Koga Yoshikata, Sebe Fumie, Minami Takamasa, Otake Keiko, Saitow Ken-ichi, Nishikawa Keiko
Department of Chemistry, The University of British Columbia, Vancouver, BC Canada, V6T 1Z1.
J Phys Chem B. 2009 Sep 3;113(35):11928-35. doi: 10.1021/jp901934c.
We study the mixing schemes or the molecular processes occurring in aqueous acetonitrile (ACN) and acetone (ACT) by near-infrared spectroscopy (NIR). Both solutions (any other aqueous solutions) are not free from strong and complex intermolecular interactions. To tackle such a many-body problem, we first use the concept of the excess molar absorptivity, epsilonE, which is a function of solute mole fraction in addition to that of wavenumber, nu. The plots of epsilonE calculated from NIR spectra for both aqueous solutions against nu showed two clearly separated bands at 5020 and 5230 cm(-1); the former showed negative and the latter positive peaks. At zero and unity mole fractions of solute, epsilonE is identically zero independent of nu. Similar to the thermodynamic excess functions, both negative and positive bands grow in size from zero to the minimum (or the maximum) and back to zero, as the mole fraction varies from 0 to 1. Since the negative band's nu-locus coincides with the NIR spectrum of ice, and the positive with that of liquid H(2)O, we suggest that on addition of solute the "ice-likeness" decreases and the "liquid-likeness" increases, reminiscent of the two-mixture model for liquid H(2)O. The modes of these variations, however, are qualitatively different between ACN-H(2)O and ACT-H(2)O. The former ACN is known to act as a hydrophobe and ACT as a hydrophile from our previous thermodynamic studies. To see the difference more clearly, we introduced and calculated the excess partial molar absorptivity of ACN and ACT, epsilon(E)(N) and epsilon(E)(T), respectively. The mole fraction dependences of epsilon(E)(N) and epsilon(E)(T) show qualitatively different behavior and are consistent with the detailed mixing schemes elucidated by our earlier differential thermodynamic studies. Furthermore, we found in the H(2)O-rich region that the effect of hydrophobic ACN is acted on the negative band at 5020 cm(-1), while that of hydrophilic ACT is on the positive high-energy band. Thus, the present method of analysis adds more detailed insight into the difference between a hydrophobe and a hydrophile in their effects on H(2)O.
我们通过近红外光谱（NIR）研究了乙腈（ACN）和丙酮（ACT）水溶液中发生的混合方案或分子过程。这两种溶液（以及任何其他水溶液）都存在强烈且复杂的分子间相互作用。为了解决这样一个多体问题，我们首先使用过量摩尔吸光系数εE的概念，它除了是波数ν的函数外，还是溶质摩尔分数的函数。根据两种水溶液的近红外光谱计算得到的εE对ν的曲线在5020和5230 cm⁻¹处显示出两个明显分开的谱带；前者显示负峰，后者显示正峰。在溶质的摩尔分数为零和单位摩尔分数时，εE与ν无关，恒为零。与热力学过量函数类似，随着摩尔分数从0变化到1，正负谱带的大小都从零增长到最小值（或最大值），然后再回到零。由于负谱带的ν轨迹与冰的近红外光谱重合，正谱带的ν轨迹与液态H₂O 的近红外光谱重合，我们认为加入溶质后“冰样性”降低，“液态性”增加，这让人联想到液态H₂O的双混合模型。然而，ACN - H₂O和ACT - H₂O之间这些变化的模式在性质上是不同的。根据我们之前的热力学研究，已知前者ACN表现为疏水剂，ACT表现为亲水剂。为了更清楚地看到差异，我们分别引入并计算了ACN和ACT的过量偏摩尔吸光系数ε(E)(N)和ε(E)(T)。ε(E)(N)和ε(E)(T)对摩尔分数的依赖性表现出性质不同的行为，并且与我们早期的微分热力学研究所阐明的详细混合方案一致。此外，我们发现在富水区域，疏水性ACN的作用体现在5020 cm⁻¹处的负谱带上，而亲水性ACT的作用体现在正的高能谱带上。因此，本分析方法为疏水剂和亲水剂对H₂O的影响差异提供了更详细的见解。
