Suspended particles stability

Many properties of colloidal suspensions, such as their stability, rheology, and phase behaviour, are closely related to the interactions between the suspended particles. The background of the most important contributing factors to these interactions is discussed in this section. 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

Wash particles (e.g., 100 mg of 1 pm carboxylated latex beads) into coupling buffer (i.e., 50 mM MES, pH 6.0 or 50 mM sodium phosphate, pH 7.2 buffers with pH values from pH 4.5 -7.5 may be used with success however, as the pH increases the reaction rate will decrease). Suspend the particles in 5 ml coupling buffer. The addition of a dilute detergent solution may be done to increase particle stability (e.g., final concentration of 0.01 percent sodium dodecyl sulfate (SDS)). Avoid the addition of any components containing carboxylates or amines (such as acetate, glycine, Tris, imidazole, etc.). Also, avoid the presence of thiols (e.g., dithiothreitol (DTT), 2-mercaptoethanol, etc.), as these will react with EDC and effectively inactivate it. 

A second consideration concerned with propellant storage and which applies specifically to slurried propellants is its mechanical stability. The suspended particles must remain in stable suspension in the gelled liquid for long times, and the mixture must not exude liquid (syneresis). To... 

Suspensions offer another dispersed platform for delivery of poorly water-soluble agents. As compared with solutions, suspensions afford higher loading. However, as with emulsions, the suspended particles must be stabilized with surfactants to prevent aggregation. A number of steroids have been available for years as suspensions for IM and intra-articular delivery. Examples include DEPO-MEDROL (Pfizer, Kalamazoo, Michigan, U.S.A.) Sterile Aqueous Suspension. The marketed product contains methylprednisolone acetate, a white, water-insoluble, crystalline... 

On account of the tendency of the suspended particles to eoalesec, colloidal solutions of sulphur are generally short-lived. The stability of the solutions can be increased by the addition of a protective colloid such as albumen or gelatine. The action of hydrochloric acid on sodium thiosulphate solution yields a colloidal solution which is more stable if the reagents are used in a concentrated condition,0 but the life of the unstable colloidal solution obtained with dilute reagents can be extended by the addition of gelatine. Also, by preparing tire sulphur in a wet way in the presence ol albumen, the... 

The expressed juice is subjected to screening before being further processed. A rotary brash sieve, followed by a centrifugal separator, may be used to bring the suspended particles into the stability range of below 1 pm diameter, at which point the juice can be pasteurised and aseptically packed for direct consumption as single-strength juice or concentrated as required. 

Experiments to distinguish between these two possibilities have often involved measurements of ultrasonic attenuation (ref. 5,9,31,32). The popularity of this approach derives in part from the fact that small impurities in liquids, such as suspended particles, have negligible influence on attenuation in comparison with even a very small concentration of microbubbles (ref. 9). (Microbubbles, in contrast to solid particles, appreciably increase the compressibility of a liquid, introducing forms of viscous losses and nonreversible energy exchanges that do not exist in the case of solid particles.) It is therefore of considerable interest that all fresh tap water samples measured by Turner (ref. 9) showed substantial and persistent abnormal (ultrasonic) attenuation, amounting to a minimum of 44% over that of distilled water it was concluded that this result stemmed from the presence of stabilized micron-sized bubbles. 

Controlling the stability of suspended particles, and the filtration characteristics of cast suspensions, are very important in the ceramics industry. Here the casting suspensions may comprise clays, silica, oxides, carbides or nitrides [357,793], In slip casting, a suspension is pored into a porous mould and allowed to stand. Water dif-... 

Since most drags are insoluble in the propellants, they are usually presented as suspensions. Micronized drag is dispersed with the aid of a surfactant such as oleic acid, sorbitan trioleate or lecithin. At concentrations up to 2% w/w the surfactant stabilizes the suspended particles by adsorption at the drag propellant interface and in addition serves as a valve lubricant. The tendency is to use minimum surfac—tant concentrations to reduce drag solubility within the propellant by solubilization (to reduce Ostwald ripening during the shelf life of the pMDI). Low surfactant concentrations will also avoid substantial reductions in the propellant evaporation rates from aerosolized drops. 

An adverse effect of adsorption of H2PO2, which is the reducing agent in the bath, is a decrease in plating speed in the presence of suspended particles. Additionally, adsorption of stabilizers results in a reduced stability of electroless bath containing suspended particles. Typically, the life-time of a Ni(P) bath is reduced from 10-15 metal turn-overs in a particle-free bath to around 5 in a particle-containing bath. Hence, through adsorption bath constituents do not only affect particle incorporation, but suspended particles also influence the metal deposition. 

Bven though dispersion stability is a rather crude method, it has a lot of relevance to the actual use of the material in the field by the farmer. Recent refinements are documented in the CIPAC method MT 180 and allow distinction between the stability of the suspended particles and the emulsified oil phase. Unfortunately, only the effects and not the cause of instability can be deduced from these measurements. 

The enzymes used for juice clarification are almost exclusively pec-tolytic. The reasons for the clarification effect are as follows. Pectin stabilizes the suspended solids in the juice. Through addition of the enzyme, the pectin is converted to a dissolved form and broken down. This is associated with a significant decrease in viscosity. In addition, pectins that have been made soluble act as protective colloids for many of the suspended particles. As soon as the protective effect of pectin is lost, these suspended particles coagulate and are precipitated. A significant decrease in viscosity is accompanied by a significant increase in filtration performance. 

The cloudy juice contains colloids which increase the viscosity of the juice (and prevent sedimentation) while at the same time binding suspended particles by virtue of electric charges. These colloids which are responsible for the haze stability of a juice are mainly pectins and starch. It is therefore a key priority in processing to retain these colloidal substances. The unripe fruit contains a lot of starch but little soluble pectin, whereas the fully ripe fruit contains little starch and a lot of soluble pectin. This means that it is possible to influence the haze stability of the juice by selection of the raw material. 

Adsorption of a non-ionic polymer (gum or cellulosic) or surfactant (polysorbate 80) of sufficient chain length creates steric hindrance and prevents adjacent suspended particles from coming close enough to join each other. Steric stabilization has the advantage over electrostatic stabilization in that it is relatively insensitive to the presence of electrolyte in the aqueous vehicle. 

The pH value of aqueous suspensions should be taken at a given temperature and only after settling equilibrium has been reached, to minimize pH drift and electrode surface coating with suspended particles. Electrolyte should not be added to the external phase of the suspension to stabilize the pH, because neutral electrolytes disturb the physical stability of the suspension. 

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