en atmospheres.

SURFACE BOUNDARY PHENOMENA

In the preceding chapter, a brief consideration of the phenomena arising at
surface boundaries was presented. It was pointed out that when any
substance exists in the colloidal, or dispersed, condition, it has
relatively enormous surface area and that, consequently, enormous surface
boundaries between the dispersed phase and the dispersion medium exist in
all colloidal mixtures. Since protoplasm is conceived to exist in the form
of a colloidal gel, having a foamlike structure, it is apparent that it
has these enormous surface boundaries between the different phases of the
system, and that the phenomena arising from this condition are of great
importance in its biological activities. The following necessarily brief
discussion will serve to give some indication of the physiological
importance of the surface boundaries in such a system.

It is easy to see that the molecules which are in the surface layers at the
interface, where two phases of a colloidal system are in contact, are under
the influence of forces quite different from those which are acting upon
the molecules in the interior of either phase. It is apparent that the
molecules in the surface layer are exposed on the inner side to the
attraction and influence of similar molecules, while on the opposite, or
outer, side they are exposed to the influence of molecules of an entirely
different kind. This results in a state of tension, known as "surface
tension," with the development of resultant forces and energy which
profoundly affect the chemical reactivity of the molecules which are
present in this surface layer. The so-called "surface energy," which
results from this surface tension, produces marked increases in the
possibility of chemical reaction between the materials which are present at
the surface boundaries. In colloidal gels, this effect is so pronounced, in
many cases, as to completely overshadow other types of influences upon
reaction velocities. Also, the surface layer of a liquid is compressed by
its surface tension, to such an extent that the solubility of substances in
this surface layer is greatly increased over that of the same substances in
the interior of the liquid, which results in greatly increased
concentration of dissolved substances in the surface layer, and so
increases the rate of chemical changes which take place there, as
contrasted with the rate of the same reactio