Water molecules are attracted electrostatically to areas of charge on dissolved
ions or polar molecules, and are also attracted to charged groups on a hydrophilic
surface such as glass. Consequently, such surfaces are wettable,
in the sense that water spreads over them and binds loosely to them. By contrast,
hydrophobic surfaces, such as those of most plastics and waxes, lack surface
charge and hence are not wettable; water on them will form isolated droplets,
but will not spread out over the surface.
The readiness of water to bind to hydrophilic surfaces explains the
phenomenon of capillarity—the tendency of aqueous
liquids to rise in narrow tubes. If the end of a narrow glass tube
is inserted below the surface of a volume of water, water will rise
in the tube to a level well above the water level outside (see Figure).
The reason is that glass is very hydrophilic, having many charged
groups on its surface. The water molecules, electrostatically attracted
to the glass, tend to creep upwards along the surface of the tube
and to pull other water molecules (linked to them by hydrogen bonds)
along with them. The water level stops rising when the pull of gravity
just counteracts the electrostatic forces that contribute to capillarity.
The larger the diameter of the tube, however, the smaller the percentage
of waster molecules in direct contact with the glass and, correspondingly,
the smaller the rise in the water in the tube. Even though the relatively
few molecules in contact with the glass have a tendency to creep upward,
they are held back by their cohesion via the network of hydrogen bonding
with the rest of the water in the tube.
Capillarity is by no means restricted to glass tubes. Water will
climb any charged surface. We are all familiar with the way it climbs
up the fibers of paper towels and spread through the fibers of many
kinds of cloth.
Water rises higher in a glass tube of small bore (left)
than in one of large bore (center) because in the smaller tube a higher
percentage of the water molecules are in direct contact with the glass
and can form hydrogen bonds with charged groups on the glass. By contrast,
water cannot “stick” to the surface of a plastic tube
(right) because plastic is uncharged.
