UV Resonance Raman Spectra and Molecular Orbital Calculations of Salicylic and Phthalic Acids Complexed to Al³⁺ in Solution and on Mineral Surfaces

Abstract

Salicylic and phthalic acids have been used to study the complexation of aromatic carboxylic acids with Al(III) in solution and on mineral surfaces. A method for studying aqueous and surface carboxylic acids/ metal complexes with UV Raman has been developed. The acids and Al(III) solutions were mixed in a 1:1 ratio at concentrations of 0.01 M. The mixtures were adjusted to pH 2.5 and 4 to be above and below the pK a1 values of the carboxylic acids. The pH values of the acid solutions were based on the pK a values for the different acids to examine the neutral and charged species. The carboxylic acids were also adsorbed onto Al 2 O 3 . Comparisons between solution and adsorbed complexes were examined. Various possible types of complexes for the metal/acid complexes were modeled with ab initio molecular orbital/density functional theory calculations. The assignments of Al(III)-acid complexes were based on the best fits between experimental and model vibrational frequencies. Results are compared to previous speciation models based on potentiometric data.

Molecular-Scale Study of Salicylate Adsorption and Competition with Catechol at Goethite/Aqueous Solution Interface

“…5,11 The shift in peak position was observed due to distortions of the salicylate structure upon adsorption on goethite surfaces. 21 The bands attributed to ν(CC) vibration were shifted from about 1623, 1593, 1487, 1458, and 1303 cm −1 in aqueous salicylate to 1610, 1574, 1477, 1462, and 1265 cm −1 once adsorbed, respectively.…”

“…This slight shift of 7 cm −1 might be associated with a weakening of the intramolecular hydrogen bond to the ph−OH group rather than surface complexation. 5,7 Hence, the most plausible configuration would be in monodentate coordination involving an oxygen atom from the carboxylate. 5,11,12 To justify the proposed surface complex, we employed the quantum chemical calculations for the adsorbed salicylate in five possible configurations including two M-M, one M-B, and two B-B structures (Figure 4).…”

“…The plausible interfacial structure was scrutinized by the agreement between the observed IR vibrations and DFT calculations (Table 2). 5,21 The statistics analysis of correlations between experimental and calculated frequencies is included in the Supporting Information (SI). The results showed that the M-M 1 configuration fits the observed complex much better than the others.…”

“…Some researchers suggest the M-B structure on TiO 2 involved both ph−OH and COO − groups. 8,9 Meanwhile, the B-B complexes were hypothesized by Jankovićet al 10 Upon salicylate adsorption on aluminum (hydr)oxides, surface configurations such as mononuclear mondentate (M-M), 5,11,12 M-B, 9 and B-B 5,12 were proposed. The inconsistency in the proposed surface complex motivated our research in molecularscale understanding of salicylate adsorption.…”

Dynamic adsorption process of phthalate at goethite/aqueous interface: An ATR-FTIR study

“…This carboxyl compound can readily adsorb onto minerals, thus altering its migration and susceptibility to transform in aquifers and soils. In addition, phthalate is deemed as an analog for natural organic matter (NOM) to investigate the mineral-NOM interactions, due to its high solubility in water, simple structure, as well as its similarity in functional groups to NOM [3][4][5][6]. Therefore, knowledge on the molecular-level mechanisms of phthalate adsorption is critical in understanding the interactions of NOM on minerals.…”