Adsorption of sodium hexanoate on α-alumina

Abstract

a b s t r a c tNeutron reflection and adsorption isotherm measurements have been used to study the adsorption behaviour of hexanoic acid onto a-alumina surfaces. Importantly, the pH dependence of the behaviour has been characterised with a pronounced maximum in adsorption identified at a pH of approximately 5, close to the pK a of the acid. The adsorbed layer is identified as a bilayer, which is reasonable given the hydrophilic nature of both side of the layer, and has a thickness of 13 Å, suggesting significant extent of interdigitation. At pH 5, the layer has much lower extent of hydration relative to the higher pH of 7, consistent with the increased total adsorption at pH 5. A number of different mechanisms for the binding of the hexanoic acid to the surface are considered. The experimental data, combined with calculations using equilibrium/binding constants of the surface and ligands, indicates that a ligand exchange reaction may be the most significant mechanism.

Synthetic Bilayers on Mica from Self-Assembly of Hydrogen-Bonded Triazines

“…This structure assumes that the triazine “head group” engages in hydrogen bonding between the NH groups and water adsorbed at the surface of the mica. A bilayer of this type is proposed for hexanoate molecules hydrogen-bonded to alumina based on neutron reflectivity measurements . The second arrangement in Figure b is a bilayer with 1.7 nm thickness and the hydrophobic alkyl chains at the exterior.…”

Neutron Reflectometry for Studying Corrosion and Corrosion Inhibition

“…As the neutron beam comes in through the substrate and is reflected back through it, the substrate must be sufficiently transparent to neutrons to prevent significant attenuation. This requirement has led to silicon/silica and alumina being more extensively studied than other materials as they have excellent neutron transmission [69][70][71][72]. They can also be obtained as single crystals of relatively large dimensions that can be polished very flat (∼<5 Å).…”

“…Unsurprisingly, therefore, aluminium has been fairly well studied using this technique, with a body of work looking at phenomena such as adsorption of organic molecules and the behaviour of the adsorbed materials under shear condition [39,[69][70][71][72][83][84][85][86]. A small handful of studies relevant to corrosion also exist, as outlined below.…”

Unusual Maximum in the Adsorption of Aqueous Surfactant Mixtures: Neutron Reflectometry of Mixtures of Zwitterionic and Ionic Surfactants at the Silica–Aqueous Interface

“…Such adsorption is illustrated by the adsorption of sodium dodecylsulfate (SDS) as a complete bilayer on sapphire, and by the adsorption of a phosphate derivative of AOT on sapphire . In the former, an adjustment of charge at the surface is thought to be associated with a maximum in the adsorption at the CMC, which was attributed to adsorption only of the surfactant ion, with the counterion being preferentially incorporated into the micelles of the solution with increasing concentration. − A maximum with pH has also been found for the weak anionic hexanoic acid on alumina, which was attributed to the replacement of OH – groups on the surface with RCO 2 – …”

“…In the former, an adjustment of charge at the surface is thought to be associated with a maximum in the adsorption at the CMC, which was attributed to adsorption only of the surfactant ion, with the counterion being preferentially incorporated into the micelles of the solution with increasing concentration. − A maximum with pH has also been found for the weak anionic hexanoic acid on alumina, which was attributed to the replacement of OH – groups on the surface with RCO 2 – …”