Interactions between molecules and solid metal-oxide surfaces play crucial role in many scientific applications. Rutile surfaces are known for their photocatalytic applications, whereas oxalic acid is used in cleaning and dishwashing products, paints and coatings, and also is a surface active agent. The adsorption of oxalate on rutile was evaluated using macroscropic pH titrations, performed from 10 to 150°C in NaCl media. Oxalate increases the magnitude of proton charge curves; moreover, oxalate adsorption is enhanced at elevated temperature. At all temperatures, adsorption of oxalate commences at pH above the pHznpc value. All experimental results were rationalized using a CD-MUSIC model combination. From the molecular point of view, oxalic acid (COOH)2 and its deprotonated forms, i.e. hydrogenoxalate (bioxalate) HOOCCOO- and oxalate (COO-)2 ions represent the only molecules with a direct bond between two carboxylic carbons. We have optimized the classical molecular dynamics parameters of these molecules to match our AIMD data. Applying the scaled charges and modified vdW parameters according to the Electronic continuum correction (ECC) was found to significantly improve model performance. Our models of nonhydroxylated and hydroxylated (110) rutile surfaces were extended to a charge density +0.2 C/m2, allowing us to study oxalate adsorption between pH values 2 and 12. The molecular level description of the surface interactions is provided by molecular dynamics simulations yielding adsorbed amounts, binding patterns and the influence of pH. Advanced sampling techniques, namely potential of mean force determination and metadynamics, were applied. Inner-sphere vs. outer-sphere binding motifs will be discussed and compared with information inferred from published ATR-FTIR spectra.

Adsorption of oxalic acid on rutile surfaces

255th National Meeting (2018)

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