Page 6 - AN INTRODUCTION TO SURFACE CHEMISTRY By ERIC KEIGHTLEY RIDEAL

CHAPTER I

THE SURFACE TENSION OF LIQUIDS
1. The existence of surface phases.
It has been found that where one distinct phase adjoins another
there exist conditions which distinguish the behaviour of a thin
layer of matter at the boundary from that of the general mass in
either of the homogeneous phases which bound it. Under certain
circumstances at least, and perhaps always, the boundary is not
simply a geometrical surface upon one side of which there is one
phase of uniform properties and on the other a second phase, every-
where distinct from the first and homogeneous in itself: nor, again,
do the contiguous phases shade, as might equally well be supposed,
rapidly but continuously one into the other, so that the boundary
would have to be considered as a more or less arbitrary mathe-
matical definition rather than a physical entity: it lies in a film or
lamina of finite though minute thickness consisting of an entirely
new phase of definite and measurable properties. To this phase
our attention will be given in the following pages. It will be seen
that two of its dimensions may be varied at will like those of a
phase in bulk, while its third dimension, which lies at right angles
to the plane of the lamina is fixed: if in any way it be altered, a
new superficial phase is formed.
A number of different types of such interfacial phases must be
considered when dealing with bulk phases in their different physical
states, Of special importance are the interfaces formed by contact
of a bulk liquid with gaseous, liquid and solid phases whilst the
problems connected with heterogeneous catalysis necessitate an
examination of the properties of the solid-gas interface.
The mathematical treatment of surfaces was developed much
earlier than the direct examination of superficial films. It is based
primarily upon the observed tendency of a volume of liquid to adopt
when unconstrained a shape with a minimum surface. The property
of having a minimum surface for a given volume is possessed by a
sphere, and this shape is assumed by a free body of liquid. A
falling raindrop is spherical: upon this fact indeed is based the
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