Contact angle of water droplet on apatite single crystals

Contact angles of water droplets on well-formed crystals of strontium and barium chlorapatites, Sr5Cl(PO4)3 and Ba5Cl(PO4)3, were observed. The contact angles of water on (1 0 1̄ 0) and (1 0 1̄ 1) faces of Sr5Cl(PO4)3 were 74±8° and 53±5° and those on (1 0 1̄ 0) and (1 0 1̄ 1) faces of Ba5Cl(PO4)3 were 52±5° and 33±1°, respectively. The surface tensions of the crystals were calculated using Neumann’s equation. They were 39.2±50 and 52.0±3.0 mJ m−2 for (1 0 1̄ 0) and (1 0 1̄ 1) faces of Sr5Cl(PO4)3, 52.5±2.9 and 63.0±0.5 mJ m−2 for (1 0 1̄ 0) and (1 0 1̄ 1) faces of Ba5Cl(PO4)3, respectively. The (1 0 1̄ 1) face has larger surface tension than (1 0 1̄ 0) face for both crystals. The chlorapatite crystals have tendency to elongate in 〈0 0 0 1〉 directions during the crystal growth process, indicating that (1 0 1̄ 0) face is more stable than (1 0 1̄ 1) face. This nature of crystal morphology is consistent with the surface tensions estimated from the water contact angles. The higher density of Ba5Cl(PO4)3 than Sr5Cl(PO4)3 is considered to cause the smaller contact angles of water droplet on Ba5Cl(PO4)3 crystal than that on Sr5Cl(PO4)3 crystal because the attractive force between the heavier atoms brings the larger surface tension of solid.