Molecular Recognition Effect in Monolayer Formation of Oleate on Fluorite

Abstract

Quantitative evaluation of self-assembled submonolayers (composition, orientation, and adsorbed amount) of oleate on polished and cleaved (111) fluorite surfaces has been carried out by means of infrared external reflection spectroscopy. We have found a preferential type interaction of the carboxyl group with calcium atoms on the cleaved fluorite surface with an orientation angle of 80° (angle between the asymmetric stretching vibration of the COO group and the surface normal), forming bidentate-like bonding. This form is characterized by the absorbance band at 1536 cm-1. It should be noticed that it is not a classical bidentate form with exactly the same O−Ca distances in the carboxylate bonding. There is still a difference in the distance between two oxygen atoms of the carboxylate group and the surface calcium. This surface product dominates (up to 75% of the total adsorbed molecules) on the fluorite surface prepared by cleavage at submonolayer coverages. This is the result of the molecular recognition of the surface calcium atoms on the (111) plane of fluorite by oleate ions from solution. The second calcium oleate surface form is characteristic of very different distances between oxygen and calcium atoms, i.e., the unidentate-like form with the absorbance band at 1575 cm-1. This form shows an average orientation angle to surface normal of about 60°. It has been found that the amount of this surface form is related to the amount of crystallographic defects present on the real cleavage plane of the mineral sample. The perturbation, found naturally or occurring during sample preparation (polishing), of the perfect cleavage plane results in the formation of both unidentate- and bidentate-like carboxylate surface complexes, as well as the intermediate forms. Other surface processes that could influence the molecular recognition interaction and the surface organization of the adsorbed molecules have been discussed.