Barman S, Vasudevan S
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-566012, India.
J Phys Chem B. 2006 Nov 16;110(45):22407-14. doi: 10.1021/jp064306p.
The melting of alkyl chains in the saturated fatty acid zinc soaps of different chain lengths, Zn(C(n)H(2n+1)COO)(2); n = 11, 13, 15, and 17, have been investigated by powder X-ray diffraction, differential scanning calorimetry, and vibrational spectroscopy. These compounds have a layer structure with the alkyl chains arranged as tilted bilayers and with all methylene chains adopting a planar, all-trans conformation at room temperature. The saturated fatty acid zinc soaps exhibit a single reversible melting transition with the associated enthalpy change varying linearly with alkyl chain length, but surprisingly, the melting temperature remaining constant. Melting is associated with changes in the conformation of the alkyl chains and in the nature of coordination of the fatty acid to zinc. By monitoring features in the infrared spectra that are characteristic of the global conformation of the alkyl chains, a quantitative relation between conformational disorder and melting is established. It is found that, irrespective of the alkyl chain length, melting occurs when 30% of the chains in the soap are disordered. These results highlight the universal nature of the melting of saturated fatty acid zinc soaps and provide a simple explanation for the observed phenomena.
通过粉末X射线衍射、差示扫描量热法和振动光谱法，研究了不同链长（n = 11、13、15和17）的饱和脂肪酸锌皂Zn(C(n)H(2n+1)COO)(2)中烷基链的熔化情况。这些化合物具有层状结构，烷基链排列成倾斜的双层，且在室温下所有亚甲基链均采用平面全反式构象。饱和脂肪酸锌皂呈现单一的可逆熔化转变，其相关的焓变随烷基链长度线性变化，但令人惊讶的是，熔化温度保持恒定。熔化与烷基链构象的变化以及脂肪酸与锌配位性质的变化有关。通过监测红外光谱中表征烷基链整体构象的特征，建立了构象无序与熔化之间的定量关系。研究发现，无论烷基链长度如何，当皂中30%的链无序时就会发生熔化。这些结果突出了饱和脂肪酸锌皂熔化的普遍性质，并为观察到的现象提供了一个简单的解释。
