Interaction between ions measured by small angle x-ray scattering (SAXS) as a function of momentum transfer Q for R4NBr aqueous solutions; note the differing Q-ranges on the abscissa. SAXS intensities of (a) Bu4NBr (concentration c = 0, 0.5, 0.75, 1.0, 1.5m from bottom to top) and (b) Pr4NBr (c = 0, 0.5, 1.0, 1.5, 2.0m) solutions. The diffraction peaks are indicated by the full-line arrow and can be related to strong cation-cation pair correlations with separation about d = 2π/Q mediated by solvent around large hydrophobic solutes as described in the text. The lack of small-angle intensity enhancement excludes the possibility of ion aggregation. SAXS intensities of (c) Et4NBr and (d) Me4NBr solutions (c = 0, 0.5, 1.0, 2.0, 3.0m). In contrast to solutions of larger Bu4NBr and Pr4NBr molecules, no obvious diffraction peaks are observed.
Comparison of (a) the nitrogen-nitrogen partial structure factors and (b) the corresponding nitrogen-nitrogen radial distribution functions from simulations of NH4Br, Me4NBr, and Bu4NBr aqueous solutions at c = 1m and T = 298 K. We used a window function in the Fourier transform to smoothen the structure factors from truncation artifacts.55
SAXS intensities of 1m solutions of (a) Bu4NBr; (b) Pr4NBr; and (c) Et4NBr at different temperatures. The contribution from pure water has been subtracted with volume correction. The lowest accessible temperature for Bu4NBr solutions is −5 °C; the other two solutions can be cooled down to −15 °C before nucleation happens.
Comparison between hydrophobic and temperature effects on the H-bonding structure of water. (a) O K-edge XAS of 1m amphiphilic solutions (Me4NBr: blue curve; Bu4NBr: red curve) and reference 1m NH4Br (grey curve). Inserted on top are difference spectra of R4NBRr solutions relative to NH4Br and normalized to the main-edge intensity of the latter showing hydrophobic effects of R4N+ cations. (b) X-ray Raman scattering (XRS) of pure liquid water at different temperatures which at the dipole limit becomes identical to XAS.36 Difference spectra referring to 22 °C at room temperature and normalized to main-edge intensity at 22 °C are plotted at the top. The difference spectra of the small (or large cations) in (a) resemble those of lower (or higher) temperature where it has been generally accepted that heating breaks or distorts H bonds while cooling enhances or promotes H bonds in water.
Crossover of hydrophobic hydration observed by XAS in amphiphilic (C n H 2n+1 )4NBr (R4NBr) aqueous solutions. (a) Difference spectra of 1m R4NBr solutions (coloured lines) relative to NH4Br (grey line) normalized to the main-edge peak height of the latter showing hydrophobic effects of R4N+ cations on H-bond structure of water in solutions. Spectra are plotted with offset and horizontal line denoting the position of zero intensity. The energy shifts of pre- and main-edge features in the difference are indicated by vertical lines. (b) Hydrophobic effects of R4N+ cations as analogous temperature change ΔT plotted against ionic radius; ΔT = 0 refers to NH4Br solution with no hydrophobic effects. The equivalent ΔT is calculated by scaling the total area of the difference spectrum in (a) to that of the temperature effect with similar profile in Fig. 1(b). The error bars are estimated by applying different background normalization and small energy shift without introducing abnormal changes to the spectrum. The reversal of the sign of ΔT between Et4N+ and Pr4N+ indicates opposite trends of structural change of water in solutions, i.e., crossover of hydrophobic hydration.5
(a) Concentration-dependent SAXS of Bu4NBr solutions after background subtraction of pure water with volume correction and normalized to the scattering intensity at Q = 1.1 Å−1 where it is proportional to the concentration. (b) XAS difference spectra between pure water and Bu4NBr solutions normalized to concentration.
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