Specific adsorbents with positive surface charges

Specific adsorbents with positive surface charges. Acidic hydroxyl groups (hydroxylated acid oxides such as silica), aprotic acid centers, or small radius cations (zeolites) on the surface. Adsorbents of this type will interact with molecules which have locally concentrated electron densities, that is. Group B and Group D molecules. 

It may occur that specifically adsorbed ions overcompensate surface charge for example, the surface is positive because pH < pHpzo hut an excess of anions lie in the plane of specific adsorption so that the global charge of the oxide particle is negative ("superequivalent adsorption" with "charge reversal" ). This is exactly the situation described by Spanos et for... 

Surfaces in contact with aqueous media are more often negatively charged than positively charged. This is a consequence of the fact that cations are usually more hydrated than anions and so have the greater tendency to reside in the bulk aqueous medium whereas the smaller, less hydrated and more polarising anions have the greater tendency to be specifically adsorbed. 

Camie and Chan30 treated an ensemble of hard-sphere ions and dipoles in contact with a hard wall. Specifically, they considered solvent molecules of radius rw with a point dipole at their center, and two kinds of ions, positive and negative, both with the same radius r,. One kind of ion can be adsorbed on the surface of the hard wall by a potential proportional to a Dirac delta function. Charge transfer between the metal (i.e. the hard wall) and the adsorbate was not... 

For oxides a common Intersection point of (T°(pH) curves at different electroljrte concentrations has also been found in the presence of specifically adsorbed lons . The trend is that if such a c,l.p. Is observed, it is to the left and to more positive charges for specific adsorption of cations and to the right and a more negative surface charge in the case of specific anion adsorption, see fig, 3.57, The physical explanation of such c.i,p, s was already anticipated In connection with the interpretation of fig, 3.34b for the thermodynamics, see . Van Riemsdljk et al. concluded, on theoretical grounds, that surface... 

It is known that surface of silica is covered with silanol or siloxane groups [1,2]. The Si-OH shows a specific interaction with water (hydrophilic), by means of the localised electron density of water molecule and the positive charge of OH groups. On the other hand, Si-O-Si are hydrophobic due to the non-polarity of these bonding [10]. Thus, it is reasonable to think that depending on the amount of silanol groups in the surface of silica, the amount of water adsorbed at very low relative pressures would be different. 

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