Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 10

2.2 SurFace ForceS (In lIquIdS)
It is important to consider the molecular interactions in liquids that are responsible
for their physicochemical properties (such as boiling point, melting point, heat of
vaporization, surface tension, etc.), which enables one to both describe and relate the
different properties of matter in a more clear manner (both qualitatively and quanti-
tatively). These ideas form the basis for quantitative structure activity relationship
(QSAR; Birdi, 2002). This approach toward analysis and application is becoming
more common due to the enormous help available from computers.
The different kinds of forces acting between any two molecules are dependent
mainly on the distance between the two molecules. The difference in distance
between molecules in liquid or gas can be estimated as follows. In the case of water,
the following data are known:

Water data
Volume per mole liquid water = V liquid = 18 mL/mol
Volume per mole water in gas state (at STP) (V gas ) = 22 L/mol

One thus finds (in general) that the ratio V gas /V liquid = ca. 1000. Hence, the approx-
imate distance between molecules will be ca. 10 = (1000) 1/3 (from simple geometri-
cal considerations of volume [% length 3 ] and length). In other words, the density
of water changes 1000 times as the surface is crossed from the liquid phase to the
gas phase (Figure 2.1). This large change means that the surface molecules must
be under an environment different from that the liquid or gas phase. The distance
between gas molecules is approximately 10 times larger than in a liquid. Hence, the
forces ( all forces increase when distances between molecules decrease ) between gas
molecules is much weaker than in the case of the liquid phase. All interaction forces
between molecules (solid phase, liquid phase, gas phase) are related to the distance
between molecules.
It is the cohesive forces that maintain water, for example, in the liquid state at
room temperature and pressure. This becomes obvious when one compares two dif-
ferent molecules, such as H 2 O and H 2 S. At room temperature and pressure, H 2 O is

1

0.001

Surface

FIGure 2.1 Change of density of a fluid (water) near the surface.