Direct experimental measurement of water interaction energetics in amorphous carbonates MCO₃ (M = Ca, Mn, and Mg) and implications for carbonate crystal growth

Interaction of carbonate surfaces with water plays a crucial role in carbonate nucleation and crystal growth. This study provides experimental evidence for the existence of two different types of water having distinct energetics in amorphous carbonates, MCO₃ (M = Ca, Mn, and Mg). The adsorption enthalpy curves obtained using a combination of gas sorption and microcalorimetry show two different energetic regions, which correspond to weakly bound restrictedly mobile and strongly bound rigid H₂O components. For weakly bound water, adsorption enthalpies of amorphous calcium carbonate (ACC) (-55.3 ± 0.9 kJ/mol), amorphous manganese carbonate (AMnC) (-54.1 ± 0.8 kJ/mol), and amorphous magnesium carbonate (AMgC) (-56.1 ± 0.4 kJ/mol) fall in the same range, suggesting their interaction modes may be similar in all amorphous phases. Water adsorption enthalpies of crystalline nanocalcite (-96.3 ± 1 kJ/mol) and nano-MnCO₃ (-65.3 ± 3 kJ/mol) measured in previous studies are more exothermic than values for ACC (-62.1 ± 0.7 kJ/mol) and AMnC (-54.1 ± 0.8 kJ/mol) and could provide a driving force for crystallization of ACC and AMnC in the presence of water. The differences in water adsorption behavior between amorphous and naocrystalline material have significant implications for crystal growth, biomineralization, and carbonate geochemistry.