Cation-exchange properties

Smectites are stmcturaUy similar to pyrophylUte [12269-78-2] or talc [14807-96-6], but differ by substitutions mainly in the octahedral layers. Some substitution may occur for Si in the tetrahedral layer, and by F for OH in the stmcture. Deficit charges in smectite are compensated by cations (usually Na, Ca, K) sorbed between the three-layer (two tetrahedral and one octahedral, hence 2 1) clay mineral sandwiches. These are held relatively loosely, although stoichiometricaUy, and give rise to the significant cation exchange properties of the smectite. Representative analyses of smectite minerals are given in Table 3. The deterrnination of a complete set of optical constants of the smectite group is usually not possible because the individual crystals are too small. Representative optical measurements may, however, be found in the Uterature (42,107). 

Dufey J.E., Genon J.G., Rufyikiri G., Delvaux B. Cation exchange properties of roots Experimental and Modelling. Proceeding of 5th International Conference on the Biogeochemistry of Trace elements, Vienna, Austria, 1999. 

The introduction of silicon into hypothetical phosphorus sites produces negatively charged frameworks with cation-exchange properties and weak to mild acidic catalytic properties. Again, as in the case of the aluminophosphate molecular sieves, they exhibit excellent thermal and hydrothermal stability. 

Micco, D.j. and Hinchey, R.j. (2003) Method for making zeolites and zeolite mixtures having enhanced cation exchange properties. US Patent 5,541,796. 

If the membrane has cation exchange properties the transport t will be functions of concentration. In fact... 

As with other hydrous metal oxides, such as alumina and zirconia, tin oxide is an amphoteric ion exchanger that exhibits cation exchange properties at basic pH. Hydrous tin oxide, however, appears to be particularly favored by virtue of its high Sr(ll) distribution coefficient (Kd Sr-bound/Sr-free) and high separation factor, Kd[Sr(ll)/Rb(l)], over a broad pH range (Table II Figure 1). 

Elimination of coextracted materials and concentration of tetracyclines have also been accomplished using mixed-phase extraction membranes with both re-versed-phase and cation-exchange properties (294,295), or solid-phase extraction columns packed with cation-exchange materials such as CM-Sephadex C-25 (301), aromatic sulfonic acid (310), and carboxylic acid (283, 300). For the same purpose, metal chelate affinity chromatography has also been employed. In this technique, the tetracyclines are specifically absorbed on the column sorbent by chelation with copper ions bound to small chelating Sepharose fast flow column (278-281, 294-296). 

Gillman, G.P. 1980. The effect of crushed basalt scoria on the cation exchange properties of a highly weathered soil. Soil Science Society of America Journal 44 465-468. 

Changes in mobile-phase components such as pH, ionic strength, and water content have been systematically studied [3,310,316,317]. These studies indicate that retention of basic analytes is mediated primarily by the cation-exchange properties of the silica [2]. Interestingly, it has been suggested from retention data of various pharmaceuticals that the retention mechanisms of silica with aqueous eluents and reversed-phase systems are similar [317,318]. Due to the ion-exchange properties of silica, mobile-phase pH adjustments are useful in changing the retention of ionic compounds. 

The IUPAC Compendium of Chemical Technology defines Normal Phase as an elution procedure in which the stationary phase is more polar than the mobile phase . In practice, the most widely used stationary phases for preparative HPLC are based on silica and the polarity of the underlying silyl ether and silanol provides the required hydrophilic surface. Amino and cyano bonded silica are also commonly used in normal phase mode though the latter also has some reversed phase properties. The predominant mechanism of interaction is hydrogen bonding. However, the silanol is mildly acidic so the silica surface will also have mild cation exchange properties. 

In 1978, the same year that the structure of ZSM-5 was first described, Flanigen and her co-workers reported the synthesis, structure and properties of a new hydrophobic crystalline silica molecular sieve (Flanigen et al., 1978). The new material, named Silicalite (now generally called Silicalite-I), has a remarkably similar channel structure to that of ZSM-5 but contains no aluminium. It was pointed out by the Union Carbide scientists that, unlike the aluminium-containing zeolites, Silicalite has no cation exchange properties and consequently exhibits a low affinity for water. In addition, it was reported to be unreactive to most acids (but not HF) and stable in air to over 1100°C. 

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