長鎖アルコールの相転移に及ぼす結晶厚みの効果

摘要

1-heptadecanol （C_（17）H_（35）OH）と1-octadecanol（C_（18）H_（37）OH）の薄膜の相転移とモルフォロジーを高感度DSCとAFMで検討した。C_（17）H_（35）OH薄膜試料の昇温過程で観察される固相転移温度と融解温度は，質量0.03 mg以下になると低温側にシフトし固相転移温度のシフト量が融解温度よりも大きい。質量0.03 mg以上の薄膜試料の固相転移と融解の転移エンタルピーはバルクと同じ値であつたが，質量0.03 mg以下になると転移エンタルピーは減少した。%Phase transition behaviors of tnin film of n-alkyl alcohols (1-heptadecanol and 1-octadecanol) with sample masses from 2 μg to 1 mg were investigated by the nigh 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 = R ln 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 oolycrystalline blocks (＞ 20 μm) and plate shaped crystals (＜ 20 μm) for the 0.1mg 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.