Thermodynamics for complex formation of boric acid and borate with hydroxy acids and diols

Abstract

The complex formation of boric acid (B(OH)3) and borate (BOH4-) with glycolic acid, lactic acid, 2-hydroxybutyric acid, 3-hydroxybutyric acid, malic acid, and (s)-1,2-propanediol in aqueous solutions has been investigated using 11B NMR and mass spectroscopy. The formation constants and the thermodynamic parameters, ΔH° and ΔS°, of the 1:1 and 1:2 complexes of boric acid for the hydroxy acid (B-HA) systems were compared with those for the polyol (B-OL) systems previously reported, together with those for a polyol of (s)-1,2-propanediol presently examined. The 1:1 complex formation was enthalpically driven in both B-HA and B-OL systems. In the former, the enthalpic gain is caused by the structural change of boric acid, accompanied by the dehydration condensation of boric acid with hydroxy acid. In the latter, the enthalpic gain only arises from the dehydration condensation of borate with polyol. The ΔH° values for the B-HA and B-OL systems are comparable with each other, despite the different numbers of dehydration of water molecules, i.e., one and two water molecules, respectively. The formation of 1:2 complex was entropically driven in the B-HA systems, while that in the B-OL systems was enthalpically progressed. This difference between the two systems is attributed to the flexibility of isolated ligand molecules. Since the isolated hydroxy acids form the intramolecular hydrogen bond, the entropic gain of the hydroxy acids is generated as the hydroxy acid is bound to 1:1 complex due to the breaking of the intramolecular hydrogen bond. Thus, the stabilities of the complexes in the B-HA systems mainly depend on the gain for the freedom of ligand.