thus, in order to predict sedimentation it is necessary to measure the viscosity at very low stresses (or shear rates). These measurements can be carried out using a constant stress rheometer (Carrimed, Bohlin, Rheometrics, Haake or Physica). Usually, a good correlation is obtained between the rate of creaming or sedimentation, v, and the residual viscosity rj 0), as will be described in Chapter 21. Above a certain value of ri(0), v becomes equal to 0. Clearly, in order to minimize sedimentation it is necessary to increase rj 0) an acceptable level for the high shear viscosity must be achieved, depending on the application. In some cases, a high rj[0) may be accompanied by a high rj (which may not be acceptable for apphcation, for example if spontaneous dispersion on dilution is required). If this is the case, the formulation chemist should seek an alternative thickener. 

According to Rehbinder and Shchukin the above condition has a simple physical meaning if the particles are of colloidal dimensions, and the interfacial tension is low, the spontaneous separation of particles from a macroscopic phase becomes possible, since the work required to form a new surface is compensated by the gain in energy upon the involvement of the newly formed particles in thermal motion. There is a critical value of the specific surface free energy, ocr, below which the spontaneous dispersion process becomes possible. This critical value is given by... 

Thus, thermodynamically equilibrium lyophilic colloidal systems consisting of particles significantly larger than those of molecular dimensions may form by the spontaneous dispersion of macroscopic phase at sufficiently low, but finite positive values of a, i.e., when a acr (RS = 5-10). For example, lyophilic colloidal system containing particles with diameter, d 10 8 m, may form when a does not exceed several hundredths of mJ/m2. Since the critical value, acr, is a function of particle size, d, (see eq. (VI. 1)) the formation of colloidal system consisting of particles with larger size is possible only at lower values of a, and vice a versa. 

One may also consider a reverse transition from macroscopic heterogeneous system (surfactant crystals in water) to micellar solutions existing above the Krafft point, via gel formation and its spontaneous dispersion stages. In this case, the swelling of soap upon the penetration of water between the lamella formed with polar ( strongly hydrated) groups occurs prior to the formation of colloidal solution. At sufficient dilutions separation of individual particles, e.g. lamella, from crystals occurs due to the... 

Consequently, spontaneous degradation of aggregate into a sol (spontaneous dispersion) becomes thermodynamically favorable, when... 

In some cases under the conditions similar to those corresponding to the formation of lyophilic colloidal systems, a spontaneous formation of emulsions, the so-called self-emulsification, may take place. This is possible e.g. when two substances, each of which is soluble in one of the contacting phases, react at the interface to form a highly surface active compound. The adsorption of the formed substance under such highly non-equilibrium conditions may lead to a sharp decrease in the surface tension and spontaneous dispersion (see, Chapter III, 3), as was shown by A.A. Zhukhovitsky [42,43], After the surface active substance has formed, its adsorption decreases as the system reaches equilibrium conditions. The surface tension may then again rise above the critical value, acr. Similar process of emulsification, which is an effective method for preparation of stable emulsions, may take place if a surfactant soluble in both dispersion medium and dispersed liquid is present. If solution of such a surfactant in the dispersion medium is intensively mixed with pure dispersion medium, the transfer of surfactant across the low surface tension interface occurs (Fig. VIII-10). This causes turbulization of interface... 

Spontaneous dispersion becomes possible in the absence of external mechanical action (see Chapter IV) only under the conditions of a very strong lowering of the surface energy down to values close to critical, given by the condition a [Pg.703]

In our concluding remarks we can emphasize that depending on the nature of interactions between the components that constitute the medium and the solid, as well as on a combination of external conditions, one may observe the effects of various types and intensity. These include the facilitation of plastic flow of solids, or, alternatively, brittle fracture due to the action of lowered stresses mechanochemical phenomena in the zone of contact mechanically activated corrosion (the stress corrosion) the processes that are close to the spontaneous dispersion (the so-called quasi-spontaneous dispersion), and the true spontaneous dispersion, leading to the formation of thermodynamically stable lyophilic system. A great variety of types of interactions that exist between the stressed solids and the medium in contact with it requires careful and thorough examination of conditions under which... 

Contrary to this general notion that dispersions of finely divided solid particles cannot be thermodynamically stabilized, Stol and de Bruyn [34] discussed the conditions necessary for obtaining a thermodynamically stable dispersion (TSD) of solid particles in an aqueous solution medium. They stressed the role of the adsortion of potential-determining ions in lowering the interfacial free energy y to promote spontaneous dispersion and subsequently reasoned that for simple inorganic solids a decrease in y by about 200 mN/m relative to its value at the PZC may be sufficient to yield a TSD. However, the existence of thermodynamically stabilized dispersions of inorganic solids has not yet been demonstrated experimentally. 

Microemulsions are composed of two mutually immiscible liquid phases, one spontaneously dispersed in the other with the assistance of one or more surfactants and cosurfactants. While microemulsions of two nonaqueous liquids are theoretically possible (e.g., fluorocarbon-hydrocarbon systems), almost all of the reported work is concerned with at least one aqueous phase. The systems may be water continuous (o/w) or oil continuous (w/o), the result being determined by the variables such as the surfactant systems employed, temperature, electrolyte levels, the chemical nature of the oil phase, and the relative ratios of the components. 

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