Physicists have
finally solved a key mystery in how hydrogen bonding works
A quantum universe in a drop of water.
It’s easy to feel like the quantum world is
incredibly distant from your everyday experience, so here’s something you can
do to bring it closer to home. Go grab a coin and put it under slowly dripping
water. If you have a pipette, that’s the way to go. Otherwise, a dripping
faucet will work.
Try enough times, and eventually you’ll be
able to get the water to pile up on your coin in
a big, bulbous blob. According to a new study, part of the reason
the drop holds together like this is because water molecules act like little,
quantum-tunnelling gears. Okay great, you can sit down now.
Water molecules are made of a big oxygen atom
and two smaller hydrogen atoms, with electrons buzzing around the whole group.
On average, the electrons spend more time buzzing around the oxygen and less
time buzzing around the hydrogen, so the oxygen tends to be negatively charged,
while the hydrogens tend to be positively charged.
If you put two water molecules next to each
other, the oxygen of molecule 1 tends to attract the hydrogens in molecule 2,
and the molecules will end up with the oxygen and one hydrogen really close
together. If you put a whole bunch of water molecules together, they’ll arrange
themselves so one molecule’s oxygen is always next to another’s hydrogen.
And then, because molecules are always
jiggling around, they’ll occasionally switch from lining up with one set of
neighbours to lining up with another set - the common metaphor being that water
molecules are dancers who like to switch partners. The whole process of
attraction and partner-switching is known as hydrogen bonding, and it’s the
underlying reason for surface tension - the tendency of water molecules to
clump together instead of spread apart. That’s why water drops can get so
big.
But there are a couple of holes in this
explanation. If all of the water molecules are in groups, how does one find
another partner without disrupting the whole dance? And what happens if they’re
not jiggling enough to keep switching? Does the drop just collapse?
These were the questions asked and answered by physicists at the
University of Cambridge in the UK, by looking at supercooled arrangements of
just six molecules.
First, they checked what happens when one
of the molecules switches partners, and found that you don’t just get one
molecule at a time doing the switch. The molecules always work in pairs, like
interlocking gears. When one turns, it frees up a hydrogen bond that can be
taken by the other and there’s never an awkward partnerless period.
But that’s not all. The molecules in these
experiments weren’t jiggling enough to do the switching on their own, so the
team turned to simulations to see how the gears were working.
Quantum particles (okay, all things in the
Universe, but let’s not go there) don’t have a well-defined position. Instead,
their positions are kind of spread out across space: it’s most
likely that they’ll be where you expect them to be, but they could also end up
somewhere else, even if they don’t have enough energy to get over there. It’s
like if you threw a ball at a wall and the ball, instead of hitting it and
bouncing back at you, just went through without breaking the wall. Your ball
would seem to have accessed some sort of tunnel between your and the other side
of the wall when no such tunnel exists.
This is how water is able to switch partners,
according to the new simulations,published in Science this week. The molecules aren’t jiggling
enough to do it on their own, so they have to rely on quantum tunneling in
order to set this molecular clockwork in motion. Instead of actually searching
for a new partner, they just appear next to the new partner and switch
immediately. The two molecules that work together in the gears coordinate their
tunneling so that none is ever without a partner.
Not bad for a little bulb of water.