Physical Chemistry , 1st ed.

razor blade moves down 0.250 mm. To do this, we use the classical physics

definition of work, force times distance:

work force distance

work (1.1462  10 ^4 N) (0.250 mm) 

100

1

0

m

mm

2.87  10 ^8 J

Therefore, we will get 2.87  10 ^8 J of gravitational work out of the system

as the razor blade drops 0.250 mm. Is this enough to overcome the energy re-

quired to pass through the water’s surface? In order to do so, the razor blade

must make a “hole” 19.9 mm by 38.9 mm large, or

area 19.9 mm 38.9 mm 

1

1

0

c

m

m

m



2

area 7.74 cm^2

Given a surface tension of 72.75 erg/cm^2 , we can calculate the energy needed

to increase the area of the water by that much:

energy 72.75 

c

e

m

rg

 (^2) (7.74 cm
(^2) ) 
1 

1

10

J

 (^7) erg
We are including the conversion from erg to joule. The energy needed to in-
crease the water’s area is
energy 5.63  10 ^5 J
This is several orders of magnitude more energy than is given off by the ra-
zor blade dropping through the surface. This suggests that a razor blade will,
indeed, float on the surface of water.

22.3 Interface Effects

In the previous section, we showed that the surface tension was related to some
thermodynamic functions, namely work and the Gibbs energy. What other ther-
modynamic manifestations are there for liquid surfaces? Several of these mani-
festations involve the interactions of two (or more) phases at their surfaces.
One thermodynamic variable, pressure, shows some unusual effects due to
the presence of a surface. Consider a liquid in contact with another phase, like
another liquid or even a gas or vacuum. Let us define region I as the liquid,
and region II as the other phase in contact with the liquid. Together, these two
regions represent our system, to be considered thermodynamically.
If the combination of these two regions is considered an isolated system at
equilibrium, then the overall change in energy of the system,dU, is zero. (This
is directly from the first law of thermodynamics.) We can define dU[Ias the
change in internal energy of region I and]dUIIas the change in internal energy
of region II. But there is also a surface energy due to surface tension at the
boundary between the two regions. This boundary is called the interface.
Although we previously considered surface tension’s relationship to the Gibbs
free energy, the surface-tension–area work is also equal to the change in inter-
nal energy as long as other thermodynamic variables (Sand V, the natural vari-
ables of internal energy) are kept constant:

dU dA (22.8)

where dU represents an interfacial internal energy.

22.3 Interface Effects 771