The study of aqueous isopropanol solutions at various concentrations: low frequency Raman spectroscopy and molecular dynamics simulations

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Abstract

The low-frequency Raman spectra of aqueous isopropanol mixtures at different proportions have been recorded. The influence of the isopropanol concentration on the position of the peak at ν̄2=190 cm−1 associated to hydrogen bond between water molecules has been analyzed. The small shift to lower frequencies of the ν̄2 band indicates that isopropanol has a small effect of a structure breaker of hydrogen bond between water molecules. This result is correlated to the degree of self aggregation at a microscopic level of isopropanol.

Introduction

Urea aqueous solutions have the property to denature a protein [1], [2] as well as to increase the solubility of hydrocarbon [3]. To understand how urea affects these properties, it is interesting to compare the effect of urea with similar iso-steric molecules possessing the same planar Y-shape as urea but differing in both polarity and hydrophobicity. The isopropanol molecule was chosen to allow this comparison because it has the same Y-shape geometry and a large dipole moment with respect to urea. This comparison will be drawn on the basis of the analysis of the low frequency Raman spectra of isopropanol aqueous solutions. This paper is organized as follows: in Section 2 we will give a theoretical and experimental basis for the interpretation of the low frequency Raman spectra and in Section 3 we describe shortly the experimental methods. In Section 4, the low frequency Raman spectra and the results of the fitting procedure are presented and discussed. Finally, we draw some general conclusions.

Section snippets

Hypothesis for the interpretation of the low frequency Raman spectra

Molecular dynamics simulations [4], [5] have shown that the low frequency Raman spectrum of water can be interpreted in terms of the translational dynamics of water molecules. In fact, this contribution lies in the frequency range between 15 and 300 cm−1. The contribution of the rotational dynamics is in the frequency range centered at about 600 cm−1. These results show that rotational and translational motions are uncoupled. The contribution of the reorientational dynamics has a contribution

Experimental methods

The Raman experiments were performed at room temperature by the use of 514.5 nm radiation from an argon laser operating at power levels in the range of 250–350 mW. The Raman signal was analyzed with the use of a Coderg T800 triple monochromator coupled to a photomultiplier and photon counting electronics. As the instrumental spectral width was usually 0.5 cm−1, much smaller than that of the band profiles observed in these experiments, its influence was assumed to be negligible. The 90° geometry

Results and discussions

The Low frequency Raman spectra of isopropanol solutions at different mole fractions of isoprpanol are presented in Fig. 1. We report in Fig. 2, the evolution of the ν̄2 at different mole fractions of the isopropanol. We suppose that in the mole fraction range of isopropanol between 0.0 and 0.05, the main contribution to the low frequency Raman spectra is due to water molecules. Thus, the shift to lower frequencies of the ν̄2 band indicates that they act as a structure breaker of hydrogen bonds

Conclusion

The low frequency Raman spectra of isopropanol aqueous solutions have been recorded. The small shift to lower frequencies of the peak at 190 cm−1 associated with hydrogen bond between water molecules indicates that these solutes have the effect of a small structure breaker of hydrogen bond between water molecules. This result is corroborated by the shift of the ν̄OH to higher frequencies.

This suggests here that the property of a structure breaker of the isopropanol (and of other solutes in

Acknowledgements

Institut du Développement et des Ressources en Informatique Scientifique is thankfully acknowledged for the CPU time allocation. The Centre d'Etudes et de Recherches Lasers et Applications is supported by the following organisations, Ministère Chargé de La Recherche, Région Nord/Pas de Calais, and Les Fonds Européen de Développement Economique des Régions.

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