Netsu Sokutei, 41 (3), p. 93, (2014)



Effect of Crystal Thickness on Phase Transitions of n-Alkyl Alcohols

Phase transition behaviors of thin film of n-alkyl alcohols (1-heptadecanol and 1-octadecanol) with sample masses from 2 μg to 1 mg were investigated by the high sensitive DSC. The shape of endothermic peaks changed at 0.1 and 0.03 mg for both alcohols, accompanied with the transition temperature shift to the lower side. The transition enthalpy values of the γ-α transition and the melting for 1-octadecanol were determined as the average over the samples larger than 0.05 mg, the γ-α transition (329.6 K, 23.9 kJ/mol), and the melting (330.1 K, 42.5 kJ/mol). The relationship between thermodynamic quantities and alkyl carbon number showed the odd-even effect for the transition temperature and the transition enthalpies for both the solid phase transition and the melting. The entropy increment at one methylene unit for the melting was 9.6 J/Kmol, showing a good correlation with ΔS = Rln 3 = 9.13 J/Kmol. The morphological observation of crystals at 0.1 and 0.005 mg samples by a differential interference contrast microscope and an atomic force microscope indicated the coexistence of polycrystalline blocks (> 20 μm) and plate shaped crystals (< 20 μm) for the 0.1 mg sample, and only plate shaped crystals (< 20 μm) for the 0.005 mg sample. These facts suggested that the former crystals showed thermal behavior of bulk sample, and the latter crystal showed the effect of surface on transition temperatures of thin sample.