Isotope effects observed in diluted D2O/H2O mixtures identify HOD-induced low-density structures in D2O but not H2O

Abstract

Normal and heavy water are solvents most commonly used to study the isotope effect. The isotope effect of a solvent significantly influences the behavior of a single molecule in a solution, especially when there are interactions between the solvent and the solute. The influence of the isotope effect becomes more significant in D₂O/H₂O since the hydrogen bond in H₂O is slightly weaker than its counterpart (deuterium bond) in D₂O. Herein, we characterize the isotope effect in a mixture of normal and heavy water on the solvation of a HOD molecule. We show that the HOD molecule affects the proximal solvent molecules, and these disturbances are much more significant in heavy water than in normal water. Moreover, in D₂O, we observe the formation of low-density structures indicative of an ordering of the solvent around the HOD molecule. The qualitative differences between HOD interaction with D₂O and H₂O were consistently confirmed with Raman spectroscopy and NMR diffusometry.

Figure 1

Temperature-dependence of density-molality relationship observed in a single dilution experiment for D₂O in H₂O (left) and H₂O in D₂O (right). The density dependence on molality measured at six temperatures of 20 to 45 °C was analyzed according to Eqs. (6–9). All data snapshots obtained for four independent series of dilution experiments for D₂O in H₂O and H₂O in D₂O are shown in Supplementary Figs. S1 and S2, respectively.

Figure 2

The temperature dependence of the density of pure H₂O and D₂O determined in the range of 20–45 °C. The relationships obtained by us (solid lines) are compared with the literature data for H₂O (black diamonds) and D₂O ^, (black circles). The values estimated from binary H₂O/D₂O solvents are denoted in magenta.

Figure 3

Temperature dependence of the partial molar volume determined from density measurements. The data obtained in H₂O and D₂O are in red and blue, respectively. Triangles represent the HOD molecule, while circles denote bulk solvent (H₂O or D₂O).

Figure 4

Black, solute-correlated spectra for HOD in D₂O (left) and H₂O (right). Red, the contributions from the isolated OD/OH stretching vibration. Blue, the spectrum assigned to solvent molecules perturbed by HOD.

Figure 5

Temperature dependence of self-diffusion coefficients. The activation energies and diffusion coefficients at 25 °C are listed in Table 1. The models fitted to the individual data are shown in Fig. S6.