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EFFECT OF NAPHTHALENE ADLAYER ON THE DESORPTION ENERGIES OF A
HOMOLOGOUS SERIES OF 1-CHLOROALKANE ON ααααα-ALUMINA
A.M. Nishimura† , Xianzhang Geng*, Rachel J. DeHoog*, Andrew D. Olson*, and Trevor M. Ban*
Department of Chemistry, Westmont College, Santa Barbara, CA 93108
Abstract
The effect of an organic adlayer, naphthalene, on the desorption energy for a homologous series of 1-chloroalkane is reported. The
multilayer desorption energy of 1-chloroalkane increased by ~ 5 kJ/mol per methylene bridge. When naphthalene was deposited
above the chloroalkane as a bilayer, the desorption energy of the chloroalkanes increased by ~ 3 kJ/mol per methylene bridge. The
desorption energy of chloropropane was raised the most from that of the multilayer by ~ 9 kJ/mol when naphthalene was deposited
above it and 1-chloroheptane was affected the least. The observed trend has tentatively been attributed to the gradual melting and
the formation of molecular clusters in the naphthalene adlayer at temperatures > ~200 K that allowed the desorption of the
chloroalkane.
Keywords: naphthalene, 1-chloroalkane, desorption, activation energy, temperature programmed desorption, TPD
Introduction
During the heating process in temperature programmed
desorption (TPD) experiments, vapor deposited adlayers of
organic fluorophores were reported to exhibit enhanced
ordering in the presence of non-emitting 1-chloroalkanes (1-
4). In these studies, the order of deposition of the
spectroscopically inert compound was stated to be
inconsequential, whether on top or on the bottom of the
fluorophoric adlayer. The purpose of this short paper is to
answer the following questions: 1) under what condition is
the location of the chloroalkane relative to the fluorophoric
adlayer important and 2) is there a predictable trend in the
difference in desorption temperatures between multilayer and
bilayer with a fluorophoric adlayer for the homologous series
of 1-chloroalkane from 1-chloropropane to 1-chloroheptane.
Experimental
The 1-chloroalkanes and naphthalene were purchased from
Sigma-Aldrich (St. Louis, MO). The samples were placed in
their respective stainless steel ampoules, outgassed by
freezing-pump-thaw cycles, and introduced into the vacuum
chamber through leak valves. Deposition temperature was
100 ± 10 K.
Details of the experimental set-up have been previously
published (1,2), and only a briefest outline is given here.
Experimental hardware integration and control for the TPD
with the residual gas analyzer tuned to the parent peak was
done with LabVIEW (Austin, TX). The heating of the
α-alumina crystal for the TPD was accomplished by sending
current through the electrically resistive tantalum foil that
had been placed in thermal contact with the crystal. The
temperature was monitored with a thermocouple that was in
direct contact with the crystal and provided the feedback for
the linear temperature ramp that was programmed at 2 K/s.
The adlayer coverages were determined by using a diode
laser that was directed at the ?-alumina surface during
deposition (2). The resulting optical interference was used to
calibrate the integrated mass spectral peaks in the TPD in
terms of the surface coverage, Θ, in units of monolayers, ML.
The error in the coverage was about ± 5%. In the TPD
experiments described in this study, attempt was made to hold
the total coverages of naphthalene constant at about 250 ML
with a reproducibility of about ± 25 ML. In the bilayer
experiments, the coverage of 1-chloroalkanes was
approximately 15 ML, with a reproducibility of about ± 5
ML.
The activation energy for desorption, Ea, was calculated
by Redhead analysis in which a first-order desorption kinetics
as described by King was assumed and was based on the mass
spectral peak desorption temperature, Tp (5-7). The
uncertainties in the desorption temperatures and the
propagated error in the activation energies were ± 2% as
determined from at least 4 replicate measurements.
Results and Discussion
Table 1 is a summary of the activation energies for
desorption of the homologous series of 1-chloroalkane, from
1-chloropropane to 1-chloroheptane. The Ea’s for multilayer
chloroalkane formed the baseline by which the desorption
energies for the chloroalkanes that had been deposited with
the naphthalene adlayer were compared and are tabulated in
the last two rows as ∆Ea. In addition, the Ea’s for the
chloroalkanes as multilayer, on top of and under naphthalene
are plotted in Figure 1. As shown in Table 1 (and Figure 1),
the multilayer Ea of 1-chloroalkane increased by ~ 5 kJ/mol
per methylene bridge and this increase is a measure of the
Table 1. Ea for homologous 1-chloroalkanes.
Ea (kJ/mol) chloroheptane chlorohexane chloropentane chlorobutane chloropropane
multilayer 67.3 63.6 60.8 53.3 47.1
above naphthalene 67.2 63.7 61.8 53.6 48.0
below naphthalene 67.7 65.2 63.9 60.5 56.3
∆Ea (kJ/mol)
above - multilayer - 0.1 0.1 1.0 0.3 0.9
below - multilayer 0.4 1.6 3.1 7.2 9.2

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dispersion force of a methylene bridge. When naphthalene
was deposited above the chloroalkane as a bilayer, the
desorption energies of decreased by ~ 3 kJ/mol per methylene
bridge. Relative to the multilayer, the desorption energy of
chloropropane was raised by ~ 9 kJ/mol when naphthalene
was deposited above it; ∆Ea for 1-chloroheptane was affected
the least. (Cf. Table 1 and Figure 1).
Figure 2 are plots of the peak desorption temperature (left
vertical axis, with marker I) and the corresponding activation
energy for desorption for 1-chloropropane (right vertical axis,
with marker N) as a function of naphthalene coverage in ML.
Here, 1-chloropropane was deposited below naphthalene. At
low coverages of naphthalene, 1-chloropropane desorbed at
temperatures close to multilayer as well as when it was
deposited on top of naphthalene (Cf. Figure 1, data shown by
N and by I, respectively.) When the naphthalene coverage
exceeded about 120 ML, the presence of naphthalene was
felt by 1-chloropropane. As seen from Figure 2, at 230 ML,
the effect of naphthalene adlayer has peaked. It should be
noted that the data shown in Figure 1 and Table 1 were in this
high coverage regime.
Shown in Figure 3 are plots of the residual gas pressure for
multilayer naphthalene as a function of the TPD temperature
at desorption, at two limiting coverages as discussed above
in reference to Figure 2: one at 43 ML and the other at 205
ML. A close examination of the leading edges of the mass
spectral TPD shows no apparent difference for both high and
low coverages. Since the desorption kinetics at high coverages
is known to be first-order (8,9), the low coverages must also
be first-order. Tro et al. have modeled this first-order
desorption kinetics and showed that the first-order kinetics
resulted from the desorption from molecular clusters that form
cylinders or island-type geometry (8,9).
From these results, a postulated explanation of the data in
Table 1 and Figure1follows. From Figure 2, at the lower
naphthalene coverages below 120 ML, the naphthalene
molecules were sufficiently dispersed such that they do not
influence the desorption of 1-chloropropane. This dispersion
would be even greater, if naphthalene formed clusters as the
temperature was raised. However, when the naphthalene
coverage was greater than 230 ML, the effect of the
naphthalene adlayer was noticeable, (Figure 2). Upon vapor
deposition, the adlayer is amorphous.As the temperature was
ramped in a TPD experiment, the naphthalene adlayer was
so closely packed that the chloroalkane, when deposited under
naphthalene, could not desorb at the normal multilayer
desorption temperature. As the temperature rose past Tp for
multilayer chloroalkane, the naphthalene in the adlayer above
began to undergo thermally induced translational movement.
This movement has been observed at temperatures as low as
~ 50 K before desorption and can be attributed to the melting
of the naphthalene adlayer (1-4). This surface dynamics
caused the formation of molecular clusters, e.g. analogous to
islands. The reason for this formation was that the affinity
for naphthalene to form molecular clusters was higher than
that of the naphthalene to spread more uniformly on the
surface because of the polar surface of Al2O3 (8,9). At the
initial stages of the thermally induced cluster formation, the
density of smaller clusters was sufficiently high to prevent
even the smaller chloroalkanes from percolating through.
Eventually the growth of naphthalene clusters opened gaps
through which the chloroalkanes can escape. However, the
size of the pores was dependent on the temperature, and
therefore, the smaller chloroalkanes can get through the
openings created by the clustering of naphthalene at a lower
temperature than the larger chloroalkanes. In fact, desorption
energy of the homologous series of chloroalkanes of about 2
kJ/mol per methylene bridge from that of the multilayer, when
Figure 3. Residual gas pressure of multilayer naphthalene at low
and high coverage (Cf. Figure 2) as a function of TPD temperature.
Figure 1. Ea’s for multilayer 1-chloroalkane is plotted as a function
of decreasing carbon chain length from 1-chloroheptane to 1-
chloropropane. The difference in the Ea’s when the chloroalkane
was deposited on top and on the bottom of naphthalene adlayer
from that of the multilayer are also plotted.
Figure 2. Peak desorption temperatures, Tp, on the left axis and
Ea on the right axis are plotted for 1-chloropropane deposited
on the bottom as a function of naphthalene coverage.

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naphthalene was deposited above it, reflected this gap size as
a function of temperature. Table 1 (Ea. under naphthalene,
and ∆Ea, last row) and Figure 1 (triangles).
Conclusion
The multilayer desorption energy of 1-chloroalkane
increased by ~ 5 kJ/mol per methylene bridge that can be
attributed to the increase in the dispersion forces with
increasing carbon chain length. Starting with chloroheptane,
the desorption energy of the homologous series of
chloroalkanes decreased by ~ 3 kJ/mol per methylene bridge
from that of the multilayer, when naphthalene was deposited
above it. The difference in desorption energy with and without
the naphthalene adlayer was the largest for chloropropane by
~ 9 kJ/mol, while chloroheptane was affected the least. At
naphthalene coverages > 230 ML, thermally induced
molecular clusters were postulated to form gaps that
accommodate the passage of chloroalkanes. The size of the
gaps varied with temperature, and therefore, larger
chloroalkanes required higher temperatures in order to
percolate through. In summary, the observed decrease in the
desorption energy with increasing length of chloroalkane with
naphthalene has tentatively been attributed to the gradual
melting of the naphthalene adlayer at > 200 K that formed
larger clusters while leaving gaps in the adlayer that allowed
the desorption of the larger sized chloroalkanes.
Acknowledgement
The authors would like to thank the Camille and Henry
Dreyfus Foundation for the student stipends that were made
available through the Senior Scientist Mentor Program.
References:
(1). B.B. Baer, S. Lau, H.E. Ryan, K.A. Martin and A.M. Nishimura, J.
Spectro., 2013, 2013, 1-6.
(2). R.J. DeHoog, M.A. Shew, K.A. Martin and A.M. Nishimura, Trends in
Phys. Chem., 2014, 15, 1-11.
(3). M.A. Shew, R.J. DeHoog, K.A. Martin and A.M. Nishimura, J.
Undergrad. Chem. Res., 2014, 13, 55-59.
(4). R.J. DeHoog, M.A. Shew, X. Geng, K.A. Martin and A.M. Nishimura,
J. Undergrad. Chem. Res., 2015, 14, 35-39.
(5). P.A. Redhead, Vacuum, 1962, 12, 203-211.
(6). F.M. Lord and J.S. Kittelberger, Surf. Sci., 1974, 43, 173-182.
(7). D.A. King, Surf. Sci., 1975, 47, 384-402.
(8). C.M. Aubuchon, B.S. Davison, A.M. Nishimura and N.J. Tro, J. Phys.
Chem., 1994, 98, 240-244.
(9). R.M. Slayton, C.M. Aubuchon, T.L. Camis, A.R. Noble and N.J. Tro, J.
Phys. Chem., 1995, 99, 2151-2154.