Grupo de Nanomateriales y Materia Blanda, Departamento de Física de la Materia Condensada, Universidade de Santiago de Compostela, Campus Vida s/n E-15782, Santiago de Compostela, Spain.
J Phys Chem B. 2012 Sep 13;116(36):11302-12. doi: 10.1021/jp3066822. Epub 2012 Sep 5.
We report a systematic study of the effect of the cationic chain length and degree of hydrogen bonding on several equilibrium and transport properties of the first members of the alkylammonium nitrate protic ionic liquids (PILs) family (ethylammonium, propylammonium, and butylammonium nitrate) in the temperature range between 10 and 40 °C. These properties were observed by means of several experimental techniques, including density, surface tension, refractometry, viscosimetry, and conductimetry. The dilatation coefficients and compressibilities, as well as the Rao coefficients, were calculated, and an increase of these magnitudes with alkyl chain length was detected. Moreover, the surface entropies and enthalpies of the studied PILs were analyzed, and the temperature dependence of the surface tension was observed to be describable by means of a harmonic oscillator model with surface energies and critical temperatures that are increasing functions of the cationic chain length. Moreover, the refractive indexes were measured and the thermo-optic coefficient and Abbe numbers were calculated, and the contribution of the electrostrictive part seemed to dominate the temperature dependence of the electric polarization. The electric conductivity and the viscosity were measured and the influence of the degree of hydrogen bonding in the supercooled liquid region analyzed. Hysteresis loops were detected in freezing-melting cycles and the effect of the length of the alkyl chain of the cation on the size of the loop analyzed, showing that longer chains lead to a narrowing of the supercooled region. The temperature dependence of the conductivity was studied in the Vogel-Fulcher-Tamman (VFT) framework and the fragility indices, the effective activation energies, and the Vogel temperatures obtained. A high-temperature Arrhenius analysis was also performed, and the activation energies of conductivity and viscosity were calculated, showing that these transport processes are governed by two distinct mechanisms. The exponents of the fractional Walden rule for the different compounds were obtained. Finally, the ionicities and fragilities of the studied PILs were analyzed, proving that all the studied PILs are subionic and fragile liquids, with propylammonium nitrate showing the lowest fragility and the greater ionicity of all the studied compounds.
我们报告了一项关于阳离子链长和氢键程度对一系列烷基铵硝酸盐质子离子液体（PILs）家族（乙基铵、丙基铵和丁基铵硝酸盐）第一成员的若干平衡和输运性质影响的系统研究。这些性质是通过多种实验技术观察到的，包括密度、表面张力、折射仪、粘度计和电导率仪。计算了膨胀系数和压缩系数，以及 Rao 系数，发现这些量随着烷基链长的增加而增加。此外，还分析了研究中的 PILs 的表面熵和焓，观察到表面张力的温度依赖性可以用表面能和临界温度的谐振子模型来描述，而表面能和临界温度随着阳离子链长的增加而增加。此外，还测量了折射率，并计算了热光系数和阿贝数，电致伸缩部分似乎主导了电极化的温度依赖性。测量了电导率和粘度，并分析了超冷液体区域中氢键程度的影响。在冻结-熔化循环中检测到滞后环，并分析了阳离子烷基链长度对环大小的影响，结果表明较长的链导致过冷区变窄。在 Vogel-Fulcher-Tamman（VFT）框架中研究了电导率的温度依赖性，并获得了脆性指数、有效活化能和 Vogel 温度。还进行了高温 Arrhenius 分析，并计算了电导率和粘度的活化能，表明这些输运过程由两个不同的机制控制。得到了不同化合物的分数 Walden 规则的指数。最后，分析了研究中的 PILs 的离子性和脆性，证明所有研究的 PILs 都是亚离子和脆弱的液体，其中丙基铵硝酸盐具有所有研究化合物中最低的脆性和最大的离子性。
