Femtosecond Lasers Uncover the Deep Weirdness of Plain Old Water

There's a reason we get so excited about finding water on other planets. It's everything to us as biological life and so it is to the many things that we depend upon, from our food to our atmosphere to every single level of our physiology. Also: most anything else we could imagine.

What makes water special? That's a big question. For one thing, it resists temperature changes, making it a welcome home for organisms, but also a moderating force for Earth's global climate. Without a bunch of water hanging around, our days would be unbearably hot and our nights deathly cold. Then there's the whole ice floating thing, and also water's ability to buffer against dramatic pH changes. Finally, on top of everything is water's role as a "universal solvent."

This last bit in particular is implicated in research released this week in Nature Communications describing the fine-scale molecular interactions among water molecules at very short time-spans. Once impossible to observe, these interactions imbue water with its own very short-term molecular memory thanks to fleeting local structures existing for mere thousandths of one billionth of a second.

"Water exhibits unique properties, distinctively different from other liquids, such as a high heat capacity, a high surface tension and a reduced density of the solid compared with the liquid," the paper explains. "These properties are caused by the strong intermolecular interactions between water molecules, through the so-called hydrogen bond (H-bond) network. Despite the relatively strong H-bond interaction in liquid water, the three-dimensional structure is very dynamic and H-bond breaking and reformation occurs on (sub-) picosecond timescales."

The ability of water to form structures is crucial to what makes it the force it is. Because the molecules have a preference for hydrogen-hydrogen bonds rather than hydrogen-oxygen bonds, they wind up polarized, with positive and negative charges concentrated at either end of the molecules. As a consequence they're attracted to stuff that happens to be floating around and this attractive force can be strong enough to disrupt the bonds between other molecules, like those between, say, sodium and chloride, the two components of salt.

The structures naturally formed within water become smeared out on average for timescales longer than a picosecond or so. A heterogeneous, structured arrangement of molecules becomes homogeneous, which is how we usually think of water. A uniform collection of identical units.

"Despite the relatively strong H-bond interaction in liquid water," the paper explains, "the three-dimensional structure is very dynamic and H-bond breaking and reformation occurs on (sub-) picosecond timescales."

At longer timescales, water-based structures fall victim to the H2O molecules' natural oscillations. They just vibrate away. So, catching a glimpse of these structures requires a means to probe at sub-picosecond scales, which the researchers accomplished using intense femtosecond infrared laser pulses. Said pulses were just fast enough to couple to the natural oscillation frequencies found within oxygen-hydrogen bonds before they could be shook away.

"71 percent of the earth's surface is covered with water," notes study co-author Mischa Bonn, director of the Max Planck Institute for Polymer Research. "As most chemical and biological reactions on earth occur in water or at the air water interface in oceans or in clouds, the details of how water behaves at the molecular level are crucial. Our results show that water cannot be treated as a continuum, but that specific local structures exist and are likely very important."