Salt sieving

Negative adsorption is a relatively import2mt phenomenon in concentrated disperse systems and in capillaries. It is responsible for the Donnan effect, for the exclusion of electrolytes from concentrated sols, dispersions and capillaries and the ensuing salt-sieving effect, already introduced in chapter I.l. It also plays a role in double layer relaxation as occurs in alternating fields or in particle-particle Interaction. As negative adsorption is a purely electrostatic feature and takes place far from the surface, in all these applications its computation from Polsson-Boltzmann statistics is reliable, especially at high ly l. 

Although for the present purpose the negative adsorption had to be suppressed, for others it is important Donnan exclusion, pH establishment in soils, and salt-sieving. Traditionally, these topics have greatly benefited from studies with clay colloids. 

In another vein, double layers play a role In the salt-sieving phenomenon, mentioned In the Introduction to Volume I, and already known to Aristotle. When seawater percolates over a compact sediment of slllcate-like particles, under some conditions the effluent Is potable. Basically the phenomenon is attributable to the negative adsorption of (in this case) anions, leading to the Donnan expulsion of electrolyte, see sec. 3.5b. Over-demand may lead to salt penetration the screening of the double layers around the silica particles (reduction of x ) makes the pores between them effectively wider. For this problem technical solutions had to be found. 

Besides these phenomena, plug experiments can also be carried out to measure salt-sieving, the retention of electrolytes caused by the negative adsorption of co-lons from the (overlapping) double layers, edso known as the Donnan exclusion. The phenomenon was already met on p. 1 of Volume I and further analysed in sec. 3.5b. 

Steeping. Sheet, roU, or suitably milled flock pulp is metered into a pulper along with vigorously stirred 18% sodium hydroxide solution at 50°C. The resulting slurry, containing about 5% finely dispersed pulp, passes to a buffer tank from which it is metered to a slurry press that sieves out the swollen fiber and returns the pressings soda for concentration correction and reuse. The cellulose reacts with the soda as a complex alcohol to form the sodium salt or alk-ceU. 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

Ammonium Ion Removal. A fixed-bed molecular-sieve ion-exchange process has been commercialized for the removal of ammonium ions from secondary wastewater treatment effluents. This application takes advantage of the superior selectivity of molecular-sieve ion exchangers for ammonium ions. The first plants employed clinoptilolite as a potentially low cost material because of its availability in natural deposits. The bed is regenerated with a lime-salt solution that can be reused after the ammonia is removed by pH adjustment and air stripping. The ammonia is subsequentiy removed from the air stream by acid scmbbing. 

Cobalt salts are used as activators for catalysts, fuel cells (qv), and batteries. Thermal decomposition of cobalt oxalate is used in the production of cobalt powder. Cobalt compounds have been used as selective absorbers for oxygen, in electrostatographic toners, as fluoridating agents, and in molecular sieves. Cobalt ethyUiexanoate and cobalt naphthenate are used as accelerators with methyl ethyl ketone peroxide for the room temperature cure of polyester resins. 

Product recoveiy from reversed micellar solutions can often be attained by simple back extrac tion, by contacting with an aqueous solution having salt concentration and pH that disfavors protein solu-bihzation, but this is not always a reliable method. Addition of cosolvents such as ethyl acetate or alcohols can lead to a disruption of the micelles and expulsion of the protein species, but this may also lead to protein denaturation. These additives must be removed by distillation, for example, to enable reconstitution of the micellar phase. Temperature increases can similarly lead to product release as a concentrated aqueous solution. Removal of the water from the reversed micelles by molecular sieves or sihca gel has also been found to cause a precipitation of the protein from the organic phase. 

Homoallyl alcohol (3) Metalation of (E) butene (1 05 equiv) with n BuLI (t equiv) and KOtBu (1 equiv) in THF at SO C for 15 mm followed by treatment of (E)-crotyl potassum salt with B(OiPr)3 at 79°C gave after quenching with 1 N HCI and extraction with EtjO containing 1 equiv of diisopropyl tartarate. the crotyl boronate 2 A solution of decanall (156 mg 1 mmol) was added to a toluene solution of 2 (1 1 15 equiv) (0 2 M) at 78 C containing 4A molecular sieves (15-20 mg/L) After 3 h at -78°1 N NaOH was added, followed by extraction and chromatography to afford 208 mg of 3 (90%), anti syn 99 1... 

The catalyst, 3-benzyl-5-(2-hydroxyethyl )-4-methyl-l, 3-thiazoHum chloride, is supplied by Fluka AG, Buchs, Switzerland, and by Tridom Chemical, Inc., Hauppauge, New York. The thiazolium salt may also be prepared as described below by benzylation of 5-(2-hydroxyethyl)-4-methyl-l,3-thiazole which is commercially available from E. Merck, Darmstadt, West Germany, and Columbia Organic Chemicals Co., Inc., Columbia, SC. The acetonitrile used by the checkers was dried over Linde 3A molecular sieves and distilled under nitrogen, bp 77-78°C. The same yield of thiazolium salt was obtained by the checkers when benzyl chloride and acetonitrile from commercial sources were used without purification. 

To a solution of 33 g. (O.S mole) of potassium hydroxide (Note 1) in 1.5 1. of distilled water in a 5-1. flask or other appropriate container fitted with a mechanical stirrer is added 80 g. (0.5 mole) of methyl hydrogen adipate (Note 2). With continuous stirring a solution of 85 g. (0.5 mole) of silver nitrate in 1 1. of distilled water is added rapidly (about IS minutes). The precipitated methyl silver adipate is collected on a Buchner funnel, washed with methanol, and dried in an oven at 50-60°. For the next step the dried silver salt is finely powdered and sieved through a 40-mesh screen. The combined yield from two such runs is, 213 g. (80%). 

Certain chemicals (sorbents) have the ability to absorb moisture from a gas they may be either solid or liquid. Performance of a chemical dehumidifi cation device depends on the sorbent used. The sorbent must t>e able to attract and remove the sorbate, such as water, from the gas stream, Stirbems absorb water on the surface of the material by adsorption or by chemically combining with water (absorption). If the unit is regenerative, the process is reversible, allowing water to be removed. This is achieved by a sorbent such as silica gel, alumina gel, activated alumina, lithium chloride salt, lithium chloride solution, glycol solution, or molecular sieves. In the case of nonregenerative equipment, hygroscopic salts such as calcium chloride, urea, or sodium chloride are used. 

F g / sieve, screen, riddle, fegen, n.t. sweep clean winnow. Fege-salpeter, m. saltpeter sweepings, -sand, m. scouring sand, -schober, m. (Salt) scum pan. 

A colorless, colloidal precipitate was formed and stirred thoroughly for about 15 minutes, whereupon it was filtered by suction. The raw product thus obtained was washed with water until It contained only about Va% water-soluble salts. After drying for 12 hours In a vacuum apparatus at 60°C and under a pressure of 12 mm Hg, the product had the form of hard pieces. The pieces were comminuted to powder in a ball mill and the powder was passed through a sieve (3,600 meshes per cm ). The small residue on the sieve was again pulverized and passed through the same sieve. The yield was 870 g, or 99% of theoretical, calculated on the assumed formula... 

Remove traces of ethanol and dissolve precipitate in a minimum (10 ml) of distilled water. Molecular sieving on a Sephadex G25 fine column (1300 ml, 0 5 cm). Assay fraction for /3-poly(L-malate) and chloride ions. Use only salt-free fractions. 

Zeolites are naturally occurring hydrous aluminum-sodium silicates in porous granule form. They are capable of exchanging their sodium base for calcium or magnesium and of expelling these alkaline earth metals for sodium by treatment with salt. Thus, they are a type of ion-exchange media. (Some zeolites act as molecular sieves by adsorption of water and polar compounds.)... 

A small library of thiazolidinones 138 has been prepared mixing directly a primary amine (as the HCl salt), an aldehyde and mercaptoacetic acid in EtOH in the presence of Hiinig s base and molecular sieves (120 °C for 30 min) [88]. Working with a chiral amine, a 1 2 mixture of diastereoisomers was obtained (Scheme 49). 

This is the most common method for the preparation of enamines and usually takes place when an aldehyde or ketone containing an a hydrogen is treated with a secondary amine. The water is usually removed azeotropically or with a drying agent, but molecular sieves can also be used. Stable primary enamines have also been prepared.Enamino-ketones have been prepared from diketones and secondary amines using microwave irradiation on silica gel. ° Secondary amine perchlorates react with aldehydes and ketones to give iminium salts (2, p. 1178). Tertiary amines can only give salts (12). 

---