Research ArticleChemistry

Electrolytes induce long-range orientational order and free energy changes in the H-bond network of bulk water

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Science Advances  08 Apr 2016:
Vol. 2, no. 4, e1501891
DOI: 10.1126/sciadv.1501891
  • Fig. 1 Snapshots of long-range perturbations in aqueous NaCl solutions.

    (A) Illustration of two H-bonded water molecules that are orientationally correlated. The black arrows represent different axes along which H-bonds can be broken. fs-ESHS is only sensitive to the breaking of this H-bond via rotation (black curved arrow). Nuclear quantum effects predict that the H-bond bending mode is stronger, whereas the H-bond stretching mode is weaker, for D2O compared to H2O. (B) Sketch of the fs-ESHS experiment. fs-ESHS intensities are recorded at a scattering angle (θ) of 90o. P(S) refers to a polarization direction parallel (perpendicular) to the scattering plane. (C) Top: Illustration of the different regimes probed in the experiment. At low ionic strengths (1), each ion induces long-range structural water correlations in its vicinity, forming a water-ordered domain. At higher ionic strengths, more domains appear. These domains (2) start to overlap and (3) interfere with one another, leading to a saturation of the observed signal. Bottom: fs-ESHS intensities, relative to that of H2O, measured from NaCl solutions. Four different polarization combinations were measured: PoutPinPin, PoutSinSin, SoutPinPin, and SoutSinSin. Only the PoutPinPin and PoutSinSin intensities change with increasing NaCl concentrations.

  • Fig. 2 Nonspecific long-range changes in the H-bond network of water.

    (A) fs-ESHS intensities, relative to that of pure water, of 21 different electrolyte solutions obtained at a scattering angle of 90o (PoutPinPin polarization combination). The relative intensities of all electrolyte solutions can be fitted with the same equation. The dashed line indicates the concentration of half saturation. fs-ESHS intensities for CCl4 are also plotted [the x axis should be read here as “Concentration (μM)”]. (B) fs-ESHS intensities of NaCl in H2O and D2O (PoutPinPin polarization combination). (C) Water-water orientational correlations (left) and corresponding changes in average tilt angles (right) obtained from a molecular dynamics simulation of pure water. (D) Ion-induced change in the orientational order for an 8 mM NaCl solution. Changes in the distance-weighed water-water orientational correlations are shown (left axis), as well as the corresponding changes in the distance-weighted average tilt angle per water molecule (right axis).

  • Fig. 3 Macroscopic manifestation of orientational order in the H-bond network of aqueous electrolyte solutions.

    (A) An illustration of the concept of surface tension. (B) Normalized resonant I surface second harmonic response of NaI and KI (|χ(2)|2; black and red data) (52) and normalized fs-ESHS (bulk) intensity change originating from constraints in the orientational order of bulk water (blue and green data). Ion-induced changes in the H-bond network of bulk water occur at lower concentrations (55 μM) than the surface adsorption (4 mM). The saturation of the bulk structural changes coincides with the minimum in Δγ (dashed line). The top panel shows an illustration of resonant surface SHG. VIS, visible; UV, ultraviolet; GS, ground state. (C). Measured surface tension difference (Δγ) for NaCl solutions of H2O (blue data) and D2O (brown data). Above 20 mM, Δγ increases, as indicated by the dashed line [see Jarvis and Scheiman (53)]. A cartoon illustrating the structural changes in the electrolyte solution is shown on top. The numbers correspond to the different regimes of ionic strength and are also indicated in (B).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/4/e1501891/DC1

    S1. Constants and properties of H2O and D2O

    S2. Femtosecond elastic second harmonic scattering

    S3. Phenomenological discussion of the response of spherical domains

    S4. Spherical domain size dependence of the fs-ESHS signal

    S5. Molecular dynamics simulations

    S6. fs-ESHS from pure H2O and D2O

    S7. Mean-field models

    S8. Surface tension measurements and interpretation

    Table S1. Constants and properties of H2O and D2O.

    Table S2. Domain radius, corresponding number of hydration shells, and anisotropy in case the domain size is only determined by the average ion separation.

    Fig. S1. Elastic and inelastic second harmonic scattering.

    Fig. S2. Sketch of the relevant parameters needed in the phenomenological discussion.

    Fig. S3. ΔI as a function of domain size and relative structural anisotropy.

    Fig. S4. fs-ESHS patterns for pure H2O and pure D2O.

    Fig. S5. Mean-field models and temperature dependence.

    References (6187)

  • Supplementary Materials

    This PDF file includes:

    • S1. Constants and properties of H2O and D2O
    • S2. Femtosecond elastic second harmonic scattering
    • S3. Phenomenological discussion of the response of spherical domains
    • S4. Spherical domain size dependence of the fs-ESHS signal
    • S5. Molecular dynamics simulations
    • S6. fs-ESHS from pure H2O and D2O
    • S7. Mean-field models
    • S8. Surface tension measurements and interpretation
    • Table S1. Constants and properties of H2O and D2O.
    • Table S2. Domain radius, corresponding number of hydration shells, and anisotropy in case the domain size is only determined by the average ion separation.
    • Fig. S1. Elastic and inelastic second harmonic scattering.
    • Fig. S2. Sketch of the relevant parameters needed in the phenomenological discussion.
    • Fig. S3. ΔI as a function of domain size and relative structural anisotropy.
    • Fig. S4. fs-ESHS patterns for pure H2O and pure D2O.
    • Fig. S5. Mean-field models and temperature dependence.
    • References (61–87)

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