Adsorbent powders

Gas chromatography makes use, as the stationary phase, a glass or metal column fdled either with a powdered adsorbent or a non-volatile liquid coated on a non-adsorbent powder. The mobile-phase consists of an inert-gas loaded with the vapourised mixture of solutes flowing through the stationary phase at a suitable temperature. In the course of the passage of the vapour of the sample through the column, separation of the components of the sample occurs in two ways, namely ... 

Adsorbent powders are nonabsorbable materials with a large surface area. These bind diverse substances, including toxins, permitting them to be inactivated and eliminated. Medicinal charcoal possesses a particularly large surface because of the preserved cell structures. The recommended effective antidiarrheal dose is in the range of 4-8 g. Other adsorbents are kaolin (hydrated aluminum silicate) and chalk. 

KaoUn powder and other hydrated aluminum silicate clays, often combined with pectin (a complex carbohydrate), are the most widely used adsorbent powders (e.g., Kaopectate). Kaolin is a naturally occurring hydrated aluminum silicate that is prepared for medicinal use as a very finely divided powder. The rationale behind its use in acute nonspecific diarrhea stems from its ability to adsorb some of the bacterial toxins that often cause the condition. It is almost harmless and is effective in many cases of diarrhea if taken in large enough doses (2-10 g initially, followed by the same amount after every bowel movement). The adsorbents are generally safe, but they may interfere with the absorption of some drugs from the GI tract. 

Derivation of the Gibbs adsorption isotherm. Determination of the adsorption of surfactants at liquid interfaces. Laboratory project to determine the surface area of the common adsorbent, powdered activated charcoal. 

The sulfide 1 (0.75 mmol) is dissolved in dichloromethane (2-3 mL) and adsorbed over silica supported sodium periodate (20%, 1.36 g, 1.28 mmol) that is wetted with 0.3 mL of water by thoroughly mixing on a vortex mixture. The adsorbed powdered material is transferred to a glass test tube and is inserted in an alumina bath (alumina 100 g, mesh 65-325, Fisher scientific bath 5.7 cm diameter) inside the microwave oven. The compound is irradiated for the time specified in the table and the completion of the reaction is monitored by TLC examination. After completion of the reaction, the product is extracted into ethyl acetate (2x15 mL). The removal of solvent at reduced pressure affords crude sulfoxide 2 that contains less than 5% sulfone. The final purification is achieved by column chromatography over silica gel column or a simple crystallization. 

Adsorption column preparation and loading. In order to obtain satisfactory results, the tube must be uniformly packed with the adsorbent uneven distribution may lead to the formation of cracks and channels and to considerable distortion of adsorption band shapes. If there is any doubt concerning the uniformity of particle size of the adsorbent powder it should be sifted before use to remove the larger particles fines are removed from the adsorbent using a sedimentation procedure immediately prior to column packing. In this the alumina or silica gel adsorbent is stirred into between five to ten times its volume of the selected solvent or solvent system, allowed to settle for five minutes and the supernatant liquor decanted off the procedure is repeated until the supernatant liquid is clear. 

In the Liver Dialysis Unit (HemoCleanse Technologies, Lafayette, USA), the adsorbents (powdered activated charcoal and cation-exchangers) are located in the dialysate-moving phase [31]. Dialysate content is adjusted so as to prevent... 

Kaolin and chalk are adsorbent powders. Their therapeutic efficacy is marginal as is shown by the fact that they are often combined with an opioid. Bulk-forming agents such as ispaghula, methylceUulose and sterculia (see above) are useful for diarrhoea in diverticular disease, and for reducing the fluidity of faeces in patients with ileostomy and colostomy. 

Physical chemists distinguish between adsorption and absorption. Adsorption is a surface phenomenon. Consider a solid or liquid phase (the adsorbent), in contact with another, fluid, phase. Molecules present in the fluid phase may now adsorb onto the interface between the phases, i.e., form a (usually monomolecular) layer of adsorbate. This is discussed in more detail in Section 10.2. The amount adsorbed is governed by the activity of the adsorbate. For any combination of adsorbate, adsorbent, and temperature, an adsorption isotherm can be determined, i.e., a curve that gives the equilibrium relation between the amount adsorbed per unit surface area, and the activity of the adsorbate. Powdered solid materials in contact... 

B. Decontamination of the Skin. Each soldier is given the means for preliminary decontamination of the skin, the means being based on physical adsorption or on the combination of physical adsorption and chemical inactivation. Physical adsorption can be achieved by adsorbing powders. Chemical inactivation is often effected by chlorinating compounds incorporated into adsorbing powders, ointments, solutions, or organic solvents. Mustards should not be decontaminated with water, except for the eyes, as this may spread the agent. 

F. Chemical inactivation using alkalis is effective, whereas chlorinating is ineffective against phosgene oxide. The eyes should be flushed immediately using water or isotonic sodium bicarbonate solution if available. Physical decontamination of the skin using adsorbent powders, e.g., fullers earth, is advised. 

Activated carbon (1921) n. (1) Any form of carbon characterized by high adsorptive capacity of gases, vapors, and colloidal solids. (2) A highly adsorbent powdered or granular carbon made usually by carbonization and chemical activation and used chiefly for purifying by adsorption. Whitten KW, Davis RE, Davis E, Peck... 

Fig. 3 Artificial sebum (Stefaniak and Harvey 2006) uptake capaeity in mesoporous silicon (Lee et al., 2009, Preliminary study of mesoporous silicon capacity to adsorb skin sebum, Intrinsiq Materials Ltd TM0902, unpublished report ). From left to right dry mesoporous silicon (80 % porosity), sebum volume that can be adsorbed prior to wet point, sebum adsorbed powder. Note how the volume of powder has not redueed appreeiably that would signify skeleton eollapse...

Allen, T., and Burevski, D., Models of adsorption of sulphur dioxide on powdered adsorbents. Powder Technol., 21,91-96(1978). 

The preparation conditions determine the special properties of silica gel. Physicochemical properties such as hardness and polarity of silica gel are related to particle size (fim), size distribution, shape, surface area (mVg), pore system [size (A) and distribution], and presence or absence of additives—that is, binders, contaminants, or indicators. The surfaee area of silica gel adsorbent for TLC is typically 300 ta600 m /g, pore volume is —0.75 ml/g, and pore diameters range from 40 to 80 A (most often 60 A or 6 nm). Adsorbent with larger surface area (smaller particle size) will generally give better resolution but a slower development time. The usual binder for commercial TLC adsorbent powders is 5-20% gypsum (silica gel G). Precoated plates usually have organic binders such as polyesters or polyvinyl alcohol. 

The adsorbent (powder) is then brought into contact with the adsorbate and, when constant pressure, volume and temperature conditions show that the system has attained equilibrium, the amount of gas is again calculated. The difference between the amount of gas present initially and finally represents the adsorbate lost from the gas phase to the adsorbate phase. The accurate determination of the amount of gas nnadsorbed at equilibrium depends upon the accurate determination of the dead space or the space surrounding the adsorbent particles. The dead space is determirud by expansion measurements using helium, whose adsorption can be assumed to be negligible. 

The results of these pressing studies are seen in Figure 10, where the theoretical density is plotted against pressure for unexposed-adsorbate, and exposed-adsorbate powder. The results indicate that the exposed system has slightly higher densities than the unexposed system. Based on the rheological and visual data, this behavior was unexpected. Ammonia release... 

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