Biomedical Engineering Reference
In-Depth Information
beginnings and ends of the surface region can
be determined, at least in principle, by compar-
ing properties of the bulk phase to any test
plane. Start in the middle of the material and
proceed outward in any direction, comparing
properties until at last chemical or energetic dif-
ferences are found. However, when the surface
region is a substantial part of the system because
the bulk phase has been significantly reduced in
volume (a large surface-area-to-volume ratio),
the definition in
Box 8.2
becomes ambiguous.
Such a circumstance arises in very thin films
when an upper bounding surface comes within
close proximity to the lower bounding surface
and there is little
bulk
in between these bounds.
Otherwise, for most macroscopic applications of
materials, the conceptual definition in
Box 8.2
works well. In most practical applications of
surface science it is not necessary to know the
surface-region thickness in absolute terms, but
it is important to realize that the surface region
is very different from anywhere in the bulk
phase of a material.
BOX 8.3
SURFACE ENERGY
An intensive thermodynamic property of a
material that arises from the loss of nearest-
neighbor interactions among atoms or mol-
ecules at the boundary. This excess energy
most prominently manifests itself in adhe-
sion and adsorption reactions at the surface.
[13]
had not yet been clearly identified, Lang-
muir understood that the energy expended
(work done) to separate atoms or molecules
along a plane (by cleavage, for example) left the
two surfaces so created in a state of excess
energy compared to the energetic state of identi-
cal atoms or molecules in an equivalent plane
within the bulk phase. Thus it is apparent that
surface energy is the excess energy per unit area
of boundary plane (ergs/cm
2
or mJ/m
2
) and is
an intensive thermodynamic property of materi-
als (
Box 8.3
).
Surface energetics are quite large and are not
to be ignored in many important technical appli-
cations (such as adhesives, biomaterials, col-
loids, paints, etc.), as well as in nearly any
circumstance when surfaces come in contact
with liquids, solids, or vapors.
On thinking of atoms as ball-bearing-like
sphere lying on a plane (
Figure 8.2
a), it becomes
quickly evident that a particular ball bearing can
have six nearest neighbors when close-packed
into a group that comprises a small portion of a
hypothetical condensed-phase material. This
close-packed arrangement of ball bearings is a
conceptual model of atoms at a planar surface.
That is to say, an atom or molecule at the surface
of the hypothetical material has six nearest
neighbors in the plane of the surface. But these
surface atoms would also be in contact with the
8.2.2 Surface Energy
Surfaces are in a unique energetic predicament
because atoms or molecules at the boundary are
deprived of nearest-neighbor interactions other-
wise enjoyed in the bulk phase. Irving Lang-
muir, widely regarded as the father of modern
surface science and namesake of the American
Chemical Society journal of surface science
Langmuir
, realized this by stating in his land-
mark papers of 1916
[11, 12]
: “Since energy must
be expended in breaking apart a solid, the sur-
faces of solids must contain more potential
energy than do the corresponding number of
atoms in the interior. Since this potential energy
is probably electromagnetic energy in the field
between atoms, the inter-atomic forces are more
intense on the surface than in the interior.” Even
though at this early time in the history of science
the six fundamental intermolecular interactions
