Erlangen Graduate School in Advanced Optical Technologies (SAOT), University of Erlangen-Nuremberg, Paul-Gordan-Strasse 6, D-91052 Erlangen, Germany.
Phys Chem Chem Phys. 2011 May 28;13(20):9525-33. doi: 10.1039/c1cp20208a. Epub 2011 Apr 12.
The present study shows that dynamic light scattering (DLS) is capable of measuring mutual diffusion coefficients for binary mixtures of ionic liquids (ILs) with different molecular liquids over the complete composition range. Evidence is given that the light scattering signals are related to true molecular binary diffusion. The method stands out due to its ability to work non-invasively in macroscopic thermodynamic equilibrium with reasonable accuracy and within convenient measurement periods. Compared with other techniques, mixtures with distinctly higher viscosities can be probed. For exemplary binary mixtures of 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO(4)]) with acetone, acetonitrile, dichloromethane, ethanol, or water as well as of 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][MeSO(3)]) with acetone, water, or methanol, mutual diffusivity data were measured over a wide range of composition at a temperature of 293.15 K. In general, the mutual diffusivity increases with increasing mole fraction of the molecular liquid and similarities to aqueous solutions of classical inorganic salts can be found. The characteristic behavior of the mutual diffusion coefficients is influenced by the nature of the chosen molecular liquid. For IL water mixtures, low light scattering intensities were observed despite the large refractive index difference of the pure components. The reason for this behavior may be the existence of water clusters in the mixtures. Additional measurements for IL acetone mixtures at temperatures ranging from 278.15 K to 323.15 K showed that the temperature dependence of the mutual diffusivity can be represented by Arrhenius functions and is increasing for decreasing mole fractions of acetone.
本研究表明，动态光散射（DLS）能够测量具有不同分子液体的离子液体（IL）二元混合物的相互扩散系数，涵盖整个组成范围。有证据表明，散射信号与真实的分子二元扩散有关。该方法的突出特点是能够在宏观热力学平衡中进行非侵入式测量，具有合理的准确度和方便的测量周期。与其他技术相比，该方法可以探测具有明显更高粘度的混合物。对于 1-乙基-3-甲基咪唑硫酸乙酯([EMIM][EtSO4])与丙酮、乙腈、二氯甲烷、乙醇或水以及 1-乙基-3-甲基咪唑甲烷磺酸盐([EMIM][MeSO3])与丙酮、水或甲醇的典型二元混合物，在 293.15 K 的温度下，在很宽的组成范围内测量了相互扩散系数。一般来说，相互扩散系数随分子液体摩尔分数的增加而增加，并且可以发现与经典无机盐的水溶液相似的规律。相互扩散系数的特征行为受到所选分子液体的性质的影响。对于 IL-水混合物，尽管纯组分的折射率差异很大，但观察到的光散射强度较低。这种行为的原因可能是混合物中存在水分子簇。在 278.15 K 至 323.15 K 的温度范围内对 IL-丙酮混合物进行的其他测量表明，相互扩散系数的温度依赖性可以用阿仑尼乌斯函数来表示，并且随着丙酮摩尔分数的降低而增加。
