Binding of polymers to calcite crystals in water: Characterization by isothermal titration calorimetry

The interaction of double hydrophilic block copolymers with calcite crystals is studied by isothermal titration calorimetry. These polymers are known to possess high activity as crystal growth modifiers for inorganic substances. Different titration protocols are used in order to decompose the total heat flow into several components arising from different physicochemical processes. This allows us to distinguish the effects due to the hydration of each of the polymer blocks, the interaction of the polymer with the ions, and the adsorption of the polymer to the exposed calcite surfaces. The observed concentration dependence of the heat of adsorption is consistent with a simple Langmuir type adsorption model, which leads to the numerical values -1.77 J/m² and 28 mM^-1 for the surface adsorption enthalpy and the equilibrium constant, respectively. The titration measurements reveal the existence of a strongly endothermic process, which we believe to arise from the binding of the polymer to a further solution species, presumably very small amorphous calcium carbonate clusters. This together with other thermodynamic data indicates that the structure of water plays an important, currently not fully recognized role in the control of mineralization processes.