Adsorption from organic solvents

At the interface between solids and organic solvents, however, specific attractions between the solid and the adsorbed substances may come into play and produce considerable adsorption. Some such cases are of importance in lubrication long-chain fatty acids, and some of their salts, are adsorbed from solution in hydrocarbon oils at the surface of many metals, and the result is a boundary lubricating layer (see Chap. VI). 

Traube s rule does not hold for adsorption from organic solvents. While, on charcoal, from aqueous solution, the adsorption of organic compounds increases as the hydrocarbon chains are lengthened,2 this rule is reversed in the case of fatty acids on silica. Holmes and McKelvey3 found much greater adsorption of the shortest chain acids, from toluene on silica, than of longer chain acids and Bartell and Fu4 obtained similar results with solutions in carbon tetrachloride. 

There is no reason to expect Traube s rule to hold good except for aqueous solutions it is only an expression of the tendency of long hydrocarbon chains to escape from solution in water. 

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There has been very little work on adsorption of stearic acid from aqueous solution, probably due to the low solubility of the acid. Suess has however made some measurements using carbon-14 labelled acid [16]. As with adsorption from organic solvent, a Langmuir type isotherm showing mono-layer adsorption was observed, however this was at a much lower level than for solvent adsorption. The reasons for this are not clear but were postulated to be due to adsorption as a hydrated complex salt. Presumably, this is converted to the stable coating layer during drying. 

In any study of electrosorption of neutral molecules on metallic electrodes, the ions of supporting electrolytes should not be specifically adsorbed. Nevertheless, the interaction of the electrolyte ions with the electrode surface may depend on the interaction of the ions with the solvent. Usually, the stronger the ion-solvent interaction, the weaker the adsorption of the ion. Since the ions are more weakly solvated in nonaqueous solvents than in water, the ions that are not adsorbed from aqueous solutions may still be adsorbed from organic solvents. However, even in the absence of... 

Isotherm data at high equilibrium concentration for modification from organic solvent, have also been obtained by the freeze sampling method. The adsorption isotherm of APTS on dehydrated mesoporous silica from toluene solution is given in figure 9.10. 

Summarizing, aminosilanes show a fast adsorption on the silica surface. An equilibrium monolayer coating is formed. Modification in aqueous solvent causes polymerization on top of the initial monolayer. For modification from organic solvent, the reactions can be better controlled. With the bifunctional AEAPTS, a secondary silane layer adsorbs on the free primary amine groups of the first monolayer. At high concentration and after long reaction times, for both aminosilane types, a further non-specific deposition occurs. 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

Thus, some important aspects to consider include the extent to which solvent is adsorbed, die significance of the adsorption limit or plateau sections found in most isotherms, the orientation of the adsorbed molecules, and the existence of physical and/or chemical adsorption. One should be aware that the data obtained from studies of molecular adsorption represent information on a macroscopic level. Information on the molecular adsorption mechanism will, therefore, require additional investigation of the system. In order to elucidate some areas concerning adsorption fi om dilute nonaqueous solvents onto mineral oxides, we will discuss more closely the adsorption of amphiphilic molecules from organic solvents on to various such oxides. 

Research has shown that ascorbic acid can be produced from hulls of immature walnuts by extracting the hull with 0.2% sulfur dioxide solutions, and purifyiag the extract by adsorption on and elution from anion-exchange resias (see Ion exchange). Eluates from the anion-exchange step are concentrated, purified by organic solvent fractionations, decolorized, and crystallized (35). 

Isolation. Isolation procedures rely primarily on solubiHty, adsorption, and ionic characteristics of the P-lactam antibiotic to separate it from the large number of other components present in the fermentation mixture. The penicillins ate monobasic catboxyHc acids which lend themselves to solvent extraction techniques (154). Pencillin V, because of its improved acid stabiHty over other penicillins, can be precipitated dkecdy from broth filtrates by addition of dilute sulfuric acid (154,156). The separation process for cephalosporin C is more complex because the amphoteric nature of cephalosporin C precludes dkect extraction into organic solvents. This antibiotic is isolated through the use of a combination of ion-exchange and precipitation procedures (157). The use of neutral, macroporous resins such as XAD-2 or XAD-4, allows for a more rapid elimination of impurities in the initial steps of the isolation (158). The isolation procedure for cephamycin C also involves a series of ion exchange treatments (103). 

A theoretical description of hydrogen bonding effects can be made from model of charge-controlled adsorption. It was found that the energy of adsorption of organic molecules ai e determined by the ratios between the effective chai ges of their atoms and atoms in polai solvent molecules ... 

Adsorption, which utilizes the ability of a solid adsorbent to adsorb specific components from a gaseous or a liquid solution onto its surface. Examples of adsorption include the use of granular activated carbon for the removal of ben-zene/toluene/xylene mixtures from underground water, the separation of ketones from aqueous wastes of an oil refinery, aad the recovery of organic solvents from the exhaust gases of polymer manufacturing facilities. Other examples include the use of activated alumina to adsorb fluorides and arsenic from metal-finishing emissions. 

These poly(2-alkyl-2-oxazoline) silane coupling agents were copolycondensed with tetraethoxysilane by acid-catalyst to produce poly(2-alkyl-2-oxazoline)-modified silica gel. The composite gel from 2-ethyl-2-oxazoline was also homogeneous and transparent glass. Poly(2-alkyl-2-oxazoline)-modified silica gels, especially gels based on poly(2-ethyl-2-oxazoline) absorbed water and also organic solvents such as DMF or alcohols as shown in Table 7. This result means that the obtained composite gel shows the amphiphilic adsorption property. 

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. 

In the most common approach, a water-insoluble metaUoporphyrin is deposited on the surface of a rotating disk electrode (RDE) or on the disk of a rotating ring-disk electrode (RRDE Fig. 18.7a) as a film of poorly defined morphology, either by spontaneous adsorption from a solution of the catalyst in an organic solvent or by evaporation of an aliquot of such a solution onto the electrode. It is impossible to know the... 

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