Chemistry Reference
In-Depth Information
to two similar troughs, reflected light from each was detected by separate photocells;
the photocells are arranged in a sensitive bridge circuit so that very small differences
could be determined. With nonabsorbing fatty acid monolayers, the observed change
in reflectance were 0.3-3%. Various other reflectance methods have been described
by other investigators. The ellipsometers currently in general use have much better
sensitivity than earlier models.
4.3.1.3 Fluorescence Studies of monolayer Films
The technique of fluorescence spectral measurements has become very sensitive
over the past decade. In order to obtain more information on the surface monolay-
ers, a new method based on fluorescence was developed. It consisted of placing the
monolayer trough on the stage of an epifluorescence microscope, with doped low
concentration of fluorescent lipid probe. Later, ordered solid-liquid coexistence at
the water-air interface and on solid substrates were reported. The theory of
domain
shapes
has been extensively described by this method.
The time-resolved fluorescence of monolayers have been studied by an appa-
ratus described in a recent study (Birdi, 1989). A polarized pulsed beam (20 ps,
wavelength 398 nm) from a laser is impinged upon the water surface at an angle
of 4 degrees at the surface plane. Fluorescence emission was measured along the
surface normal. In these studies, polymers (poly(ethyl)acrylate, poly(ethyl) meth-
acrylate) labeled with anthracene-based fluorescent probe were investigated. The
surface pressure versus area isotherms showed collapse at ca. 20 mN/m. The dif-
ferences in isotherms was correlated to fluorescence data. This method was found
to have a sufficiently high sensitivity for studying the mobility of the monolayers
at air-water interface. The fluorescence lifetimes were found to fit the presence of
two different lifetimes.
4.3.2 o
T h e r
c
h a n g e S
a T
W
a T e r
S
u r f a c e S
d u e
T o
m
o n o l a y e r S
The presence of lipid (or similar substance) monolayer at the surface of the aqueous
phase gives rise to many changes in the properties at the interface. The major effects
that have been investigated extensively are
Surface viscosity (η
s
)
Surface potential (ΔV)
There are also other surface properties that change, and these will be described later
(Chapter 4.4).
4.3.2.1 Surface viscosity (
η
s)
A monomolecular film is resistant to shear stress in the plane of the surface, as is
also the case in the bulk phase: a liquid is retarded in its flow by viscous forces.
The viscosity of the monolayer may indeed be measured in two dimensions by flow
through a canal on a surface or by its drag on a ring in the surface, corresponding
to the “Ostwald” and “Couette” instruments for the study of bulk viscosities. The
surface viscosity, η
s
, is defined by the relation
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