1. EW-CRDS and the Spatial Investigation of Adsorption on a Fused Glass Prism
Christopher Sue, Nick Sovronec, Jordan Kohl, Michael A. Everest
Department of Chemistry, Westmont College, Santa Barbara, CA
Abstract
The interactions between (3-aminopropyl)-trimethoxysilane
(APTMS) and phosphotungstic acid (PTA) were studied on a
fused-silica prism through the use of evanescent-wave cavity-
ringdown spectroscopy (EW-CRDS). Phosphotungstic acid has
been shown to be effective in increasing proton transfer which
could be useful in improving the efficiency of fuel cells. It was
discovered that PTA will adsorb irreversibly to higher concentration
of APTMS and that it can be removed through concentrated base.
Materials & Methods
Results
References
•Looking at interactions of Phosphotungstic acid and
N--butyltrimethoxysilane.
•Investigation of other heteropoly acids.
•Looking at the interactions between APTMS and PTA
in two dimensions.
http://en.wikipedia.org/wiki/File:Phosphotungstate-3D-polyhedra.png
Prism Preparation
Silica prisms were coated with APTMS through vapor
deposition and then annealed in an oven.
θ θ
Scheme 1: Vapor Deposition silanizes
the glass and forms a covalent bond.
Contact Angles
The contact angles across the prism face were measured with a
KSV Cam-100 using the pseudo-dynamic technique.
Figure 2: Larger angles result from
hydrophobic surfaces (left) and smaller
angles from hydrophilic surfaces (right).
Figure 3: The change in the
contact angles indicates a
relative amount of coating of
APTMS on the surface with
larger angles indicating high
concentrations of APTMS and
low angles indicating low
concentrations of APTMS.
EW-CRDS
Phosphotungstic Acid
Phosphotungstic Acid is a hetropoly acid with
the molecular formula of H3PW12O40. It is
known to be a good catalyst and proton
conductor, and has a -3 charge on its surface
when deprotenated. PTA is dissolved in a
50/50 mixture of ethanol and water at a pH of
~2.00 to ensure the stability of the PTA
complex.
Future Work
EW-CRDS was used to study the adsorption loss and scattering of
laser light through PTA and the background solution on an APTMS
covered fused-silica prism. A laser pulse creates an evanescent
wave at the prism’s face.
Figure 9: By taking measurements across the prism, the APTMS
gradients can be studied when combined with varying concentrations
of APTMS.(Left) In order to study how much PTA irreversibly adsorbs
the background is subtracted from the other measurements to display
the differences between the prism after it is coated immediately and a
prism that does not have excess PTA in solution.
Figure 5: Two mirrors formed a
cavity of length ~164.5 cm. An
evanescent wave at the prism
face allows for the study of signal
loss.
Figure 1: Vapor deposition creates
an idealized gradient of APTMS.
Figure 6: A graph displaying the
signal slowly dissipating as it
reflects through the cavity.
The angles are measured three times with advancing and
receding angles to ensure reproducibility. These are then used
in the Young–Laplace equation
Loss vs. Fraction Coverage
Figure 10: Adsorption of light by PTA
correlated to the fraction coverage of
APTMS. Equation (1) was used to fit the
data.
Loss (ppm) vs. Time (s)
Figure 8: The attachment of the PTA to APTMS is pH sensitive and an
increase in the pH causes the PTA to detach from the prism. This change in
pH causes deprotonation of the amine on the APTMS removing its positive
charge. Subsequent additions of solvent and PTA in solution lowers the pH
protonating the amine and thus allows for the PTA to bind again. Changing the
surface back to a stable, non-binded PTA structure generates more testable
PTA application analysis.
∆Gº ∆Gº
-81 kJ/mole ± 17 7 kJ/mole ± 20
APTMS Clean Glass
Figure 11: Adsorption of PTA to APTMS
is favorable under standard state
conditions
Figure 4: A phosphorus atom
surrounded by tungsten atoms
which are bonded to oxygen atoms.
Loss (ppm) vs. Distance (mm)