Page 8 - AN INTRODUCTION TO SURFACE CHEMISTRY By ERIC KEIGHTLEY RIDEAL

EXISTENCE OF SURFACE PHASES 3

Surface tension is thus exactly analogous to pressure in three
dimensions except that the sign is reversed. By taking into considera-
tion the thickness of the surface film we can imagine surface tension
as a negative pressure, averaged over the thickness of the film but it
is seldom convenient to do so, partly because the thickness of the
film is not usually known with certainty, and partly because the
calculated mean pressure has no very clear physical significance.
We may however, if we choose, regard the surface tension as an
integrated value of the tangential pressure over the thickness of
the film.
There is another point of view .from which it is often useful to
regard surface phenomena. If the surface of a soap-film in a wire
frame is increased by moving outwards against the tension one side
of the frame, a quantity of work must be done against the surface
tension equal to the product of the surface tension and the increase
of area. A definite quantity of potential energy is thus bound up
with each unit of surface, The numerical value of the surface
energy defined as energy per unit surface is clearly the same as
that of the surface tension. The dimensions in each case are
l,"} see he potential energy of the or»tom tends to

minimum, the surface must contract to a minimum area: we are
thus led to the same result as before,
It is important to observe that the surface energy a is a quantity
of the kind called "free energy" by Gibbs, and Helmholtz. It does
not represent the whole of the energy expended when a fresh
surface is formed. If the extension of surface takes place adiabati-
cally, a quantity of heat - T � ;, is absorbed from the interior, which

is thereby somewhat cooled. Allowing for the additional energy,
which may be called the latent heat of surface formation, introduced
to compensate for this cooling effect, the total energy per unit of

fresh surface is u -T :;,: this quantity is known at the Total Sur-
\
face Energy: (w), We are as a rule more concerned with the free
surface energy, and the latter quantity will be understood when for
shortness the term surface energy is used.