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WII Report Summary

Project ID: 616305
Funded under: FP7-IDEAS-ERC
Country: Switzerland

Periodic Report Summary 2 - WII (Water, Ions, Interfaces: Quantum effects, charge and cooperativity in water, aqueous solutions and interfaces)

Water, Ions, Interfaces (WII)

Water: No substance on earth is so intimately linked to our well-being. Without water, membranes -the structures that provide the architecture of our cells and organelles- cannot function. Charges and charged groups cannot be dissolved, self-assembly cannot occur, and proteins cannot fold. Apart from the intimate link with life, water also shapes the earth and our climate. Our landscape is formed by slow eroding/dissolving processes of rocks in river and sea water; aerosols and rain drops provide a means of transport of water. Our society depends on products that all relate to water and aqueous systems, such as food products, medicine, and consumer goods. The WII project aims to investigate the structural, dynamic, and biological properties of water by probing the relationship between the properties of water on different length and time scales. Structuring and quantum effects in aqueous solutions and on interfaces are studied as well as long-range ordering effects. Furthermore, the interactions between ions, hydrophilic, and hydrophobic solutes and interfaces with water and the impact they have on the interaction of water with itself are studied.
So far, the project is a big success. By using a worldwide unique toolbox developed by the WII team we have been able to investigate the structure of water, aqueous solutions and interfaces on multiple length scales as exemplified by two highlighted findings:
1. A multi-scale investigation of the bulk and surface of aqueous electrolyte solutions that extends from the atomic scale, using atomistic modeling, to nanoscopic length scales, using bulk and interfacial femtosecond second harmonic measurements, to the macroscopic scale using surface tension experiments was performed. Electrolytes were found to induce orientational order in light water (H2O) at concentrations starting at 10 micromolar, which corresponds to a distance of 21 nm or 77 hydration shells. For heavy water (D2O) the increase in orientational order occurs at ionic strengths that are a factor of 6 higher. This is unusual, since isotope effects normally are on the order of a few percent only. The increased orientational order of the extended hydration shells leads to non-specific changes in the surface tension of dilute electrolyte solutions. Here too, an isotope effect of a factor of 6 was observed. The non-specific change in the surface tension is an effect that has been discovered for H2O already in the 1930’s but its origin has remained a mystery until now. The experimental data, combined with modeling on different levels shows that aside from well-known ion-dipole interactions, collective hydrogen bond water-water interactions are crucial: The hydrogen bond network of water responds to the collective electrostatic field of ions by increasing its orientational order.
2. The surface structure of 200 nm sized water droplets in mixtures of hydrophobic oils and surfactants was obtained from vibrational sum frequency scattering measurements. The interface of a water droplet shows significantly stronger hydrogen bonds than the macroscopic planar air/water or hexane/water interface at room temperature. The observed spectral difference is equivalent to a planar air/water surface at a ~50 ºC lower temperature. Supercooling the droplets to -10 ºC does not change the surface structure.

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