Introduction: Although specific surface free energy (SSFE) of inorganic crystal is discussed theoretically in many cases, experimental measurement of SSFE of inorganic is seldom performed. The measurement of SSFE of inorganic crystal is bereaved to be difficult. Recently, we determined the SSFE of some inorganic crystals, ruby, quartz, and apatite, using contact angle of liquid droplet. This time, we considered the basic meaning of SSFE, especially, polar and dispersion component of SSFE, and chose simple alkali halide crystals.

Experimental: Four kinds of alkali halide single crystals, NaCl, KCl, NaBr, and KBr, were used as sample crystals. Each alkali halide was saturated in water of 100mL, and kept it in incubator. Two or more well formed crystals were obtained, and the contact angle of liquid droplet on (100) face of the crystals were measured by the sessil drop method. Formamide and glycol was used as liquid droplet.

Result and discussion: Dispersion component and the polar component of SSFE were calculated using Young’s equation, Fowks approximation and Wu’s Harmonic mean equation. The calculated SSFE ware 88.63, 88.86, and 80.11mN/m for NaCl, NaBr and KBr, respectively. The polar components of SSFE ware 1.15, 1.08, and 0.81 mN/m for NaCl, NaBr, and KBr respectively. On the other hand, the distance between neighbor ions is 0.279, 0.294, and 0.329 nm for NaCl, NaBr, and KBr, resoectively. Therefore, the crystals with larger distance between neighbor ions have smaller SSFE. Such relationship between the polar component of SSFE and the neighboring ion distance can be explained by the rumpling of the crystal surface.

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