Solvents for adsorption

Table 25-2 Eluotropic series and ultraviolet cutoff wavelengths of solvents for adsorption chromatography on silica...

Selection of solvents for adsorption chromatography. The choice of solvent for transferring the mixture to be chromatographed to the column will naturally depend upon the solubility characteristics of the mixture. If it is already in solution, for example as an extract, this is usually evaporated to dryness under reduced pressure and the residue dissolved in the minimum volume of the most non-polar solvent suitable. As concentrated a solution as possible is desirable to achieve a compact band at the top of the column of adsorbent, so that during subsequent development the separation will hopefully proceed with formation of discrete bands. 

The hydrophobicity of TS-1 could also explain why the oxidation of hydrocarbons in aqueous H2C>2 is faster without added organic solvent (triphase catalysis) than in organic solution (biphase catalysis) e.g. benzene hydroxylation under triphase conditions was up to 20 times faster than in acetonitrile or acetone (biphase conditions).1741 Indeed, benzene competes more favourably with water than with organic solvents for adsorption within the micropores of hydrophobic TS-1, as furthermore confirmed through adsorption experiments.1471... 

Snyder and Glajch (32) note that alcohols can be used as solvent-specific localizing solvents for adsorption on silica, with behavior predictable by their model this is not true for alumina systems (34). 

Table 8-3 presents relative eluotropic series for separations on charcoal and polyamide. The series for charcoal is almost the reverse of that shown for alumina in Table 8-1. Solvent strength on charcoal is seen to increase with the size of the solvent molecule, and there is a good correlation between solvent strength and the calculated area (A,) required by an adsorbed solvent molecule. Jermyn (24) has reported additional data on the relative strengths of various solvents for adsorption on charcoal. The...

Competition between the sample ion and the counter ion for the fixed ionic site is very similar to the competition between solute and solvent for adsorption sites in LSC. In fact, sometimes ion exchange is referred to as adsorption chromatography involving electrostatic interactions. However, as the nature of the stationary and mobile phases, as well as the samples handled, are quite unlike those used for LSC, we prefer to classify ion-exchange chromatography separately. 

A special case of adsorption in cavities is that of clatherate compounds. Here, cages are present, but without access windows, so for adsorption to occur the solid usually must be crystallized in the presence of the adsorbate. Thus quinol crystallizes in such a manner that holes several angstroms in diameter occur and, if crystallization takes place in the presence of solvent or gas... 

Reference has already been made to the choice of solvent for introducing the mixture to the column. Generally speaking, adsorption takes place most readily from non-polar solvents, such as petroleum ether or benzene, and least from highly polar solvents such as alcohols, esters and pyridine. Frequently the solvent for introducing the mixture to the column and the developer are so chosen that the same solvent serves the dual purpose. 

Graded Adsorbents and Solvents. Materials used in columns for adsorption chromatography are grouped in Table 12 in an approximate order of effectiveness. Other adsorbents sometimes used include barium carbonate, calcium sulfate, calcium phosphate, charcoal (usually mixed with Kieselguhr or other form of diatomaceous earth, for example, the filter aid Celite) and cellulose. The alumina can be prepared in several grades of activity (see below). 

Toluene is to be removed from a wastewater stream. The flowrate of the waste stream is 10 kg/s and its inlet composition of toluene is 5(X) ppmw. It is desired to reduced the toluene composition in water to 20 ppmw. Three external MSAs are considered air (S2) for stripping, activated carbon (S2) for adsorption, and a solvent extractant (S3). The data for the candidate MSAs are given in Table 3.6. The equilibrium data for the transfer of the pollutant from the waste stream to the yth MSA is given by... 

It is well known that anionic samples tend to adsorb on poly(styrene-divinylbenzene) resins. However, cationic samples tend to be repelled from the resins. The mechanism seems to be an ionic interaction, although the poly(styrene-divinylbenzene) resin should be neutral. The reason is not well clarified. Therefore, it is recommended to add some salt in the elution solvent when adsorption or repulsion is observed in the analyses of polar samples. For example, polysulfone can be analyzed successfully using dimethylformamide containing 10 mM lithium bromide as an elution solvent, as shown in Fig. 4.42. 

Because we wanted to suppress the effects of thermodynamic quality of the eluent toward the polymer probes, we therefore looked for liquids that would be thermodynamically good solvents for PMMA. At the same time, one solvent should promote polymer adsorption whereas the others should promote desorption. 

Thus, the spacing of the chains relative to the neutral, free, swollen size of the buoy blocks is, for a given chemical system and temperature, a unique function of the solvent-enhanced size asymmetry of the diblock polymer and a weak function of the effective Hamaker constant for adsorption. The degree of crowding of the nonadsorbing blocks, measured by a decrease in the left-hand side of Eq. 28, increases with increasing asymmetry of the block copolymer. 

Sophisticated and veiy sensitive methods have been developed in the food industry for detecting many other microbial toxins. For example, aflatoxin deteetion in seedstuffs and their oils is performed by solvent extraction, adsorption onto columns containing selective antibodies for them, and detected by exposure to ultraviolet light. 

General trends are focused on reduced-solvent extractions or adsorption-based methods — enviromnentaUy friendly solvents for both solid and liquid samples. In recent decades, advanced techniques like supercritical fluid extraction (SFE), ° pressurized liquid extraction (PLE)," microwave-assisted extraction (MAE), ultrasound-assisted extraction, countercurrent continued extraction (www.niroinc.com), solid... 

A sequential analysis protocol includes three steps (1) extraction in water or other appropriate solvent for the colorant, (2) purification or concentration of the colorant, and (3) separation coupled with detection of the target molecule. Different methods of extracting synthetic colorants from foods have been developed using organic solvents followed by SPE protocols using as adsorption support RP-C18, amino materials, or Amberlite XAD-2. Eor qualitative evaluations, the easiest option for separating colorant molecules from unwanted ingredients found in an extract is SPE on polyamide or wool. 

The correlation of Snyder s solvent strength e° with molecular dipolarity and polarizability (7t ) and the hydrogen-bond acidity (a) and the hydrogen-bond basicity ((3) solvatochromic parameters for adsorption chromatography can be achieved, although most papers on solvatochromic parameters deal with reversed-phase systems [18]. 

The performance of demulsifiers can be predicted by the relationship between the film pressure of the demulsifier and the normalized area and the solvent properties of the demulsifier [1632]. The surfactant activity of the demulsifier is dependent on the bulk phase behavior of the chemical when dispersed in the crude oil emulsions. This behavior can be monitored by determining the demulsifier pressure-area isotherms for adsorption at the crude oil-water interface. 

---