Morin, Mario,Al-Maznai, Hassan Mohamed.2009-03-232009-03-2319991999Source: Dissertation Abstracts International, Volume: 61-04, Section: B, page: 1963.9780612480810http://hdl.handle.net/10393/8561http://dx.doi.org/10.20381/ruor-15881Organic thiols adsorb on metallic surfaces and form so-called self-assembled monolayers (SAMs). These interfacial systems have attracted a lot of attention because of their possible use in various technologies such as electrochemical sensors, organic conductors or corrosion inhibitors. These applications make use of the ability of thiols to prevent charge transfer between the metal surface and the electrolyte solution. However, organic materials can undergo phase transitions in the range of temperature where modified electrodes are used. Few studies have been done on the thermal stability of molecular films. This thesis is a study of the electrochemical properties of alkanethiols chemisorbed on Au(111) between 5°C and 60°C. We found that soluble alkanethiols are reduced in a single step. We found that insoluble alkanethiols are reduced in two steps. The reduced products (thiolates) are insoluble and thus remain physisorbed at the electrode surface. They can be oxidatively redeposited when the potential is scanned in the positive direction. The development of a fine structure in the voltammogram of insoluble thiols is shown to be caused by the low solubility of the thiols. We propose a model in which the reductive desorption and the oxidative redeposition processes proceed through the same two-step mechanism. We have studied the effect of temperature on insoluble thiols that can be induced, by the application of an electric field, to reversibly go from a chemisorbed state to a physisorbed state. We found that the physisorbed thiolates remain at the electrode surface at temperatures as high as 70°C. Our results reveal a change in the reductive desorption/oxidative redeposition at 23°C. We assign this change to an order-disorder transition. The ion blocking properties of insoluble thiols were studied with differential capacitance and with cyclic voltammetry using ferricyanide as a redox probe. We found that monolayers with low numbers of defects are formed after a single incubation in a solution of thiols. Multiple incubations in a solution of thiols give a monolayer with such an extremely low number of defects that electron transfer occurs via tunneling. Finally, we determined the oxidation mechanism of adsorbed butanethiol in neutral and alkaline aqueous solutions. Under neutral conditions, the butanethiols are oxidized via a nine electrons process, whereas in alkaline solutions, the oxidation is greater and requires thirteen electrons. These results are found to be related to the greater electrocatalytic activity of gold in alkaline solutions. (Abstract shortened by UMI.)233 p.Chemistry, Inorganic.Thesis