Stearic acid chemisorption

Such behavior is consistent with what one would expect from the chemical nature of the adsorbate and the adsorbent surfaces. The platinum surface is unreactive and the behavior of the stearic acid film on it is typical of what is generally regarded as physical adsorption, while the NiO surface is reactive and the stearic acid is considered to be chemisorbed. In Section 10.3.2 below we shall examine the difference between physical adsorption and chemisorption in detail. 

The most reasonable explanation for the persistently retained stearic acid is chemisorption on the "metal" surface as the soap. Timmons and Zisman [lO] chose diethyl ether as the solvent for the depletion of the monolayer adsorbed on nickel oxide because it dissolves stearic acid but not nickel stearate. 

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. 

Early work by Schonhorn [119] showed that orientated monolayers of amphipathic molecules, such as stearic acid, could be employed as extremely effective adhesives in the bonding of polyethylene to aluminium indeed, so effective were these adhesives that joint strengths often exceeded the cohesive strength of the polyethylene Multilayer adsorption lowered the joint strength not because of less intrinsic adhesion of the amphipathic molecule to the substrate, but because the relatively thick layer possessed low cohesive strength and thus behaved as a weak boundary layer. Chemisorption was thought to occur for the stearic acid on the aluminium oxide [120,121], and this... 

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