Thermal and spectroscopic study to investigate p-aminobenzoic acid, sodium p-aminobenzoate and its compounds with some lighter trivalent lanthanides
Introduction
p-Aminobenzoic acid (H-pABA), whose molecular formula is C7H7NO2 and which has a molar mass of 137.14 g mol−1, is a cyclic amino acid obtained as a crystalline powder or white/white-yellowish needles. It is also known as vitamin B10. Although the human organism does not synthesize it, H-pABA is a component which is present in foods. It is produced by essential symbiotic bacteria and is metabolized constantly in the human body [1], [2], [3]. In lower concentrations it is a precursor of folic acid, and in high concentrations its role has been studied in the inhibition of various bacteria and viruses, showing anticoagulant, antioxidant and immunomodulatory properties. It is an one active ingredient in formulations against ultra-violet radiation [2], [3]. In vivo and in vitro assays have demonstrated the efficacy of H-pABA in the treatment of ocular viral herpes; its efficacy is primarily related to its capacity to induce the synthesis of endogenous interferon in the human body [2], [3].
Several studies of the coordination compounds formed by metallic ions and organic substances, such as pharmaceuticals and benzoic acid derivatives, have been reported [4], [5], [6], [7], [8], [9], [10], [11]. These types of compounds have a great structural variety and also the ability to be used in new technological applications. Such compounds present great potential to be employed in the pharmaceutical area as biomarkers and new drugs, as industrial lasers and luminescent materials and in catalysis, among other applications [8], [9], [10], [11], [12], [13].
The literature contains reports of the thermal decomposition and the spectroscopic study of m-aminobenzoic acid coordination compounds with lanthanide ions and yttrium. One such study indicates the formation of compounds presenting 4 to 6 water molecules, with coordination occurring between the amino and carboxylate groups, to form a polymeric structure [14]. The coordination compounds formed between Eu(III) and Tb(III) ions with aminobenzoates presented different luminescent properties due to the strong electron donor effect of the NH2 group in the ortho, meta and para positions of the benzene ring [15]. A fluorescence lifetime of 0.58 ms and emission quantum yield of 0.67 were obtained for the [Tb(pABA)3(H2O)2]·2H2O complex [16]. Single crystals were obtained by the association of H-pABA and lanthanides, except Gd(III), Nd(III) and Tm(III), forming polymeric (La, Ce, Pr, Sm, Eu, Tb, Dy and Er) and binuclear (Tb, Ho, Yb, Lu and Y) structures depending on the pH of the prepared solutions. The structures were determined by XRD, elemental analysis, infrared spectroscopy and thermogravimetric analysis [17].
Although many studies in the literature describe the synthesis and characterization of coordination compounds formed between p-aminobenzoic acid and lanthanide ions, there is no systematic study investigating the thermal behavior of these materials in the solid state. The present study describes the synthesis, characterization and thermal behavior of p-aminobenzoic acid, sodium p-aminobenzoate and some lighter trivalent lanthanide p-aminobenzoates in the solid-state. The characterization was performed using thermoanalytical techniques (TG–DTA, DSC, EGA) and complementary techniques (complexometry, elemental analysis, X-ray diffraction (XRD), diffuse reflectance (DR), mid-infrared spectroscopy (MID) and near-infrared region spectroscopy (NIR)).
Section snippets
Synthesis
The p-aminobenzoic acid (H-pABA) C7H7NO2 used in this study was of 99% purity and was obtained from Sigma; it was used as received. Aqueous solution of sodium p-aminobenzoate (Na-pABA) 0.1 mol L−1 was prepared by neutralization of aqueous suspension of H-pABA with sodium hydroxide solution 0.1 mol L−1.
Lanthanide chloride solutions were prepared from the corresponding metal oxides (except for cerium) by treatment with concentrated hydrochloric acid, following the procedure described in the
H-pABA
Simultaneous TG–DTA curves in dynamic dry air and N2 atmospheres of H-pABA are shown in Fig. 1a (air) and b (N2). In both atmospheres, these curves showed total mass losses in two steps between 120–550 °C (air) and 120–510 °C (N2) with losses of 95.91 and 4.09% (air) or 96.05 and 3.95% (N2). In both atmospheres the beginning of mass loss that occurred slowly between 120 and 180 °C, with loss of 4.75% (air) and 4.69% (N2), and without any thermal event on the DTA curve, was attributed to the
Conclusion
Using TG, complexometry and elemental analysis results it was possible to establish a general formula for the synthesized compounds.
The simultaneous TG–DTA and DSC curves provided previously unreported information about the thermal stability and thermal decomposition of these compounds in dynamic dry air and nitrogen atmospheres.
The X-ray powder patterns showed that all the compounds have a crystalline structure, with evidence of the formation of isomorphous compounds.
The middle and near
Acknowledgments
The authors thanks FAPESP (Proc. 2013/09022-7), CNPq, and CAPES foundations (Brazil) for financial support.
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