The production of stable hydrophobic surfaces by the adsorption of hydrocarbon and fluorocarbon carboxylic acids onto alumina substrates

The adsorption of various carboxylic acids onto alumina substrates has been studied as a model system for the study of the long range hydrophobic interaction. The adsorption was studied using classical adsorption isotherms, contact angle measurements and colloid probe atomic force microscopy. Carboxylic acids form esters with the hydroxyl groups present on the alumina surface and produce surfaces with a range of hydrophobicities and stabilities depending on the acid's functionality. Hydrocarbon carboxylic acid adsorption was found to be adequately described by the Langmuir adsorption model and produced stable surfaces of varying hydrophobicities which resisted dissolution by water. By comparison, hydrophobic surfaces produced with fluorinated acids adsorbed from aqueous solution were not always stable, although their stability was greatly improved when the chemisorption was carried out in a non-aqueous solvent at elevated temperatures. Adsorption of these fluorinated carboxylic acids produced the most hydrophobic surfaces. Interaction forces between alumina surfaces coated with adsorbed propionic acid were as expected from DLVO theory with no evidence of hydrophobic forces.