Specific surface free energy and morphology of chlorapatite crystals studied by modified Wilhelmy method

Barium chlorapatite [Ba5Cl(PO4)3] and strontium chlorapatite [Sr5Cl(PO4)3] crystals were grown from a sodium chloride flux. The aspect ratios for these crystals were distributed in the range 1–9, and maximum number of crystals was observed in the range 3–4 for both chlorapatite crystals. The contact angle of water on (1 0 1̄ 0) face of each chlorapatite crystal was observed using a modified Wilhelmy method, where the change of liquid weight was measured instead of the crystal weight. The contact angle depends on the aspect ratio of the crystal. Both the advancing and the receding contact angles showed maximum value when the aspect ratios of the crystals were approximately 4 for both Ba5Cl(PO4)3 and Sr5Cl(PO4)3 crystals. The specific surface free energy of (1 0 1̄ 0) face was calculated using Neumann’s equation. The (1 0 1̄ 0) face of the crystals of the aspect ratio 4 has a minimum specific surface free energy, indicating that they have the most stable (1 0 1̄ 0) face when the aspect ratio is 4.