Stearic acid from platinum

Direct study of the desorption of stearic acid from platinum and from NiO was carried out by Timmons and Zisman [10]. Their findings are shown in Table 10-5. The methylene iodide contact angle and the surface potential measurements indicate that a stearic acid monolayer adsorbed on platinum can be removed completely by heating to 130 C or by extraction with diethyl ether. But if the adsorbent is nickel oxide, heating to 150 C or extraction with diethyl ether fails to restore the original contact angle behavior or the surface potential of the adsorbent surface. 

Figure 9-5. Influence of temperature on lubrication by long-chain compounds. 1 Docosane on platinum. 2 Stearic acid on platinum. 3 Copper laurate on platinum. 4 1% lauric acid in paraffin oil on copper. 5 Sodium stearate on steel. Data by Bowden, Gregory and Tabor [17]. Adapted by permission from Natali, Vol. 156, No. 3952, pp. 99. Copyright 1945, Macmillian Journals Ltd.

PROBLEM 9.10 Oleic acid and stearic acid are naturally occurring compounds, which can be isolated from various fats and oils. In the laboratory, each can be prepared by hydrogenation of a compound known as stearolic acid, which has the formula CH3(CH2)7C=C(CH2)7C02H. Oleic acid is obtained by hydrogenation of stearolic acid over Lindlar palladium stearic acid is obtained by hydrogenation over platinum. What are the structures of oleic acid and stearic acid ... 

C and n-octadecane-1,2-H at a concentration of 0.35 molar in stearic acid and studied the coadsorption of stearic acid and octadecane on polished surfaces of silver, platinum, copper and iron. The films were prepared by retraction from the melt at 40 C (at room temperature the mixture was solid). The proportion of n-octadecane in the film was assayed by differential extraction with cyclohexane. The results of the investigation adequately demonstrate that n-octadecane coadsorbs with stearic acid but not necessarily as a mixed oriented monolayer. Some of the data indicate that more than a single layer is present on the surface. Thus the structure of the long-chain material on the surface may be open to conjecture, but that each constituent adsorbs and in what relative amount is directly determined by radioactive assay. 

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 formation of monolayers of cerotic, be-henic, and stearic acids adsorbed from cis-decahydronaphthalene and w-hexadecane solutions onto glass was studied by electron microscopy. Platinum preshadowed carbon replicas of the monolayered glass surfaces were examined in a JEM 6-A electron microscope. The acids adsorb initially in discrete patches the average patch diameter depends on the length of the acid molecule, becoming smaller for shorter acids. The area covered by the unimolecular film increases as the adsorption time or concentration of adsorbate solution increases. Contact angle measurements on partially monolayered surfaces show the contact angle to be dependent upon the fraction of substrate covered. Direct evidence of metastable incorporation of hexadecane in acid mono-layers was obtained. 

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