Solid–Liquid Phase Equilibrium in the Ternary Systems C6H5B(OH)2 + KCl/RbCl/CsCl + H2O at 298.15 and 348.15 K

The phase equilibria of ternary systems phenylboronic acid (C6H5B(OH)2) + KCl/RbCl/CsCl + H2O were investigated at 298.15 and 348.15 K using a direct isothermal dissolution equilibrium method with mannitol-NaOH titration. Solid phases were characterized by Schreinemakers’ wet method, XRD, Raman, IR, and TG-DSC. No new adducts were detected, in contrast to the stable adduct 2H3BO3·3CsCl formed in the H3BO3 + CsCl + H2O system. All three systems are of the simple eutectic type, with two solubility branches and one invariant point. C6H5B(OH)2 exerts a weak salting-in effect on the chlorides, independent of temperature. Conversely, the chlorides salt out C6H5B(OH)2 following the order CsCl < RbCl < KCl, an effect that increases with temperature. This pattern differs from H3BO3-alkali chloride systems (salting-in) but resembles H3BO3-alkaline earth chloride systems (salting-out). The salting-out is attributed to salt-induced reduction of water activity, which disrupts the hydration layer of the solute and promotes its dehydration condensation to triphenyl boroxine. From a crystallographic viewpoint, the layered structure of H3BO3 provides suitable sites for Cs+, whereas the three-dimensional network of C6H5B(OH)2 does not. This structural insight suggests that the layered 1,4- and 1,3-benzenediboronic acids may form similar adducts with Cs+, warranting further investigation.