Adsorption of Organosilanes on the Surface of Inorganic Materials: 2. Adsorption on the Surface of Metals

Organofunctional silanes are important surfactants capable of forming functional nanolayers on mineral surfaces, which can strengthen the interaction (adhesion) between the inorganic material (glass, silicon dioxide, carbon, etc.) and polymer (a binder in composites or a polymer coating); in the case of metal surfaces, the surfactants can inhibit metal corrosion. Articles devoted to the adsorption of organosilanes on metal surfaces under different conditions are reviewed. It is shown that organosilanes can be adsorbed onto the surface and form both mono- and polymolecular adsorption layers with a thickness of up to several hundred nanometers, which depends on the conditions of layer deposition. It is shown that the first monolayer on the freshly deposited aluminum surface is adsorbed irreversibly because of the polymolecular adsorption of ethoxysilanes from the vapor phase, and the adsorption bonds are of van der Waals nature. The isotherm of silane adsorption is described by the Langmuir and BET equations. The energy of the adsorption of silanes and the energy of landing pads of individual molecules, which are determined from the literature, are given. It is established that covalent bonds between silanes and aluminum (Al–O–Si) are formed on the metal surface in the presence of adsorbed water, while modification of a metal deposited from an aqueous solution with organosilanes leads to multilayer adsorption of triethoxysilanes. It is shown that silane (silanol and siloxane) molecules displace adsorbed water from the surface of freshly deposited aluminum when they adsorb chemically on it and form the surface layer from siloxane oligomers, which is capable of inhibiting the hydration of the metal-oxide film and, as a result, can slow down the processes of metal corrosion. The information about the effect of organosilicon surface layers on the adhesion of the polymer coating deposited on the surface of aluminum and data on the resistance of the adhesion of a coating to the influence of water are given. Both mechanisms of promoting the dry adhesion of an epoxy coating on the surface of aluminum and increasing the adhesion resistance of epoxy to water are clarified. The results of studying the adsorption of organosilanes from an aqueous solution onto the surfaces of aluminum, zinc, and iron by the method of piezoquartz nanoweighing are given. The following adsorption isotherm approaches known from the literature are used to interpret the adsorption data: the Langmuir, BET, Flory–Huggins, multisite Langmuir, Temkin, Frumkin, and Freindlich approaches. It is shown that silanes displace adsorbed water from a metal surface during adsorption and occupy from two-and-a-half (on the surface of aluminum) to more than six (on the surface of zinc) adsorption sites on the metal surface. The surface orientation of adsorbed molecules is determined. The heats of adsorption of silanes, which are calculated by various methods, are given. It is shown that silanes are adsorbed on metal surfaces by forming chemical bonds and that organosilanes and corrosion inhibitors can adsorb jointly on the metal surface. Analysis of the literature on the use of mixtures of silanes and corrosion inhibitors shows that the treatment of a metal surface with mixtures of organosilanes and corrosion inhibitors gives rise to the formation of a surface layer with a chemical structure different from that of the layers obtained by the treatment of the surface with solutions containing individual components of the mixture, which effectively inhibits the corrosion of metal under the coating and substantially increases the adhesion of the coating to the metal, even under the conditions of high humidity.