Supercritical water density fluctuations

In supercritical fluids, the possibility of local composition enhancements of cosolvent about a solute suggests that we should see enhancement of anion fluorescence if the water cosolvent clusters effectively about the 2-naphthol solute. Although in liquids the water concentration must be >30% to see anion emission, the higher diffusivity and density fluctuations in SCFs could allow stabilization of the anion at much lower water concentrations provided that the water molecules provide sufficient structure. Therefore the purpose of these experiments was to investigate 2-naphthol fluorescence in supercritical CO 2 with water cosolvent in the highly compressible region of the mixture to probe the local environment about the solute. 

In this subsection, we presented an approximate scheme to evaluate the contribution S/u. of the electron density fluctuation to the excess chemical potential. Although we saw that this contribution is minor for a QM water molecule in ambient and supercritical water, it should emphasized that 8p, can be treated quantitatively in the method of energy representation. Actually, the treatment of the electron density fluctuation is not directly possible in the PCM and RISM-SCF methods. Furthermore, the approximate 8p, is exact to second order in the solvent density and in the electron density fluctuation. Thus, when the effect of the electron density fluctuation is weak, the calculation of 5p, is expected to be accurate. 

It is interesting to note that the useful properties of supercritical water arise from the breakdown of the extensive HB network that is at least partly responsible for many of the anomalies of liquid water. We have discussed how the use of the idea inherent in the Widom line helps in understanding the large-scale fluctuations observed in supercritical water. Because of the large separation of timescales between vibrational relaxation and density relaxation, the vibrational line widths are influenced significantly by the transient density inhomogeneity present near the critical temperature. 

In this chapter, we briefly review some of our recent studies on structure and dynamics of normal and supercritical water based on first principles simulations. The work reviewed here was based on the methods of ab initio molecular dynamics for trajectory generation and time series analysis for frequency calculations. We consider normal water at room temperature and also supercritical water at three different densities ranging from 1.0 to 0.35 g cm at a temperature of 673 K. More details of the work reviewed here can be found in [16,17]. The next section of this chapter contains a brief account of the ab initio simulations and time series analysis. The results of the hydrogen-bonded structures and their relations to the vibrational frequencies and also the dynamics of the hydrogen bond and vibrational frequency fluctuations in normal and supercritical water are discussed in Section 14.4. This chapter is concluded in Section 14.5. 

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