Water adsorption, supported aqueous-phase

Supported aqueous-phase catalysts (SAPC) can be seen as a special case of adsorption, whereby a water-soluble catalyst dissolved in a very polar solvent is adsorbed on a hydrophilic support forming a water film on the inner surface of the support [30,31]. In the case of supported liquid-phase catalysis (SLPC),the water film on the inner surface is replaced by a solvent of low vapor pressure (e.g.jphthalic acid esters) [2]. The reaction itself takes place in the supportedUq-uid or at the interface of the supported liquid film, or in the gas phase or organic phase when dealing with SLPC or SAPC, respectively. The use of SLPC catalysts is generally restricted to the synthesis of low-boiling compounds. 

Liquid/liquid partition chromatography was explored by Willstatter from 1913. The process was extensively developed by Martin and Synge (ca. 1941-1948) who partitioned amino acid derivatives between chloroform and water using precipitated silica as support for the aqueous phase. The preparations of silica were again very variable and it was difficult to prevent adsorption which interfered with the expected behavior of the aminoacids. At first methyl orange was added to the water phase to visualize the amino acids the separation of the acids then caused a red band to move down the columns. The quantitative reaction with ninhydrin was introduced by Moore and Stein in 1948 for both the detection and estimation of the amino acids. Consid-... 

We will chiefly be concerned with "wet" interfaces, between solid supports and an aqueous phase. Unfortunately, most of the methods that allow precise interface structural studies are not applicable in these conditions however, several spectroscopic techniques may be used, preferably in combination with macroscopic adsorption measurements, to arrive at a reasonable degree of local characterisation of the solid/water interface. 

Liquid-partition chromatography was a hybrid of Tswett s column-adsorption chromatography and a technique widely employed in industry called countercurrent extraction. The latter involved passing two immiscible liquids moving in opposite directions in contact with each other to separate components differing in their relative solubilities (partition coefficients) in the two solvents. Martin and Synge discovered that a solid support such as silica gel can be wetted by water to form, in essence, a stationary aqueous phase. The surface of the wetted silica gel has, surprisingly, the same solvent properties as pure water. When the stationary... 

The original concept of cellulose as an inert support for the aqueous phase has been modified since the liquid-liquid hypothesis is inadequate for a complete explanation of the process (60,266,267). The mechanism is complex (186,187) and both adsorption and partition probably play a part in the same system. Hanes and Isherwood (122) consider the operation of paper chromatograms to depend upon the relative ease with which solutes pass from the mobile phase into the cellulose-water complex. While its exact nature is controversial, it is thought that cellulose in its hydrated form is a complex gel composed of alternate submicroscopic crystalline and amorph-... 

The technique and instrumentation for liquid-liquid chromatography are much the same as for adsorption chromatography the main distinction lies in the nature of the stationary phase. In the original development of the method, Martin and Synge effected a separation of acetylated amino acids by using silica gel as support for an aqueous stationary phase. The mobile phase was a mixture of chloroform, butanol, and water. The method was used for analysis of the amino acids of various proteins. ... 

Another type of extraction commonly used on liquid samples is based upon the partitioning of an analyte between the hquid in which it is dissolved and a sohd support. Such extractions are typically based upon adsorption of the solute onto the solid. An example of such an extraction is the adsorption of hydrocarbons in aqueous solution onto activated charcoal. This process has long been used in such areas as pollution control (e. g. oil spills in water), and now is beginning to be implemented more in trace analysis procedures as a technique called solid phase micro-extraction. The main disadvantage of extraction techniques is typically the time that is required to recover the majority of the solute. Because of this problem the use of extraction techniques in quantitative analyses is typically performed as a last choice. 

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