Specific surface free energies of strontium chlorapatite single crystal determined by contact angles of liquid droplets

Contact angles of water and formamide droplets on a well-formed Sr5Cl(PO4)3 crystal with an aspect ratio of 3.2 were observed. The specific surface free energies of the ( 1 0 1 ¯ 0 ) and ( 1 0 1 ¯ 1 ) faces, calculated using the approximation of Fowkes, are 35 ± 2 and 61 ± 1 mN/m, respectively. The stability of the ( 1 0 1 ¯ 0 ) face corresponds to the morphology of this crystal, which tends to elongate in the 〈0 0 0 1〉 direction. The wide distribution of the measured contact angles is ascribed to the variation in the steps on the surface. An ideal flat surface without step should have uniform specific surface free energy, estimated to be γter ⩽ 26 and ⩽50 mN/m for the ( 1 0 1 ¯ 0 ) and ( 1 0 1 ¯ 1 ) faces, respectively.