Concentrated electrolytes, sols

Fig. 49. Minimum concentrations of individual electrolytes causing precipitation of iron hydroxide (coagulation threshold, mg/1) as a function of concentration of sol.

To study the effect of electrolytes, sols of several concentrations were also used (150, 300, 750, and 1500 mg/1), and the experiments were made at various controlled values of pH (2, 4, 6, 8, and 10). A special series of experiments in a neutral medium (pH = 6) was conducted separately to compare the coagulating effect of the individual ions of sea water. From the preliminary results of these experiments, the following conclusions can be drawn ... 

Frolov, Shabanova, and co-workers (37-39) studied the transition of a sol into a gel and the aggregate stability of colloidal silica. Their aim was to develop a technology for the production of highly-concentrated silica sols and to use them as binders, catalyst supports, polymer fillers, adsorbents, and so forth. Kinetic studies were made of polycondensation and gel formation in aqueous solutions of silicic acids. At the stage of particle growth, poly condensation proceeds in the diffusion-kinetic region. With changes in pH, temperature, concentration, and the nature of electrolytes,... 

An approximate measure of the influence of an electrolyte on the flocculation of a lyophobic sol is the critical coagulation concentration (c.c.c.), which is the minimum concentration of that electrolyte leading to rapid coagulation under specified conditions of concentration of sol, rate of addition of electrolyte, etc. 

Silica Concentration. The fluoride method [5] was used to determine silica in the electrolyte-sol. 

Electrolyte concentration 0.06 N sodium sulfate Electrolyte-sol volume 101 Cell outflow pH 9... 

Nesbitt, H.W., 1982, The Stokes and Robinson hydration theory A modification with application to concentrated electrolyte solutions. J. Sol. Chem., 11 415-22. 

Some very remarkable sols have been described by Voet He prepared stable sols of different metals (Pt, Pd) and salts (sulphides halides) in very concentrated electrolytes like sulphuric acid, phosphoric acid, saturated calcium chloride in water etc Dilution with water made these sols coagulate in a short time. There is no theory as to the cause of their stability. One might think of an increasing dissociation caused by the addition of water ... 

Flachisu S and Kobayashi Y 1974 Kirkwood-Alder transition in monodisperse latexes. II. Aqueous latexes of high electrolyte concentration J. Colloid Interface Sol. 46 470-6... 

In the absence of a suitable soHd phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous siUca to quartz at high temperatures. Particle size influences the stabiUty of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkaU (1,33). 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

In steric stabilization the colloids are covered with a polymer sheath stabilizing the sol against coagulation by electrolytes. In sensitization or adsorption flocculation, the addition of very small concentrations of polymers or polyelectrolytes leads to destabilization (Lyklema, 1985). 

SOL. 1.1. Prigogine, La structure de solutions d electrolytes forts en solution concentree (Structure of solutions of strong electrolytes in concentrated solution). Bull. Soc. Chim. de Belg. 50, 89—98 (1941). 

In more complex solutions of high ionic strengths with more than one electrolyte at significant concentrations, e.g., (Na, Mg, Ca " ) (Cl, SOl ), Pitzer s equation may be used to estimate the osmotic coefficient the necessary interaction coefficients are known for most systems of geochemical interest. 

Concentration of an added electrolyte above which a particulate sol undergoes coagulation instead of gelation. 

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