Sedimentation, aqueous suspensions

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

Transport in solution or aqueous suspension is the major mechanism for metal movement from the land to the oceans and ultimately to burial in ocean sediments. In solution, the hydrated metal ion and inorganic and organic complexes can all account for major portions of the total metal load. Relatively pure metal ores exist in many places, and metals from these ores may enter an aquatic system as a result of weathering. For most metals a more common sequence is for a small amount of the ore to dissolve, for the metal ions to adsorb onto other particulate matter suspended in flowing water, and for the metal to be carried as part of the particulate load of a stream in this fashion. The very insoluble oxides of Fe, Si, and A1 (including clays), and particulate organic matter, are the most important solid adsorbents on which metals are "carried."... 

Sharom MS, Miles JR, Harris CR, et al. 1980a. Behavior of 12 insecticides in soil and aqueous suspensions of soil and sediment. Water Res 14 1095-1100. 

Negative atmospheric pressure chemical ionization (APC) low-energy collision activation mss spectrometry has also been employed for the characterization of flavonoids in extracts of fresh herbs. Besides the separation, quantitative determination and identification of flavonoids, the objective of the study was the comparison of the efficacy of the various detection systems in the analysis of flavonoids in herb extracts. Freeze-dried herbs (0.5g of chives, cress, dill, lovage, mint, oregano, parsley, rosemary, tarragon and thyme) were ground and extracted with 20 ml of 62.5 per cent aqueous methanol. After sedimentation the suspension was filtered and used for HPLC analyses. Separations were carried out in an... 

The transport of disulfoton from water to air can occur due to volatilization. Compounds with a Henry s law constant (H) of <10 atm-m /mol volatilize slowly from water (Thomas 1990). Therefore, disulfoton, with an H value of 2.17x10" atm-m /mol (Domine et al. 1992), will volatilize slowly from water. The rate of volatilization increases as the water temperature and ambient air flow rate increases and decreases as the rate of adsorption on sediment and suspended solids increases (Dragan and Carpov 1987). The estimated gas- exchange half-life for disulfoton volatilization from the Rhine River at an average depth of 5 meters at 11 °C was 900 days (Wanner et al. ] 989). The estimated volatilization half-life of an aqueous suspension of microcapsules containing disulfoton at 20 °C with still air was >90 days (Dragan and Carpov 1987). 

Paul FA, Qark FE (1989) Soil microbiology and biochemistry. Academic Press, New York Pecher K, HaderUne SB, Schwarzenbach RP (2002) Reduction of polyhalogenated methanes by surface— b Fe(ll) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741 Penrose WR, Metta DN, Hyfko JM, Rinkel LA (1987) The reduction of plutonium (V) by aquatic sediments. J Environ Radioact 5 169-184... 

Low concentrations, i.e. 2-5%, of Kollidon 90 F suffice to stabilize aqueous suspensions. Fig. 4 demonstrates that it can completely prevent sedimentation. The example taken was a crospovidone suspension. 

Most of the Pu in seawater is not in solution but is adsorbed on suspended sediment. When waves break at the sea surface, the spray is enriched in sediment, and in Pu, relative to the bulk seawater, by a mechanism which is the basis of the technique of froth flotation used by chemical engineers to extract small particles from aqueous suspensions. 137Cs, being in solution in the seawater, is not enriched relative to Na in the spray. 

Monodispersed colloidal gold particles allowed to sit in perfectly quiescent aqueous suspension at 25°C form a sedimentation equilibrium in which their number density decreases by a factor of 2 in 10 cm of elevation. What is the average number of gold atoms in each colloidal particle ... 

We use here the term sorption as the retention of a compound on the surface of a solid particle that removes it from the aqueous medium. This phenomenon affects the composition of water by transferring the compound or ion from the aqueous medium to a solid (mainly a sediment in suspension or a colloid). Then, it may no longer be present in water, especially if the sediment settles. Sorption may be identified and associated with adsorption, surface precipitation, surface complexation, and/or ion exchange (or even absorption). 

In the literature, the reported use of zeta potential measurement for non-aqueous suspensions is relatively infrequent because non-aqueous suspensions only represent a small percentage of all medicated suspensions. Su and others evaluated the flocculation-deflocculation behavior of cefazolin sodium in non-aqueous media and the effect of surfactants as measured by zeta potential along with sedimentation and porosity measurements. A significant difference in zeta potential was observed when the particles were dispersed in peanut oil and ethyl oleate. The addition of lecithin reduced the zeta potential of cefazolin sodium, resulting in a deflocculated state accompanied by a decrease in sedimentation volume. The effect of surfactant... 

In soft gelatin capsules, powdered cellulose may be used to teduce the sedimentation rate of oily suspension fills. It is also used as the powder base material of powder dosage forms, and as a suspending agent in aqueous suspensions for peroral delivery. It may also be used to reduce sedimentation during the manufacture of suppositories. 

The relatively poor solubility of natural and synthetic eumelanins (Table IV) is a considerable obstacle in structural determination. The rate of particle sedimentation in aqueous suspensions of synthetic melanins is... 

Thus there is a lai e discrepancy between the theoretical predictions of the collision efficiency for aerosol coagulation by differential sedimentation (taking into account inter-particie fluid motion) and experimental measurements for coagulation by turbulent shear in aqueous suspensions. We do not know whether this discrepancy is due to the ba.sic difference in the coagulation mechanisms (differential sedimentation vs. turbulent shear), different phenomena operating in the different fluid media, or some other as yet unidentified effect. 

In this chapter, both colloidal silica and coarser silica materials such as chromatographic supports are examined. The particle sizes range from 0.01 to 20 pm. An arsenal of FFF subtechniques was used, including sedimentation FFF, flow FFF, and thermal FFF. Although most of these studies involve aqueous suspensions, thermal FFF is shown to be capable of fractionating and characterizing nonaqueous suspensions of silica as well. 

Brown examined the aqueous suspension of pollen particles in the microscope, and found that the particles were in continuous motion. To rule out the possibility that the life cycle of pollen cells was responsible for the observed phenomenon, he conducted similar observation with finely divided matter of both organic and inorganic nature. These experiments confirmed the existence of chaotic motion, which could be observed with highly crumbled matter of any nature. Further in this chapter we will return to the discussion of the theory of Brownian motion, but first let us address the macroscopic laws that govern particle motion in the gravity field (sedimentation) and during diffusion. 

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