Solids stearic acid adsorption

Still another manifestation of mixed-film formation is the absorption of organic vapors by films. Stearic acid monolayers strongly absorb hexane up to a limiting ratio of 1 1 [272], and data reminiscent of adsorption isotherms for gases on solids are obtained, with the surface density of the monolayer constituting an added variable. 

Equation 10.27 is generally known as Freundlich equation. Equation 10.27 with concentration replaced by pressure was also used to describe the adsorption isotherms of gases on solids, suggesting the incorrect idea that adsorption from solution by a solid could be paralleled with gas or vapor adsorption on the same adsorbents. Whereas in some cases the restriction to dilute solutions was imposed by the solubility of solids (e.g., benzoic acid in water or stearic acid in benzene) it was not imposed on the investigation of mixtures of completely miscible liquids, e.g., acetic acid in water. 

It is, however, worthwhile to mention that the orientation of a molecule in the adsorbed phase cannot be determined directly. It can only be assumed that the orientation adopted by a given adsorbate molecule is the same as in the case of vapor phase adsorption on the concerned solid although this may not be completely trae in all cases of adsorption from binary solutions. For example, stearic acid is oriented with the major axis perpendicular to the surface as close packed films on... 

Adsorption from solution on solids is, in some respects, more complex than adsorption of gases. There are several complications related to the adsorption of solutes on solid surfaces. The most important one is the role of the solvent. For example, the adsorption of stearic acid on carbon black reaches different limiting values in different solvents ranging from 10 mmol kg when benzene is the solvent up to more than 40 mmol kg when cyclohexane is the solvent. The stearic acid is probably adsorbed with the acyl chain parallel to the surface and the measured adsorption values are consistent with that arrangement. The role of the solvent is indeed very important. 

In early work on oxides [22,23], it was suggested that oleic acid and butyric acid adopted a perpendicular orientation on titania, as did stearic acid on aluminum hydroxide [24]. In these experiments it was not clear whether adsorption was physical or chemical in nature. This now seems an important distinction to draw, especially with basic solids. In chemisorption the orientation of the solute molecules generally presents no problem, as the functional group determines the point of attachment. Thus the long chain fatty acids are attached to the surface by the carboxyl group, - OOH, with the hydrocarbon chain perpendicular to the surface. 

---