Subunit contribution model for thermodynamic properties of borates and its application in hydrothermal synthesis of MgBO₂(OH) nanowhiskers

Pure monoclinic MgBO₂(OH) nanowhiskers were synthesized by room temperature co-precipitation followed by hydrothermal treatment at 220-240°C for 6.0-30.0 h, using MgCl₂·6H₂O, H₃BO₃, and NaOH as raw materials with molar ratio of Mg:B:Na=2:3:4. Enlightened by mechanical mixture model and group contribution method, a novel subunit contribution model for the assessment of thermodynamic properties such as Δ_fH_m and Δ_fG_m of borates was developed, by introducing correction factor k and reasonably decomposing the complex double salt metal borates into constitutional subunits according to mass conservation. The as-developed model was utilized to assess Δ_fH_m and Δ_fG_m at various temperatures for precipitate Mg₇B₄O₁₃·7H₂O and hydrothermal product MgBO₂(OH), based on which the values of Δ_rH_m and Δ_rG_m of the co-precipitation of Mg₇B₄O₁₃·7H₂O and hydrothermal conversion to MgBO₂(OH) were obtained. The results show that, the room temperature co-precipitation can occur spontaneously whereas the hydrothermal conversion can only be performed spontaneously at high temperature. The feasibility for the co-precipitation as well as the hydrothermal conversion turns to be higher with the increase in the temperature and time. The subunit contribution model is beneficial for the analysis and prediction of controllable synthesis of other complex double salt nanostructures via the soft-chemistry based method.