Biomedical Engineering Reference
In-Depth Information
a teaspoonful of oil on Clapham Pond and found its surface becoming mir-
ror smooth. The actual event is estimated to have taken place between 1769
and 1771, and the account appeared in 1774 in Philosophical Transaction of
the Royal Society [3-6]. Following the account of this experiment, many illus-
trious names in 19th century science are associated with the phenomena of
layering amphiphilic substances on water surfaces such as Lord Rayleigh [7],
Kelvin [8], Pockels [9], and others. The intriguing history is presented with
a dramatic flair by Giles, Giles and Forrester [4-6] and Tanford with his
usual inimitable perspective [2]. Monolayers at A/W of macromolecules of
biological and synthetic origins had begun to be studied in the middle of
the last century. The first examinations of polymer monolayers are traced
to Crisp [10, 11] in 1946 in J Colloid Sci and subsequently to Gabrielli and
Pugelli in 1971 [12] when they compared how the area per monomer unit
changes from A/W to an oil/water interface. Resurgence of interest in polymer
monolayers, particularly its dynamics in recent decades has technological
underpinnings. The interest is in part derived from a search for optimum
conditions for transfer of monolayers to solid substrates to form Langmuir-
Blodgett films [13-15] in various non-linear optical applications [16, 17]. The
technological potential has diminished mainly due to the life-time problems
of devices, while fundamental questions remain relative to the rheology of
monolayers as two-dimensional objects.
2
Static Properties of Polymer Monolayers
Inordertosetthestageforthisreviewofthepolymerdynamicsonmonolay-
ers at interfaces with emphasis on A/W, we need to lay out its static properties
first. Surface tension
represents a fundamental property of a liquid surface.
The change in Gibbs free energy d
G
for a multi-component system including
the surface contribution is written as
σ
d
A
s
+
d
G
=-
S
d
T
+
V
d
p
+
σ
i
µ
i
d
n
i
,
(1)
where
S
is the total entropy of the system,
T
temperature,
V
volume,
p
pres-
sure,
A
s
surface area, and
µ
i
and
n
i
correspond, respectively, to the chemical
potential and number of moles of the ith component. Thus, the surface ten-
sion is defined as the incremental change in the Gibbs free energy with
respect to the surface area change,
=
∂
G
σ
(2)
∂
A
S
T
,
p
