Contact angle and adsorption behavior of carboxylic acids on α-Al2O3 surfaces

The hydrophilic character of aluminum oxide surfaces may be altered through coating such surfaces with carboxylic acids. The initially hydrophilic nature of the solid substrate changes towards a less hydrophilic character as the bulk concentration and the chain length of the acids increases. The acids employed in this work (propionic, valeric and enanthic) show a certain affinity to the liquid–gas, solid–liquid and solid–gas interfaces, being the relative adsorption on them competitive. The adsorption behavior of these carboxylic acids is experimentally investigated combining pendant drop tensiometry, contact angle measurements on α-Al2O3 polycrystalline ceramics and adsorption on particles in aqueous suspensions, as a function of the hydrocarbon chain length of the acids and their bulk concentration, at pH equal to the acids’ pKa. The hydrophilic character of the coated alumina decreases with the acids concentration upon a certain concentration beyond that, it increases. The minimum of hydrophilicity is reached right before bi-layer arrangements on the adsorption pattern of the acids on the solid substrates take place.

Sketch of the pattern of the adsorption of carboxylic acids on the different interfaces, when varying the acids bulk concentration.Figure optionsDownload high-quality image (47 K)Download as PowerPoint slideResearch highlights
► Carboxylic acids continuously adsorb at the interfaces between alumina, water and air.
► The bulk concentration of the acids determines their preferential adsorption.
► Maxima of contact angle correspond to a full monolayer of acids on the solid–gas interface.
► An adsorbed bi-layer of acids is responsible of further decreasing contact angles.
► Carboxylic acids show the lowest preference to adsorb on the solid–liquid interface.