Organic component, preferential adsorption

One of the promising new technologies for separation of very fine particles is selective flocculation. The selective flocculation process has been used effectively to separate very finely disseminated minerals from mixed ore suspensions (5.). The process is based on the preferential adsorption of an organic flocculant on the wanted minerals, thereby flocculating them, while leaving the remainder of the suspension particles dispersed. The dispersion of certain components in the suspension such as pyrite can be enhanced by using more selective or powerful dispersants. Methods for achieving selective flocculation and dispersion have been recently described by Attia (6j. 

In reversed-phase HPLC the binary mixture of water and organic components is the common eluent. It is obvious that organic modifier would be preferentially adsorbed on the surface of hydrophobic stationary phase, and this adsorption has been studied for more than 30 years [24-26]. 

A more recent study has been made of the surface reactivity of several carbon fibres towards gaseous adsorbents. Heats of adsorption for water and for various organic gases were found not to vary significantly, but calculated adsorption coefficients showed that pre-oxidation enhanced the adsorption of polar adsorbates such as water, pyridine, and aniline. Kipling et a/. have shown that pre-treatment by oxygen can result in preferential adsorption of more polar components of a binary mixture. Care must be exercised, however, in that different surface complexes may promote specific adsorption of individual chemicals. ... 

The preferential sorption of the organic component of the solvent is a well known effect in RPLC. Based on their interpretation of adsorption isotherm measurements with water/acetOTiitrile mixtures, Kazakevich and coworkers have inferred that the excess adsorption occurs purely at the surface of the alkyl chains and that acetOTiitrile forms up to five molecular layers atop the alkyl chains. Clearly, from the snapshots and the density profiles in Fig. 3, this layering does not appear. The simulations indicate that the excess adsorption occurs both within the chain region and at the chain surface. Furthermore, the adsorbed solvent at the surface is not pure acetonitrile it is simply a local enhancement of the acetonitrile concentration. 

In these mixtures, the vapor or organic compound can either adsorb preferentially on the zeolite pores or undergo capillary condensation in pores of small diameter, therefore blocking the membrane for the other components in the mixture (i.e., permanent gas). The separation selectivity toward the blocking molecule decreases with temperature due to the decrease in adsorption and capillary condensation. 

---