The degree to which the surface tension decreases depends on the
structure of the amphiphile and the packing density of the resulting
monolayer. If the concentration of the amphiphiles is low, then we can
expect that the number of molecules on the surface is also relatively small.
In a way, this dilute surface approximates a gaseous phase in which
individual molecules are far apart and are free to move over the surface in
a random fashion. As the concentration increases, however, the packing
of the amphiphilic molecules at the surface becomes denser and the
surface tension consequently decreases. This behavior continues with
rising concentration until the point at which a saturated monolayer is
formed. Beyond this concentration the surface tension value does not
change. The phase behavior of amphiphiles at the air–water interface is
discussed in Chapter 8.
7.1.3 Contact angles and wetting phenomena
When a drop of water is placed on a planar solid surface, it may at one
extreme completely spread to cover the entire surface, or at the other
extreme form a spherical droplet on the surface. These situations represent either complete wetting or complete dewetting. Usually the degree of
wetting is intermediate between these extremes and depends largely on
the interfacial energy (or surface tension) between the liquid and the solid
surface. The contact angle is the angle at which the liquid–vapor interface
meets the solid surface (Figure 7.4). If the planar surface is horizontal and
the droplet is not moving, this angle is called the static contact angle
(Figure 7.4a). If the liquid is in motion because the surface is tilted, then
we can identify two dynamic contact angles, the advancing contact angle
and the receding contact angle (Figure 7.4b). This picture is similar to a
raindrop running down the surface of a window. Usually the advancing
contact angle is much larger than the receding angle, and the difference
between the two values is called contact angle hysteresis.
Figure 7.4(a) shows a nonwetting drop (conventionally called a “sessile
drop”) on a planar solid surface making a contact angle, q, which must
be greater than zero. The various interfaces are described by their surface tensions: the liquid–vapor tension (g LV ), the solid–vapor tension
(g SV ), and the solid–liquid tension (g SL ). The Young equation provides a
relationship between these various surface tensions and the static contact
angle:
g SV = g SL + g LV cos q
(7.2)
FUNDAMENTALS OF SURFACE SCIENCE 223
structure of the amphiphile and the packing density of the resulting
monolayer. If the concentration of the amphiphiles is low, then we can
expect that the number of molecules on the surface is also relatively small.
In a way, this dilute surface approximates a gaseous phase in which
individual molecules are far apart and are free to move over the surface in
a random fashion. As the concentration increases, however, the packing
of the amphiphilic molecules at the surface becomes denser and the
surface tension consequently decreases. This behavior continues with
rising concentration until the point at which a saturated monolayer is
formed. Beyond this concentration the surface tension value does not
change. The phase behavior of amphiphiles at the air–water interface is
discussed in Chapter 8.
7.1.3 Contact angles and wetting phenomena
When a drop of water is placed on a planar solid surface, it may at one
extreme completely spread to cover the entire surface, or at the other
extreme form a spherical droplet on the surface. These situations represent either complete wetting or complete dewetting. Usually the degree of
wetting is intermediate between these extremes and depends largely on
the interfacial energy (or surface tension) between the liquid and the solid
surface. The contact angle is the angle at which the liquid–vapor interface
meets the solid surface (Figure 7.4). If the planar surface is horizontal and
the droplet is not moving, this angle is called the static contact angle
(Figure 7.4a). If the liquid is in motion because the surface is tilted, then
we can identify two dynamic contact angles, the advancing contact angle
and the receding contact angle (Figure 7.4b). This picture is similar to a
raindrop running down the surface of a window. Usually the advancing
contact angle is much larger than the receding angle, and the difference
between the two values is called contact angle hysteresis.
Figure 7.4(a) shows a nonwetting drop (conventionally called a “sessile
drop”) on a planar solid surface making a contact angle, q, which must
be greater than zero. The various interfaces are described by their surface tensions: the liquid–vapor tension (g LV ), the solid–vapor tension
(g SV ), and the solid–liquid tension (g SL ). The Young equation provides a
relationship between these various surface tensions and the static contact
angle:
g SV = g SL + g LV cos q
(7.2)
FUNDAMENTALS OF SURFACE SCIENCE 223