Introduction to Surfaces and Colloids - Surface and Colloid Chemistry: Principles and Applications - page 2

Vapor Phase

Liquid Surface

Liquid Phase

FIGure 1.1 Surface molecules (schematic).

instances such as oil spills, the importance of the role of the ocean surface can be easily
realized. It is also well known that the molecules situated near or at the interface (i.e.,
liquid-gas) will be interacting differently with respect to each other than the molecules
in the bulk phase (Figure 1.1). This important aspect will be described extensively in
this topic. The intramolecular forces acting would thus be different in these two cases.
In other words, all processes occurring near any interface will be dependent on these
molecular orientations and interactions. Furthermore, it has been pointed out that, for
a dense fluid, repulsive forces dominate the fluid structure and are of primary impor-
tance. The main effect of these repulsive forces is to provide a uniform background
potential in which the molecules move as hard spheres. The molecules at the interface
would be under an asymmetrical force field, which produces the so-called surface ten-
sion or interfacial tension (Figure 1.2) (Chattoraj and Birdi, 1984; Birdi, 1989, 1997,
1999, 2002; Adamson and Gast, 1997). This leads to the adhesion forces between liq-
uids and solids, which is a major application area of surface and colloid science.
The resultant force on molecules will vary with time because of the movement of
the molecules; the molecules at the surface will be pointed downward into the bulk

Surface

FIGure 1.2 Intermolecular forces around a molecule in the bulk liquid (dark) and around
a molecule in the surface (light) layer (schematic).