240 Chapter 7. Entropic forces at work[[Student version, January 17, 2003]]
shape can be very strong.^9
7.5 Special properties of water
Suppose you mix oil and vinegar for your salad, mix it thoroughly, and then the phone rings. When
youcome back, the mixture has separated. The separation is not caused by gravity; salad dressing
also separates (a bit more slowly) on the Space Shuttle. We might be tempted to panic and declare
aviolation of the Second Law. But by now we know enough to frame some other hypotheses:
1.Maybe some attractive force pulls the individual molecules of water together (expelling the
oil), to lower the total energy (as in the water-condensation example of Section 1.2.1 on page
7). The energy thus liberated would escape as heat, increasing the rest of the world’s entropy,
perhaps enough to drive the separation.
2.Maybe the decrease of entropy when the small, numerous oil molecules combine is offset by a
muchlargerincrease of entropy from some even smaller, even more numerous, objects, as in
the depletion interaction (Section 7.2.2 on page 221).Actually, many pairs of liquids separate spontaneously, essentially for energetic reasons like point
#1 above. What’s special about water is that its dislike for oil is unusually strong, and has an
unusual temperature dependence. Section 7.5.2 will argue that these special properties stem from
an additional mechanism, listed as #2 above. (In fact, some hydrocarbons actuallyliberateenergy
when mixed with water, so #1 cannot explain their reluctance to mix.) Before this discussion,
however, we first need some facts about water.
7.5.1 Liquid water contains a loose network of hydrogen bonds
The hydrogen bond The water molecule consists of a large oxygen atom and two smaller
hydrogen atoms. The atoms don’t share their electrons very fairly: All the electrons spend almost
all their time on the oxygen. Molecules that maintain a permanent separation of charge, like water,
are calledpolar.Amolecule that is everywhere roughly neutral is callednonpolar. Common
nonpolar molecules include hydrocarbon chains, like the ones making up oils and fats (Section 2.2.1
on page 41).
Asecond key property of the water molecule is its bent, asymmetrical shape: We can draw a
plane slicing through the oxygen atom in such a way that both the hydrogens lie on the same side of
the plane. The asymmetry means that an external electric field will tend toalignwater molecules,
partly countering the tendency of thermal motion to randomize their orientations. Your microwave
ovenuses this effect. It applies an oscillating electric field, which shakes the water molecules in
your food. Friction then converts the shaking motion into heat. We summarize these comments
bysaying that the water molecule is adipole,and that the ability of these dipoles to align (or
“polarize”) makes liquid water a highlypolarizablemedium. (Water’s polarizability is the origin of
the large value of its permittivityε,which is about eighty times greater than that of air or vacuum;
see Section 7.4.1.)
(^9) T 2 Interestingly, though, we can adjust the strength of this binding experimentally, by changing the salt con-
centration in the surrounding solution. At high enough salt concentration, Equation 7.40 supersedes Equation 7.27,
and the binding energy begins to go down.