Oversaturated solution

This closeness of 0 to zero explains the existence of a gas-oversaturated solution area in the polymer melt, when P < Pg, but the entire volume of gas remains in the solution. The degree of oversaturation, particularly upon free foaming (not in flow) can be 2- to 3-fold. In real polymer compositions, there are always solid admixtures, which have poor wetting areas. This reduces the degree of oversaturation at the interface melt-molding tool. Moreover, bubble nuclei can result from fragmentation of gas bubbles in the polymer [16]. Another factor that promotes the formation of bubble nuclei is the presence of localized hot points in the polymer melt they act as nuc-leation centres. Hot points appear either after a chemical reaction in the melt polymer [17], or in overheated areas on the surface of metal equipment [18]. Density of nucleation can be improved via introduction of various agents that reduce tension of the polymer [19]. 

B n = 3) were isolated in the solid state from aqueous solutions under different conditions A by evaporation of the solution to dryness B by crystallization from an oversaturated solution. 

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

The obtained saturated solution is filtered in pressure filter 18, which heated with vapour to avoid crystallisation. The oily liquid remaining on filter cloth is collected and added to the product in collector 15 the hot filtered crystal solution enters tank 19 for partial distillation of light petroleum and preparation of oversaturated solution. The hot solution is sent from the tank into crystalliser 22. 

The proposed model serves to resolve another old puzzle, related to the rate of crystal growth from slightly oversaturated solutions [157]. It is argued that growth on a perfect crystal face depends on spontaneous nucleation, exactly like the appearance of first embryos, and hence that crystal... 

There was no trace then of the horror which I had myself felt.. . but his face showed rather the quiet and interested composure of the chemist who sees the crystals falling Into position from his oversaturated solution. 

While this book treats several topics similar to those found in an analytical chemistry text, it endeavors to consider the spatial and temporal scales of the reactions in nature as distinctly different from those of the laboratoiy. For example, in chemical analysis, precipitates (frequently of metastable and active compounds) are formed from strongly oversaturated solutions, whereas in natural water systems, the solid phase is often formed under conditions of slight supersaturation often crystal growth and aging may continue over geological time spans. Interfacial phenomena are particularly important because chemical processes of significance often occur only at phase discontinuities. 

Figure 7.2. Solubility and saturation. A schematic solubility diagram showing concentration ranges versus pH for supersaturated, metastable, saturated, and undersaturated solutions. A supersaturated solution in the labile concentration range forms a precipitate spontaneously a metastable solution may form no precipitate over a relatively long period. Often an active form of the precipitate, usually a very fine crystalline solid phase with a disordered lattice, is formed from oversaturated solutions. Such an active precipitate may persist in metastable equilibrium with the solution it is more soluble than the stable solid phase and may slowly convert into the stable phase.

Various processes are involved in the formation of a solid phase from an oversaturated solution (Figure 13.22). Usually three steps can be distinguished ... 

Preparation of the Saturated MCT Oil with C-Labeled PPP. Oversaturated solutions of C-PPP in MCT oil (20 mL) were prepared by adding an excess amount of " C-PPP (supplied in organic solvents) to the oil. The organic solvents were then vaporized over several hours, and the dispersion of C-PPP crystals in MCT oil was centrifuged for about 30 min. The upper phase was then removed and used as the liquid phase in the experiments. Saturated solutions of C-PPP in MCT oil were prepared at temperatures from 7 to 25°C. 

Figure 5.8 Schematic representation of the dissolution-dissociation-recrystallisation process of a cyclodextrin complex with a poorly soluble guest. The complex rapidly dissolves, and a metastable oversaturated solution is obtained. The anomalously high level of dissolved guest drops bock but remains higher than the level that can be obtained with noncomplexed drug. Solid curve = complexed drug broken curve = noncomplexed drug.

Honey is produced by honeybees from the flower nectar of plants. Fresh honey is a clear, very aromatic, dark-amber-colored liquid. It is very sticky and hygroscopic, with a density of about 1.40 g/cm3. Honey is an oversaturated solution of glucose and fructose, easy crystallizing. After crystallization its color is brighter. It is a very stable product. At a temperature of 8-10°C and a humidity of 65-75% it may be stored for many years. Honey is a high-caloric food easy assimilated by the human organism. 

Recent studies have shown the formation of altered surface layers thicker than one or two unit cell layers on feldspar surfaces, in apparent support of the leached layer theory. Thick (> 100 nm), silica-rich surface layers were detected by XPS on feldspar samples weathered in solutions having pH < 3 (Casey et at, 1988b) or >9 (Heilman et al., 1990). At these extreme pH values, the rate of release of Al and charge-balancing cations to solution is much faster than the rate of hydrolysis of silica. Under these conditions, oversaturation with respect to amorphous silica could occur, and a highly hydrated, residual leached or precipitated layer of silica could form. However, this layer is probably too porous and discontinuous to be a diffusion-limiting mechanism, but would still account for incongruence under these conditions (Hellmann et al., 1990). Whether the altered layer formed by a leached layer process or by simple precipitation from oversaturated solutions was not determined. 

NEC/ALT] in oversaturated solution. In both cases (solubility studies from underand oversaturation) solid-liquid equilibrium is followed by measuring the thorium concentration and pH as a function of time. In equilibrium with Th oxide or hydroxide the total thorium concentration is given by ... 

Urine, when voided, is often an oversaturated solution, e.g. urates and phosphates. In addition, cells and cell conglomerates may act as centres for crystallisation. Urine therefore tends to precipitate on storage. Cooling or warming may cause additional precipitate formation. Bacterial infection in the urinary tract may increase the tendency to precipitation. Trace elements in the urine may coprecipitate with other elements, or adsorb onto the surface of the precipitates. 

The concentration of dissolved silica in salt water solutions in contact with solid amorphous silica may decrease for a time period of several weeks to several months after the initial dissolution, and then after this initial aging process is completed the concentration remains stable for months or years (2. Several studies have reported this equilibrium solubility. Jorgensen (16) found that three to five months were required to achieve equilibrium in his experiments, and after that time the same solubility was determined from imdersaturated or oversaturated solutions in contact with silica for time periods up to two years. Jones and Pytkowicz ( ) found the same solubility for aged silica after 66 or 123 days of equilibration time. 

Before the nucleation, dissolution and minerogenesis processes nm actually in a homogeneous medium, without the formation of a solid substance. At this stage, a solution maybe oversaturated, and for stable existence of a mineral it is necessary to overcome the resistance of surface tension. Here, mineral micro-particles, nuclei, as if form and straight away dissolve, not having reached significative size. Such a relatively stable state of oversaturated solution without minerogenesis is called a metastable state. 

Steels with a chromium content of 13-17% but with a carbon content of 0.2-0.4% can be hardened by a heat treatment forming the martensitic structure, an oversaturated solution of carbon in ferritic crystals. The hardening is also improving the corrosion resistance. 

Of these, the 3.1.8 and 5.1.8 phases may exist at ambient temperatrrre, whereas the 2.1.4 and 9.1.5 phases are stable only at temperatures above 100°C (Cole and Demediuk, 1955 Demeditrk et al 1955). The 5.1.8 phase tends to precipitate from an oversaturated solution faster than the 3.1.8 phase. Two additional hydrate phases exist in the system Mg0-MgCl2-H20 at ambient temperature Mg(OH)2 and MgCl2.6H20. 

Recently, a modification ofWakai s model has been created [49], based on the fact that the detailed examination of a nucleation process requires the addition of two new terms to the Wakai s free energy [81]. One term is related to the free energy increases due to the molecular volume change when the solute (which initially was dissolved in the liquid phase) precipitates into the crystal at a step. The second term arises from the fact that an oversaturated solution of crystalline material in the glassy phase, giving rise to precipitation phenomena, is not in equilibrium with the crystal, at least locally in the surroundings of the step [85, 86] (also see Ref. [49] for further details). 

After diluting the infusion or injection concentrate with an aqueous solution or with blood the active substance may precipitate. This is due to the concomitant dilution of the co-solvent(s) and oversaturated solutions. Whether or not precipitation occurs in mixtures of water and co-solvents can be calculated in analogy to the calculation of the chance of precipitation in water, see Sect. 18.1.1. 

Acrylnitrile is catalytically formylated with CO/H2 and the resultant aldehyde is transformed through a Strecker reaction into glutamic acid dinitrile which yields D,L-glutamic acid after alkaline hydrolysis. Separation of the racemate is achieved by preferential crystallization of the L-form from an oversaturated solution after seeding with L-glutamic acid ... 

Figure 7.39 Saccharose-water state diagram t = temperature (°C), c = concentration, T , = equiiibrium formation-meiting of ice (equilibrium melting temperature of ice), 7, = equilibrium saturated solutlon-oversaturated solution, = glass transition temperature, = glass transition temperature of maximum concentrated liquid phase with saccharose content...

Nanobubbles have indeed been observed [1076,1221-1223]. Thqrdo, however, not form spontaneously. Only when a hydrophobic surface is exposed to an oversaturated solution containing dissolved gas the bubbles form. Once they are formed, they are surprisingly stable and can stay for hours or days even if the concentration of dissolved gas is reduced again (Figure 10.7) [1222,1224—1226]. It is one of the open questions in surface science why these nanobubbles are stable. 

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