Incorporation of growth-inhibiting diphosphates into steps on the calcite cleavage plane surface

The interaction between the calcite cleavage plane surface and three different diphosphonate molecules has been studied employing methods based on interatomic potentials. The diphosphonates studied include 1-hydroxyethylidene-1,1-diphosphonate (HEDP) (which is known empirically to poison crystal growth) together with two more symmetrical analogues. We show that the diphosphonate molecules enter into a calcite surface or step (to which they are more strongly bound) by replacing two carbonate ions, provided that the carbonate-carbonate distance matches the distance between the phosphonate groups reasonably well. We suggest a microscopic mechanism of growth inhibition on the basis of calculations of the binding energy for a calcium carbonate unit in the vicinity of the inhibitor.