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Spectrum of Excess Partial Molar Absorptivity. I. Near Infrared Spectroscopic Study of Aqueous Acetonitrile and Acetone

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posted on 03.09.2009, 00:00 by Yoshikata Koga, Fumie Sebe, Takamasa Minami, Keiko Otake, Ken-ichi Saitow, Keiko Nishikawa
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, εE, which is a function of solute mole fraction in addition to that of wavenumber, ν. The plots of εE calculated from NIR spectra for both aqueous solutions against ν 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, εE is identically zero independent of ν. 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 ν-locus coincides with the NIR spectrum of ice, and the positive with that of liquid H2O, we suggest that on addition of solute the “ice-likeness” decreases and the “liquid-likeness” increases, reminiscent of the two-mixture model for liquid H2O. The modes of these variations, however, are qualitatively different between ACN−H2O and ACT−H2O. 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, εNE and εTE, respectively. The mole fraction dependences of εNE and εTE show qualitatively different behavior and are consistent with the detailed mixing schemes elucidated by our earlier differential thermodynamic studies. Furthermore, we found in the H2O-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 H2O.