Grand Séminaire d'Institut

Vendredi 2 Mars 2018 à 11h00.

Aqueous nanoscale systems: from long range interactions in water to confinement

''Water is important for many processes on earth. In living organisms, membrane formation and functioning, protein folding and activity are driven by hydration. In our environment, water shapes our landscape and determines our climate. To obtain molecular level knowledge on multiple length scales of aqueous solutions and nano- and micron scale curved interfaces in solution, we develop and use nonlinear light scattering and imaging tools. With those methods, we study the interaction of ions and water, the structure of confined water droplets and the formation and functioning of membranes.
By probing the interaction of ions with water with fs elastic second harmonic scattering, a background-free method sensitive to ordering effects on the nanoscale, we find universal long-range effects that can be observed already at an ionic strength of 10 micromolar (~70 hydration shells). 20 different electrolytes all increase the orientational order of bulk water in the same way. This modification of the hydrogen bond network of water is due to the interaction of the total electric field of all the ions with the water network, leading to a modulation of the water-water interactions. This entails a change in the energy stored in bulk water and manifests itself macroscopically as a small amplitude minimum in the surface tension. Light and heavy water show a remarkably different behavior, pointing to the importance of nuclear quantum effects [1-3].
Nanoscale water droplets with a radius of 100 nm embedded in a hydrophobic liquid environment are exemplary of (marine) aerosols. We have determined the surface water structure of these small droplets at room temperature, under supercooled conditions and in frozen form and find that the hydrogen bond network at the interface exhibits more order than an equivalent extended planar interface made of the same chemical. The increased amount of order is equivalent to a reduction of the surface temperature by 50 K [4].

References
[1] - Y. Chen, H. I. Okur, N. Gomopoulos, C. Macias-Romero, P. S. Cremer, P. B. Petersen, G. Tocci, D. Wilkins, C. Liang, M. Ceriotti, S. Roke, Sci. Adv. 2016, 2, 4, e1501891
[2] - D. M. Wilkins, D. E. Manolopoulos, S. Roke, and M. Ceriotti, J. Chem. Phys. 2017 146, 181103
[3] - H. I. Okur, Y. Chen, D. M. Wilkins, S. Roke, submitted
[4] - N. Smolentsev, W. J. Smit, H. J. Bakker and S. Roke, Nat. Commun, 2017, 8, 15548, DOI: 10.1038/ncomms15548
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