Adsorption from Solution and Effects of Surface Functionalities

ADSORPTION FROM SOLUTION AND EFFECTS OF SURFACE FUNCTIONALITIES 

Adsorption from liquid solution is more complicated than that from the gas phase. In a two-component liquid solution, both the solvent and the solute will be adsorbed to different extents. Usually the adsorption of the solute is of interest. The experimental procedure and expression for the amount adsorbed both differ from those used for gas adsorption. The basis for expressing the amount adsorbed from liquids is the concept of surface excess given by Gibbs in 1878. Gibbs surface excess is the difference in the amount of a given component in the surface layer (per unit surface area) over that in the bnlk Uqnid. 

The actual experiment involves measurements of the initial and final concentrations of the solution. Let n be the total number of moles in the initial solution, and n° =nl+ n, where subscripts 1 and 2 denote the solute and solvent, respectively. Let x° and x denote the initial and final mole fractions, respectively. Then the Gibbs adsorption or the apparent adsorption is given, for 1 g of sorbent, by  

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Chemical structure of the solute and its interactions with the solvent The structure (hydrocarbon chain length, branching, nature and location of polar functional groups) of the solute and its interactions with the solvent (solubility, complexation, micellization) have a marked effect on its adsorption. For example, it is well known from Traube s rule that for aqueous surfactant solutions the surface activity and hence the adsorption at the liquid-air interface increases with an increase in the chain length of the solute molecule. The solutes of interest, surfactants, are also capable of forming association structures in solution (micelles or reverse micelles depending on the solvent), which is a measure of their solvophobicity. 

The effects of oxygen functionalities on the adsorption of aliphatic compounds from their aqueous solutions have also been studied (Cookson, 1978 Jankowska et al., 1991 Radovic, 1996). The adsorption capacities of butyl disulfide and decane were both decreased by surface oxides (Cookson, 1978). Hence it was concluded that surface oxides hindered adsorption of nonpolar aliphatic compounds. 

Radovic et al. (1996) investigated the effects of nitriding the surface on adsorption from solution. Reacting with ammonia at elevated temperatures introduced pyridine functional groups on carbon. Reaction at 200 °C forms amides, imides, imines, amines, and nitriles while reaction at 250 °C results in bonding of ammonia to the carbon double bonds (Vinke et al., 1994). The effects of nitriding (at 250 °C) were similar to that of oxidation. Nitriding also hindered the adsorption of benzoate and aliphatic anions, oxalate, and fumarate. 

Silica Gel. Silica gel is prepared by the precipitation of a silicate solution with acid, or by hydrolysis of silicon derivatives. The surface area and diameter of the silica gel particles depend on the method of precipitation. Variations in pH during precipitation can produce silica gels with surface areas ranging from 200-800 m /g. It has been shown [4] that silica gel provides three types of surface hydroxyl groups Bound, reactive, and free. Relative reactivity and adsorption follow the order bound > free > reactive. Thus, control of the distribution of surface functions can have a significant effect on the chromatographic properties of a silica. 

In modern practice, inhibitors are rarely used in the form of single compounds — particularly in near-neutral solutions. It is much more usual for formulations made up from two, three or more inhibitors to be employed. Three factors are responsible for this approach. Firstly, because individual inhibitors are effective with only a limited number of metals the protection of multi-metal systems requires the presence of more than one inhibitor. (Toxicity and pollution considerations frequently prevent the use of chromates as universal inhibitors.) Secondly, because of the separate advantages possessed by inhibitors of the anodic and cathodic types it is sometimes of benefit to use a formulation composed of examples from each type. This procedure often results in improved protection above that given by either type alone and makes it possible to use lower inhibitor concentrations. The third factor relates to the use of halide ions to improve the action of organic inhibitors in acid solutions. The halides are not, strictly speaking, acting as inhibitors in this sense, and their function is to assist in the adsorption of the inhibitor on to the metal surface. The second and third of these methods are often referred to as synergised treatments. 

Figure 6.16. Different modes of adsorption of CHjOH on Pt under ultra-high vacuum (left) and in aqueous solutions (right) showing the effect of local electrostatic field and surface work function on the mode of adsorption.100 Reprinted with permission from the American Chemical Society.

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