Ultra-High Vacuum Studies of Model Boundary Layer Lubricants
Parker, Bryan Robert
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https://hdl.handle.net/2142/72292
Description
Title
Ultra-High Vacuum Studies of Model Boundary Layer Lubricants
Author(s)
Parker, Bryan Robert
Issue Date
1993
Doctoral Committee Chair(s)
Gellman, A.J.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Abstract
Surface Analytical Techniques have been used to probe the structure and surface chemistry of model boundary layer lubricants adsorbed onto single-crystal metal surfaces under ultra-high vacuum conditions. Chapter 3 presents results from the adsorption of a set of straight-chain carboxylic acids and perfluorinated carboxylic acids onto the Ag(110) surface. The carboxylic acids are found to desorb reversibly from the clean Ag(110) surface. For monolayer coverages of the acids, the corresponding peak temperature maximum in the Temperature Programmed Desorption spectrum increases by roughly 10-15 K as the alkyl chain length is increased. This corresponds to an increase in the activation energy of desorption by 0.94 $\pm$ 0.01 kcal/mol/-CH$\sb2$ group. On the preoxidized Ag(110) surface, the carboxylic acids undergo an acid-base reaction with the adsorbed oxygen to form water and the corresponding carboxylate. The carboxylates then decompose under heating to form CO$\sb2,$ hydrocarbons, and other products. The perfluorinated acids adsorbed onto the clean Ag(110) surface spontaneously deprotonate to form the perfluorocarboxylate. Perfluorination of the alkyl chain lowers the barrier for deprotonation of the acid which increases the rate of formation of the perfluorocarboxylate intermediate. The perfluorocarboxylates decompose near 600 K to give CO$\sb2$ and fluorine-containing products.
Chapter 4 presents results for the adsorption of straight-chain alkylthiols (CH$\sb3$(CH$\sb2)\sb{\rm n}$SH, n = 0-6) on the clean Ni(100) surface. On the clean Ni(100) surface the alkylthiols decompose at low temperatures (T $<$ 140 K) to form the adsorbed alkylthiolates (CH$\sb3$(CH)$\sb{\rm n}$S-$\sb{(\rm ad)}).$ Further decomposition at higher temperatures results in the formation and desorption of alkanes, olefins, and hydrogen, and leaves sulfur adsorbed to the surface. Since no sulfur-containing species desorb, the alkylthiolate coverage was measured by measuring the sulfur coverage following decomposition. These measurements were made for saturated monolayers of methylthiolate through hexylthiolate. The saturation coverage remains constant at $\Theta$ = 0.28 $\pm$ 0.04 monolayers as the number of methylene groups in the alkyl chain increases. This suggests that the thiolates have self-assembled into a structure in which the alkyl chains are oriented away from the surface.
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