Self-assembled monolayers on gold generated from terminally perfluorinated alkanethiols bearing propyl vs. ethyl hydrocarbon spacers

Article abstract of DOI:10.1016/j.jfluchem.2014.09.003

This paper examines the structural and interfacial properties of terminally perfluorinated self-assembled monolayers (FSAMs) on gold generated from the adsorption of a new series of terminally perfluorinated propanethiols F(CF₂)_n(CH₂)₃SH, where n = 8, 10, and 12. Analysis of these FSAMs by ellipsometry and X-ray photoelectron spectroscopy (XPS) confirmed the formation of the monolayer films. The contact angles of water and n-hexadecane on these FSAMs indicated a high degree of hydrophobicity and oleophobicity. Polarization modulation infrared reflection-adsorption spectroscopy (PM-IRRAS) analysis of the films revealed that the fluorinated chains are oriented largely perpendicular to the gold surface. In addition, the FSAMs formed from the new adsorbates were compared to known FSAMs derived from F(CF₂)_n(CH₂)₂SH, where n = 8, 10, and 12, to examine the influence of the number of CH₂ groups in the short alkyl spacer upon the conformational order and packing structure of the films. Analysis of the XPS spectra for the normalized peak intensity of the F 1s and S 2p binding energies for both types of films suggest a slight increase in packing density for the chains having the propyl vs. the ethyl hydrocarbon spacer. This conclusion is consistent with the observed decrease in the wetting behavior of hexadecane on the FSAMs formed from the new adsorbates. However, the preponderance of the data indicates that these two series of partially fluorinated alkanethiols form films with highly similar structure/packing characteristics with no discernible "odd-even" effect between the two series.

Full text of DOI:10.1016/j.jfluchem.2014.09.003

Journal of Fluorine Chemistry 168 (2014) 128–136
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Self-assembled monolayers on gold generated from terminally
perfluorinated alkanethiols bearing propyl vs. ethyl hydrocarbon
spacers
*
Oussama Zenasni, Andrew C. Jamison, Maria D. Marquez, T. Randall Lee
Departments of Chemistry and Chemical Engineering and the Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5003, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
This paper examines the structural and interfacial properties of terminally perfluorinated self-assembled
monolayers (FSAMs) on gold generated from the adsorption of a new series of terminally perfluorinated
propanethiols F(CF2)n(CH2)3SH, where n = 8, 10, and 12. Analysis of these FSAMs by ellipsometry and
X-ray photoelectron spectroscopy (XPS) confirmed the formation of the monolayer films. The contact
angles of water and n-hexadecane on these FSAMs indicated a high degree of hydrophobicity and
oleophobicity. Polarization modulation infrared reflection-adsorption spectroscopy (PM-IRRAS) analysis
of the films revealed that the fluorinated chains are oriented largely perpendicular to the gold surface. In
addition, the FSAMs formed from the new adsorbates were compared to known FSAMs derived from
F(CF2)n(CH2)2SH, where n = 8, 10, and 12, to examine the influence of the number of CH2 groups in the
short alkyl spacer upon the conformational order and packing structure of the films. Analysis of the XPS
spectra for the normalized peak intensity of the F 1s and S 2p binding energies for both types of films
suggest a slight increase in packing density for the chains having the propyl vs. the ethyl hydrocarbon
spacer. This conclusion is consistent with the observed decrease in the wetting behavior of hexadecane
on the FSAMs formed from the new adsorbates. However, the preponderance of the data indicates that
these two series of partially fluorinated alkanethiols form films with highly similar structure/packing
characteristics with no discernible ‘‘odd-even’’ effect between the two series.
Received 19 May 2014
Received in revised form 26 August 2014
Accepted 2 September 2014
Available online 11 September 2014
Keywords:
Self-assembled monolayers
SAMs
Fluorinated alkanethiols
Fluorinated thin films
Conformational order
Packing density
ß 2014 Elsevier B.V. All rights reserved.
1. Introduction
chemically bound to the surface offers an advantage over their
polymeric counterparts when used as nanoscale boundary films to
Fluorinated thin films serve as vital tools for modifying the
surfaces of nanoscale devices. Such modifications include the
adsorption of fluorinated amphiphiles on coinage metals as a
means of corrosion prevention [1], on metal oxides to reduce
stiction in microelectromechanical systems (MEMS) [2], and on
medical implants as biomaterial coatings [3,4]. The motivation to
use these adsorbates is tied to the ability of perfluorocarbon
segments to transform the physical properties of an interface by
influencing the wettability, friction, and barrier properties of the
resulting surfaces [5–7]. Such modifications allow forthe generation
of films that exhibit low surface energy with coefficients of friction
that are a quarter of that of polytetrafluoroethylene (PTFE) [8]. In
addition, the fact that the molecules of these thin films are
reduce friction.
One of the most widely explored classes of nanoscale boundary
lubricants includes films known as self-assembled monolayers
(SAMs), which are derived from the adsorption of either
fluorinated or non-fluorinated adsorbate molecules, as illustrated
in Scheme 1 [9–12]. In perhaps the most well known example, the
adsorption of alkanethiols on Au(1 1 1) proceeds spontaneously
with the sulfur headgroups binding covalently to the surface of the
metal in a highly ordered array [9,12–15]. The alkyl spacer tilts to
maximize interchain van der Waals (vdW) interactions, stabilizing
the film. Normal alkanethiolate SAMs (HSAMs) formed on gold
offer key advantages over other adsorbate/substrate combinations
because of the inertness of gold as well as the moderately stable
S–Au bond that enables the adsorbate molecules to migrate on the
surface and maximize packing [9]. Both of these characteristics
allow for precise control over the chemical and structural nature
of the interface. This combination of features offers a more
*
Corresponding author. Tel.: +1 713 743 2724; fax: +1 713 751 4445.
E-mail address: trlee@uh.edu (T.R. Lee).
http://dx.doi.org/10.1016/j.jfluchem.2014.09.003
0022-1139/ß 2014 Elsevier B.V. All rights reserved.

O. Zenasni et al. / Journal of Fluorine Chemistry 168 (2014) 128–136
129
Scheme 1. Illustration of the differences in the tilt angle of thiolated chains in HSAMs as compared to FSAMs on gold.
comprehensive assessment of the effect of adsorbate chain
modification on film properties as compared to other systems.
Thus, concerning terminally perfluorinated adsorbates, the
ability to control the number of fluorocarbon moieties in the
tailgroup makes possible the production of fluorinated SAMs
(FSAMs) with specific tunable properties [12,16].
relatively long fluorocarbon tailgroup: F(CF2)n(CH2)3SH; the FnH3
series where n = 8, 10, and 12; F8H3, F10H3, and F12H3,
respectively. These adsorbates are illustrated in Scheme 2 in the
form of thiolates on gold. We compare the FSAMs formed from
these new adsorbates to those formed from F(CF2)n(CH2)3SH; the
FnH2 series where n = 8, 10, and 12; F8H2, F10H2, and F12H2,
respectively, whose characteristics have been extensively investi-
gated [26,30]. Prior work on FSAMs formed from FnH2 have shown
that such adsorbates typically possess certain defects at the metal-
thiol interface due to the spacing restrictions imposed by the larger
vdW diameter of the fluorocarbon helix being so close to the
substrate.
Thus, by extending the methylene spacer to three, we sought
to evaluate the effect of such a change on the conformational
order of the adsorbate chains as well as upon the interfacial
properties of the resulting films. Further, we wished to examine
whether these terminally fluorinated films would exhibit the
SAMs generated from the adsorption of selectively fluorinated
alkanethiols on gold have been used by many researchers as model
thin films for evaluating the structural influences that determine
the properties of these unique organic films [12,17,18]. Prior
research has shown that the relatively large van der Waals
diameter of the perfluorocarbon segments in FSAMs of the form
F(CF2)n(CH2)2SH (FnH2), where n ꢀ 6, causes the chains to be
˚
spaced ꢁ5.7 A apart, which is larger than that of normal HSAMs on
˚
gold (ꢁ4.9 A) [19–21]. Such differences in chain spacing are
partially attributable to the helical arrangement of the CF2 units in
the perfluorinated segments, as opposed to the trans-extended
chain alignments adopted by CH2 units in well-ordered n-
alkanethiol-based monolayer films—an arrangement that max-
imizes the packing density of the chains [12]. Therefore, as
illustrated in Scheme 1, FSAMs pack with their helical perfluori-
nated chains oriented roughly perpendicular to the surface, with a
tilt angle ꢁ118 from the surface normal, as opposed to well-packed
HSAMs where the tilt angle is ꢁ308 [22,23]. Such highly fluorinated
thiolates form monolayers that adopt either a c(7 Â 7) or p(2 Â 2)
hexagonal lattice, as compared to HSAMs with a (H3 Â H3)R30
hexagonal lattice [24,25].
The physical properties of such films are also directly affected
by the size of the fluorocarbon segment [16,26–28]. Films of FnH2
where n = 8 are poorly wettable by both water and oil [26]. This
phenomenon can be attributed in part to the relatively large van
der Waals radii of the perfluorinated segments, which has also
been credited with influencing the effective interfacial energy
(dispersive energy) for the exposed surface through a mechanism
in which the packing density of the terminal CF3 groups diminishes
with increasing length of the perfluorinated segments [16]. Wet-
tability studies of FSAMs generated from thiols of the form
F(CF2)n(CH2)mSH (FnHm, where the length of the fluorocarbon and
hydrocarbon segments were varied, but the total chain length was
held constant) vs. FSAMs generated from thiols of the form
F(CF2)n(CH2)11SH (FnH11, where the length of the fluorocarbon
segment was varied but the hydrocarbon chain was held constant
at eleven methylenes), indicated little additional influence from
these extended fluorocarbon segments upon the measured contact
angles with changes in the underlying SAM structure [16,27]. How-
ever, the interaction between a polar contacting liquid with each of
the monolayers in the two series decreased with an increasing
number of fluorocarbons until approximately n = 5. Previous
studies have attributed this phenomenon to a burying of the
dipole associated with the fluorocarbon–hydrocarbon (FC–HC)
junction within the interface [16,28,29].
Scheme 2. Illustrations of the two series of adsorbates of the terminally
perfluorinated FSAMs on gold that possess either three (FnH3) or two (FnH2)
methylene spacers. The structural drawings on the right emphasize the presence of
the alkyl segments between the surface bound headgroups and the rigid
perfluorinated chains.
This report explores the structural and interfacial properties of
FSAMs associated with a new class of highly fluorinated thiol-
based adsorbates having a propyl group as an alkyl spacer and a

130
O. Zenasni et al. / Journal of Fluorine Chemistry 168 (2014) 128–136
Table
1
well known ‘‘odd-even’’ effects that are characteristic of highly
ordered SAMs on gold, including those with CF3-terminated
hydrocarbon tailgroups [28,29,31]. Prior reports of thiolate SAMs
with short alkyl spacers bearing rigid aromatic ring structures
have pointed to a number of influences that might contribute to
parity effects: the bending potential associated with the bonds of
the headgroup to the substrate and to the alkyl spacer, the
relative length of the alkyl spacer, and the rotation of the
tailgroup around the bond with the alkyl spacer [32–35]. To
explore the similarities and differences of the FSAMs in the
present study, we characterized the films formed from both the
FnH3 series and the FnH2 series using ellipsometry, contact
angle goniometry, polarization modulation infrared reflection-
adsorption spectroscopy (PM-IRRAS), and X-ray photoelectron
spectroscopy (XPS). SAMs formed from octadecanethiol (C18)
were used as reference standards for these studies.
Ellipsometric data for FSAMs formed from the FnH3 and FnH2 series as compared
to the C18 SAMs.
Adsorbate
1.45 Refractive index
1.33 Refractive index
F8H3
F10H3
F12H3
F8H2
F10H2
F12H2
C18
11
13
16
10
12
15
21
13
16
20
13
15
19
–
thicknesses, which is expected given the decrease in the value of
the refractive index used in the calculation [26].
2.2. Analysis of the films using X-ray photoelectron spectroscopy
2. Results and discussion
Analysis of the chemical composition of organic films by X-ray
photoelectron spectroscopy not only provides an understanding of
the elemental content of the monolayer, but also insight into the
ordering of these systems. For the current study, the binding
energy of the sulfur headgroup to the substrate was used as a
reference point to determine the type of sulfur moieties present in
the monolayer. All of the FSAMs of the FnH3 series exhibited S 2p
binding energies of 162.0 eV (2p2/3) and 163.2 eV (2p1/2), as shown
in Fig. 1a, peak positions that have been assigned to thiolate sulfur
species bound to gold [37,38]. Absent from these spectra are the
characteristic peaks associated with sulfur, either as an unbound
thiol or a disulfide (peaks typically found at 163.5–164.0 eV)
[24,38,39] or in an oxidized state (generally located at ꢁ168 eV)
[40]. Therefore, analysis of the XPS spectra confirms the formation
of the monolayers, the absence of any non-adsorbed species
present in the film, and the integrity of these adsorbates upon the
assembly of the FSAM.
After successfully synthesizing the new adsorbates for the
FnH3 series, we tested their performance in forming terminally
perfluorinated monolayer films on gold and compared these
FSAMs to those of the FnH2 series. We anticipated that the increase
in the spacer length could offer better packing for the molecules,
yet allow the fluorinated chains to still maintain their upright
orientation with respect to the gold surface. To evaluate the FSAMs,
we utilized XPS to analyze the atomic concentration on the gold
surface and PM-IRRAS to determine the perfluoro-helix orientation
for both classes of FSAMs examined. Furthermore, we measured
the thicknesses and evaluated their wetting behavior to provide a
complete set of data for determining the influence of the length of
the short hydrocarbon spacer on the structure and interfacial
properties.
2.1. Ellipsometric thicknesses of the films
The C 1s spectra for the FnH3 FSAMs provided in Fig. 1b
reveal three C 1s peak positions corresponding to the carbons of
CF3 at ꢁ293.2 eV, CF2 at ꢁ290.8 eV, and CH2 at ꢁ284.8 eV, as
shown in Table 2. Note that the intensity of the peaks
corresponding to the CF2 carbons increases in a manner
proportional to the increase in the number of CF2 units in the
monolayer. This trend is also observed in the peaks correspond-
ing to the F 1s binding energy, Fig. 1c, where the intensity of the
peaks increase proportionally to the increasing number of
fluorine atoms on the chain. Furthermore, these trends are also
found in the XPS spectra for the FSAMs of the FnH2 series, as
shown in Table 2 and Fig. 2.
To evaluate the packing density of the FnH3 FSAMs, we
examined the ratio of the S 2p/Au 4f peak intensities for the FnH3
FSAMs compared to the FnH2 FSAMs [41]. Table 3 shows these
data along with the composite peak intensity ratios for C 1s/Au 4f
and F 1s/Au 4f for the FnH3 and FnH2 FSAMs. The ratios of the
normalized sulfur peaks increase as the number of fluorocarbons
increases: 0.93 for n = 8, 1.01 for n = 10, and 1.05 for n = 12, which
indicates that the packing of the FnH3 FSAMs with the longer
perfluorinated segments is denser than that of the FnH2 series,
particularly in light of the increased attenuation of the sulfur signal
on the FnH3 series. To provide an alternative perspective on the
surface packing, we also generated the C 1s/Au 4f ratios for the two
series, which gave 1.07 for n = 8, 1.13 for n = 10, and 1.01 for
n = 12. For these data, we anticipated values that reflected an
additional carbon atom for the FnH3 series. However, the value for
the F12H3 FSAM fails to align with the data obtained with the S
2p/Au 4f ratios. On the other hand, the depth and number of F 1s
electrons is the same for a given length of fluorocarbon chain for
two analogs (e.g., F8H3 vs. F8H2). Therefore, changes in the
Variation in the thickness of organic thin films is often detected
utilizing ellipsometry, with numerous studies showing how this
technique can be used to examine trends in thin film thickness for a
homologous series of thiolate SAMs [15,36]. Therefore, we
examined the two series of FSAMs by ellipsometry to determine
if they produced similar trends and if the thickness values were
consistent with the difference of a single methylene unit. As for the
SAM formed from the adsorbate standard, C18, its thickness was
compared to prior measurements collected on our ellipsometer,
˚
which gave an average thickness of 21 A for data collected on seven
samples over a one year timeframe. Table 4 shows the thicknesses
of the FnH3 series of FSAMs and the corresponding FnH2 series,
along with the data for the SAMs formed from C18. For the
measurements taken utilizing a refractive index value of 1.45, a
value typically used for alkanethiolate SAMs [26], the ellipsometric
data indicate an increase in the thickness of the film going from
˚
˚
˚
F8H3 (11 A), to F10H3 (13 A), to F12H3 (16 A); an increment of
˚
ꢁ1.2 A per CF2 unit. A similar trend is apparent in the data for the
˚
˚
˚
FnH2 series: 10 A, 12 A, and 15 A for n = 8, 10, and 12, respectively.
Both sets of data align with prior published research, which
˚
reported a change of ꢁ1.23 A per CF2 unit—a value associated with
an average increase in the length of the perfluorinated segment for
a
homologous series of FSAMs [36]. When comparing the
thicknesses of the FnH3 series to that of the FnH2 series, the
˚
data indicate that the FnH3 thiols produce films that are ꢁ1 A
thicker, a number that is in line with the increase in chain length by
one methylene unit [36]. Additionally, we collected ellipsometric
data using 1.33 as the refractive index—a value that has been used
in prior studies of highly fluorinated thin films [26]. This second set
of data is included in Table 1 for comparison, showing higher

O. Zenasni et al. / Journal of Fluorine Chemistry 168 (2014) 128–136
Table
131
2
Peak positions for the FnH3 and FnH2 FSAMs for the XPS spectra displayed in Figs. 1
and 2.
Peak
FnH3 (eV)
n = 8
FnH2 (eV)
n =8
n = 10
n = 12
n = 10
n = 12
C 1s (CF3)
C 1s (CF2)
C 1s (CH2)
S 2p3/2
293.2
290.8
284.6
162.0
688.2
293.2
290.8
284.8
162.0
688.2
293.2
290.9
284.7
162.0
688.2
293.0
290.7
284.8
162.0
688.1
293.2
290.9
284.7
162.0
688.2
293.2
291.0
284.8
162.0
688.2
F 1s
Table
3
Ratios of the S 2p, C 1s, and F 1s XPS peak intensities for the FnH3 FSAMs as
compared to those of FnH2, using the peak intensity for Au 4f to normalize the
valuesa.
Fn
(S 2p/Au 4f)FnH3
/
(C 1s/Au 4f)FnH3
/
(F 1s/Au 4f)FnH3/
(S 2p/Au 4f)FnH2
(C 1s/Au 4f)FnH2
(F 1s/Au 4f)FnH2
n = 8
0.93
1.01
1.05
1.07
1.13
1.01
1.00
1.05
1.01
n = 10
n = 12
a
The ratios in this table were derived from the averages of two data sets.
2.3. Wettabilities of the films
For fluorinated thin films, the wettability of the surfaces have
typically been probed with contacting liquids such as water
(H2O) and n-hexadecane (HD) to provide a measure of the
interfacial hydrophobicity and oliophobicity, respectively. Fur-
thermore, because contact angle measurements on SAMs are
highly sensitive to small differences in the interfacial structure/
composition of the films, additional insight into the nature of
the packing and/or orientation of the adsorbates within the film
can sometimes be gleaned from contact angle data. For FSAMs,
the interpretation of the data can be complicated if the dipole
arising from the fluorocarbon–hydrocarbon junction lies near
the interface and influences the contact angles; however, for the
current FSAMs, this dipole is sufficiently buried within the film
that it should have no impact on the contact angles
[16,28,29,42]. Assuming that the films expose perfluorinated
surfaces of equivalent adsorbate density and surface ordering,
the contact angle data should reveal only the influence of the
fluorinated film, with a possible minor role from the underlying
gold substrate [43].
The advancing contact angles (ua) of H2O (a polar protic liquid)
and HD (a nonpolar hydrocarbon liquid) measured on both series
of FSAMs are shown in Table 4. FSAMs of the FnH3 series and those
of the FnH2 series show the same high degree of hydrophobicity
and oleophobicity, with contact angle values ꢀ1158 for H2O and
ꢀ738 for HD. Note that all of the FSAMs are less wettable than the
C18 SAMs for both liquids, even though the latter is a well-packed
film formed from the nanoscale equivalent of paraffin wax. For
both series of films, the contact angles of water are generally
equivalent for chains of equal perfluorocarbon length, but show a
systematic increase in value with a lengthening of the perfluoro-
carbon segment (i.e., with increasing n). The effect of the
monolayer thickness on wettability might reflect a reduction in
the attractive van der Waals force between the contacting liquid
and the underlying gold substrate as the thickness of the film
increases (and the separation between them consequently
increases) [43]. However, comparison of the contact angle data
for the FnH3 FSAMs vs. the FnH2 FSAMs suggests that there is no
strong influence from the underlying gold substrate, nor are there
any ‘‘odd-even’’ effects in these contact angle data. The absence
of such parity effects likely arise from the helical conformation of
these perfluorinated termini and the low surface energy of
Fig. 1. XPS spectra for each of the FSAMs of the FnH3 series: (a) the S 2p spectral
region, (b) the C 1s spectral region, and (c) the F 1s spectral region.
normalized F 1s ratios per fluorocarbon chain size would better
correlate to changes in the packing density of the perfluorinated
helix as the number of methylenes increased from two to three. For
our data (1.00 for n = 8, 1.05 for n = 10, and 1.01 for n = 12), the
ratios indicate a slight improvement in packing associated with the
longer propyl spacer.

132
O. Zenasni et al. / Journal of Fluorine Chemistry 168 (2014) 128–136
Table
4
Contact angles of FSAMs formed from the FnH3 and FnH2 series and the C18 SAMs.
Adsorbate
H2O (adv./rec.)
HD (adv./rec.)
F8H3
F10H3
F12H3
F8H2
F10H2
F12H2
C18
115/105
116/106
118/106
115/104
116/106
117/106
114/104
76/56
76/60
78/59
73/56
76/60
77/60
49/40
Fig. 3. Advancing contact angles (ua) for hexadecane on the FnH3 ( ) and FnH2 (^)
monolayer films. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
In particular, the data in Table 4 and Fig. 3 show that the average
value of ua for HD on F8H3 (768) is higher than that for F8H2 (738),
which might be an indication that the FSAMs formed from the
shortest perfluorocarbon chains benefit from an increase in
distance between the relatively larger perfluorocarbon segments
(as compared to the hydrocarbon segments) and the preferred
lattice bonding sites on gold for thiols (vide supra). This difference
is not evident in the F10 and F12 films from both series, perhaps
because the interchain packing forces between the longer
perfluorocarbon segments overcome the disorder introduced by
having a short ethyl hydrocarbon spacer.
2.4. Analysis of the films using surface infrared spectroscopy
Surface IR spectra generally reveal information regarding SAM
organization and orientation of the individual chains within the
film. Due to the small number of methylene units in these chains,
the C–H vibrational bands proved to be too weak to make reliable
measurements, which limited our ability to use the collected PM-
IRRAS data to determine the relative conformational order of the
alkyl segments within these thin films, and also limited our insight
into how the methylenes in the two systems might orient
themselves to allow the overlying perfluorinated chain segments
to afford similar exposed interfaces. However, analysis of the C–F
stretching region in the PM-IRRAS spectra proved more fruitful. In
Fig. 2. XPS spectra for each of the FSAMs of the FnH2 series: (a) the S 2p spectral
region, (b) the C 1s spectral region, and (c) the F 1s spectral region.
Fig. 4, the bands between 1240 to 1280 and 1330 to 1380 cmÀ1
,
CF2
fluorinated thin films formed from perfluorocarbon segments
where n ꢀ 6 [16,42].
designated as npd
(vibrational mode for CF2 with a transition
CF2
dipole perpendicular to the helical axis) and nax
(vibrational
Similar conclusions can be drawn from the contact angle data
for HD shown in Table 4. There are, however, slight indications that
the FnH3 FSAMs might be better organized than the FnH2 FSAMs.
mode for CF2 with a transition dipole parallel to the perfluoro-
carbon helical axis), respectively, provide evidence that all of the
chains are oriented largely perpendicular to the surface for these

O. Zenasni et al. / Journal of Fluorine Chemistry 168 (2014) 128–136
133
Fig. 4. PM-IRRAS spectra for the C–F stretching region of the films generated from the FnH3 series: (a) F8H3, (b) F10H3, and (c) F12H3.
generate self-assembled monolayers on gold. These new FSAMs
were compared to those of known analogous FSAMs having short
ethyl hydrocarbon spacers. Ellipsometric measurements con-
firmed the formation of monolayer films from all of the adsorbates
and revealed that the FnH3 FSAMs produced films that were
˚
consistently ꢁ1 A thicker than the FnH2 FSAMs. Analysis by XPS
provided additional support for a conclusion that FSAMs were
successfully formed by showing that all of the sulfur atoms in the
films were bound to the surface of gold. Analysis by XPS also
revealed a slight increase in packing density at the gold surface for
the FnH3 FSAMs compared to the FnH2 FSAMs. Contact angle
measurements were further consistent with a model in which the
propyl spacer allows for a slightly denser packing of the adsorbates
CF2
on the surface of gold. The relative intensity of the npdCF2 to nax
bands in the PM-IRRAS spectra indicated that the perfluorocarbon
tailgroups were oriented largely perpendicular to the surface. In
stark contrast to prior reports for thiolate SAMs bearing rigid
aromatic tailgroups on short alkyl spacers, none of the data showed
a discernible ‘‘odd-even’’ effect for FnH3 FSAMs compared to FnH2
FSAMs.
Fig. 5. Peak positions of the vaxCF2 bands as a function of fluorocarbon units in the
FSAMs formed from the FnH3 and FnH2 series: first and second axial bands for
FnH3 series, (*) and (^), respectively, and first and second axial bands for FnH2
series, ( ) and ( ), respectively. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
4. Materials and methods
4.1. Materials
FSAMs [27,44]. The data also suggest a slight decrease in the tilt of
the chains as the size of the perfluorocarbon helix increases, based
Gold shot (99.999%) was purchased from Americana Precious
Metals. Chromium rods (99.9%) were purchased from R. D. Mathis
Company. Polished single-crystal silicon (1 0 0) wafers were
purchased from Silicon Wafer Enterprises and rinsed with
absolute ethanol (Aaper Alcohol and Chemical Co.) before use.
n-Octadecanethiol (C18) was purchased from Sigma-Aldrich and
used as received. The starting materials 1-iodo-1H,1H,2H,2H-
perfluorodecane (94%), 1-iodo-1H,1H,2H,2H-perfluorododecane
(94%), and 1-iodo-1H,1H,2H,2H-perfluorotetradecane (90%) were
purchased from SynQuest Laboratory. Iodoperfluorooctane (98%),
iodoperfluorodecane (98%), and iodoperfluorododecane (97%)
were purchased from SynQuest Laboratory. Triethylamine (NEt3),
potassium thioacetate (KSAc), lithium aluminum hydride (LiAlH4),
allyl alcohol (99%), tributyltin hydride (Bu3SnH), and methanesulfonyl
chloride (MsCl) were purchased from Sigma-Aldrich Co. and used as
purchased. Azobisisobutyronitrile (AIBN) was purchased from Sigma-
Aldrich Co. and was recrystallized from methanol prior to use. Solvents
used in the synthesis, 1,2-dichloroethane (DCE), tetrahydrofuran
(THF), diethyl ether (Et2O), toluene, and hexanes, were purchased from
either Sigma-Aldrich or Avantor Performance Materials and used
as received, unless stated otherwise. Magnesium sulfate (MgSO4)
was purchased from J. T. Baker Co., and sulfuric acid (H2SO4)
was purchased from MACRON. Column chromatography
CF2
on the relative intensity of the two bands (nax to npdCF2) for the
spectra in Fig. 4. Previous studies haveproposed that such a decrease
CF2
CF2
in intensity of npd compared to that nax in the spectra of the
surface IR is associated with a decrease in the tilt angle of the
perfluorinated helix from the surface normal due to the orthogonal
natureofthevibrationalmodes[27,44]. However, theexacttiltangle
cannot be determined from this technique (from the as-is spectra)
since we are unable to quantitatively compare the absolute
intensities of a given vibration mode between spectra due to the
signal modulation associated with the PM-IRRAS method [45–47].
Comparing the peak positions of the FnH3 FSAMs to those of
FnH2, the naxCF2 bands appear at exactly the same wavenumber for
a given perfluoroalkyl segment regardless of the number of
underlying methylene units. As shown in Fig. 5, the peak positions
of these bands shift with an increasing number of fluorocarbon
units and appear to be specific to the size of the helix—a
phenomena that has been noted previously [36].
3. Conclusions
A series of new terminally perfluorinated alkanethiols having
short propyl hydrocarbon spacers were synthesized and used to
was performed using silica gel (40–60
mm), and thin-layer

134
O. Zenasni et al. / Journal of Fluorine Chemistry 168 (2014) 128–136
Scheme 3. Synthetic strategy used to prepare the fluorinated alkanethiol adsorbates of the form F(CF2)n(CH2)3SH, where n = 8, 10, and 12 (the FnH3 series).
chromatography (TLC) was carried out using 200
m
m-thick silica
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,13-Henicosafluoro-
tridecan-1-ol (1b). This intermediate was prepared using a method
analogous to that used to prepare intermediate 1a. 1H NMR
gel plates, both obtained from Sorbent Technologies, Inc. The
developed TLC plates were visualized using molybdenum blue
staining solution. Contacting liquids were of the highest purity
available; n-hexadecane (HD) was purchased from Aldrich
Chemical Co. and water was generated from a Milli-Q Water
(500 MHz, CDCl3):
d 3.75 (q, J = 5.73 Hz, 2H, CH2O), 2.22 (m, 2H,
CH2CF2), 1.87 (m, 2H, CH2CH2).
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,15,15,15-Pen-
tacosafluoropenta-decan-1-ol (1c). This intermediate was prepared
using a method analogous to that used to prepare intermediate 1a.
System with resistance of 18.2 M
V (Millipore Corporation).
1H NMR (500 MHz, CDCl3):
d 3.75 (q, J = 5.54 Hz, 2H, CH2O), 2.22
4.2. Synthesis of the adsorbates
(m, 2H, CH2CF2), 1.87 (m, 2H, CH2CH2).
4.2.1. Synthesis of terminally perfluorinated alkanethiols
The fluorinated alkanethiols having propyl hydrocarbon
spacers were synthesized using the method illustrated in Scheme
3. The perfluorinated alkanethiols with spacers formed from two
methylene units were synthesized according to a procedure found
in the literature [30]. Because 1H,1H,2H,2H-perfluorotetradecanethiol
has not been reported previously, its synthesis is also included in
this subsection. Appendix A contains NMR spectra of the newly
synthesized thiols.
4.2.3. Synthesis of the mesylate intermediates
An aliquot of NEt3 (4.4 mL; 32 mmol) was added to a solution of
alcohol 1a (5.00 g; 10.5 mmol) in anhydrous THF (50 mL) at room
temperature while stirring. The resultant mixture was cooled to
0 8C under argon. After cooling, MsCl (8.1 mL; ꢁ10 mmol) was
added while stirring. The reaction was allowed to warm to room
temperature and stirred for 6 h. The reaction was then quenched
with ice-cold water (50 mL). The product was extracted with Et2O
(3 Â 100 mL), and the combined organic phases were washed with
1 M aqueous HCl (1 Â 100 mL), water (1 Â 100 mL), and brine
(1 Â 100 mL). The organic layer was dried over anhydrous MgSO4,
followed by removal of the solvent by rotary evaporation to yield
the crude product as a crystalline solid. Triturating these crystals in
hexanes (50 mL) afforded the pure mesylate.
4.2.2. Synthesis of the alcohol intermediates
In
a 100-mL pear-shaped Schlenk flask, the starting 1-
iodoperfluorooctane (5.00 g, 9.16 mmol), AIBN (10 mol%) and allyl
alcohol (0.80 mL; ꢁ12 mmol) were dissolved in DCE (30 mL). The
system was degassed with three cycles of a standard freeze-pump
thaw procedure. After warming to room temperature, the reaction
mixture was heated to 85 8C for 8 h. The reaction was then cooled
to room temperature, and an additional equivalent of AIBN was
added under argon, followed by the same degassing method. The
system was heated again to 85 8C for 8 h. The reaction percent
conversion of perfluorinated iodide was monitored via 19F NMR.
The reaction was then transferred to a 250-mL round-bottom flask,
and the solvent was removed by rotary evaporation. The crude
product was dissolved in anhydrous toluene, and AIBN (10 mol%)
was added. The reaction was heated to 60 8C and then Bu3SnH
(3.5 mL; 13 mmol) was added dropwise for 15 min. The reaction
was then further heated to 85 8C and stirred at that temperature for
12 h. After removal of the volatiles, the crude product was
dissolved in anhydrous Et2O (200 mL). Excess of Bu3SnH was
removed under vacuum. Tributyltin iodide was converted to
tributyltin fluoride by adding KF (1.246 g; 13.24 mmol), followed
by stirring the resultant mixture at room temperature for 12 h. The
mixture was filtered, and the solvent was removed by rotary
evaporation. The crude product was carried to the next step
without additional purification.
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-Heptadecafluoroundecyl-
methanesulfonate (2a). Obtained in 69% yield. 1H NMR (500 MHz,
CDCl3):
d 4.32 (t, J = 6.02 Hz, 2H, CH2O), 3.04 (s, 3H, CH3), 2.25 (m,
2H, CH2CF2), 2.09 (m, 2H, CH2CH2).
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,13-Henicosafluoro-
tridecylmethanesulfonate (2b). This intermediate was prepared
using a method analogous to that used to prepare intermediate
2a. Obtained in 80% yield. 1H NMR (500 MHz, CDCl3):
J = 6.01 Hz, 2H, CH2O), 3.04 (s, 3H, CH3), 2.24 (m, 2H, CH2CF2), 2.09
(m, 2H, CH2CH2).
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,15,15,15-Pen-
tacosafluoropentadecyl-methanesulfonate (2c). This intermediate
was prepared using a method analogous to that used to prepare
intermediate 2a. Obtained in 78% yield. 1H NMR (500 MHz, CDCl3):
d 4.32 (t,
d
4.32 (t, J = 6.02 Hz, 2H, CH2O), 3.04 (s, 3H, CH3), 2.25 (m, 2H,
CH2CF2), 2.08 (m, 2H, CH2CH2).
4.2.4. Synthesis of the thioacetate intermediates
In
a three-necked round-bottom flask equipped with a
condenser and an addition funnel, mesylate 2a (1.0 g; 1.8 mmol)
was dissolved in a blend of THF/ethanol (1:1) (100 mL) under
argon. KSAc (0.616 g; 5.40 mmol) was dissolved in absolute
ethanol (20 mL) (previously degassed), and added dropwise to
the stirred mesylate solution under argon over 10 min. The
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-Heptadecafluoroundecan-1-
ol (1a). 1H NMR (400 MHz, CDCl3):
d 3.74 (m, 2H, CH2O), 2.21 (m,
2H, CH2CF2), 1.86 (m, 2H, CH2CH2).

O. Zenasni et al. / Journal of Fluorine Chemistry 168 (2014) 128–136
135
reaction was refluxed for 7 h. After the reaction was cooled to room
temperature, water (100 mL) was added, and the resulting mixture
was extracted with Et2O (3 Â 100 mL). The organic phases were
combined and washed with water (1 Â 100 mL), brine
(1 Â 100 mL), and then dried over MgSO4. After evaporation of
the solvent, the crude product was purified by column chroma-
tography on silica gel (hexanes/DCE, 9/1).
C-3), 24.70 (s, C-2), 23.88 (s, C-1). Broad peaks at
are characteristic of a long perfluorocarbon chain [22]. GC-MS, m/z:
641 (M+–HF–SH), 119 (C2F5+), 69 (CF3+), 61 (C2H5S+).
d
108.64–120.41
1H,1H,2H,2H-perfluorotetradecanethiol (F12H2). In
a three-
necked round-bottom flask equipped with a condenser and an
addition funnel, 1-iodo-1H,1H,2H,2H-perfluorotetradecane (1.0 g,
1.3 mmol) was dissolved in a blend of THF/ethanol (2:1) (100 mL)
under argon. An aliquot (0.29 g; 2.6 mmol) of KSAc was dissolved
in absolute ethanol (20 mL) (previously degassed), and added to
the stirred solution of starting material under argon over 10 min.
The reaction was then refluxed for 7 h. After cooling, water
(100 mL) was added to dissolve the potassium iodide salt. The
mixture was extracted with Et2O (3 Â 100 mL), and the combined
organic layers were washed with brine (1 Â 100 mL), and then
dried over MgSO4. The solvent was removed by rotary evaporation,
and the crude compound was dried under high vacuum for 12 h.
The resulting crude thioacetate was dissolved in dried THF (50 mL)
and added slowly to a stirred suspension of LiAlH4 (0.10 g;
ꢁ2.6 mmol) in THF (10 mL) at 0 8C. The mixture was then stirred at
room temperature for 10 h under argon, after which the reaction
was quenched with water (25 mL) (previously degassed) at 0 8C
and acidified with 1 M aqueous H2SO4 solution (25 mL) (previously
degassed). The mixture was then extracted with Et2O
(3 Â 100 mL), and the combined organic layers were washed with
water (1 Â 100 mL), brine (1 Â 100 mL), dried over MgSO4 and
then filtered. The solvent was removed by rotary evaporation, and
the crude product was purified by column chromatography on
silica gel (hexanes) to give 1H,1H,2H,2H-perfluorotetradecanethiol
(F12H2) as a white solid in 68% yield from the starting iodide (mp:
S-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-Heptadecafluoroundecyl)
ethanethioate (3a). Obtained in 73% yield. 1H NMR (500 MHz,
CDCl3):
d 2.95 (t, J = 7.16 Hz, 2H, CH2S), 2.35 (s, 3H, CH3C(O)), 2.16
(m, 2H, CH2CF2), 1.90 (m, 2H, CH2CH2).
S-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,13-Henicosa-
fluorotridecyl) ethanethioate (3b). This intermediate was prepared
using a method analogous to that used to prepare intermediate
3a. Obtained in 67% yield. 1H NMR (500 MHz, CDCl3):
d 2.95 (t,
J = 7.16 Hz, 2H, CH2S), 2.35 (s, 3H, CH3C(O)), 2.14 (m, 2H, CH2CF2),
1.90 (m, 2H, CH2CH2).
S-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,15,15,15-
Pentacosafluoropentadecyl) ethanethioate (3c). This intermediate
was prepared using a method analogous to that used to prepare
intermediate 3a. Obtained in 82% yield. 1H NMR (500 MHz, CDCl3):
d
2.95 (t, J = 7.16 Hz, 2H, CH2S), 2.35 (s, 3H, CH3C(O)), 2.14 (m, 2H,
CH2CF2), 1.90 (m, 2H, CH2CH2).
4.2.5. Synthesis of the thiol final products
The perfluorothioacetate 3a (0.5 g; 0.9 mmol) was dissolved in
dry THF (50 mL) and added dropwise to a stirring slurry of LiAlH4
(0.106 g; 2.80 mmol) in THF (10 mL) at 0 8C. The reaction was
stirred at room temperature for 6 h under argon. The reaction was
then quenched at 0 8C with water (25 mL, previously degassed),
and was acidified with 1 M H2SO4 solution (previously degassed).
The mixture was then extracted with Et2O (3 Â 100 mL). The
combined organic phases were washed with water (1 Â 100 mL)
and brine (1 Â 100 mL), dried over MgSO4, filtered, and evaporated
to dryness. The crude thiol product was purified by column
chromatography on silica gel (hexanes).
103.5 8C). 1H NMR (500 MHz, CDCl3 at 50 8C):
CH2SH), 2.44 (m, 2H, CH2CF2), 1.61 (t, 1H, J = 8.25 Hz, SH); 19F NMR
(471 MHz, CDCl3 at 50 8C):
-80.72 (3F, CF3), À113.95 (2F, CF2CH2),
d 2.79 (m, 2H,
d
À121.37 to À121.55 (14F), À122.41 (2F, CF2), À123.24 (2F, CF2),
À125.82 (2F, CF2); 13C NMR (125 MHz, CDCl3 at 50 8C):
d 36.14 (t,
JCF = 22.25, C-2), 15.56 (s, C-1). Broad peaks at d 107.62–120.41 are
characteristic of a long perfluorocarbon chain [22].
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-Heptadecafluoroundecane-
1-thiol (F8H3) was obtained as a colorless liquid in 73% yield. 1H
4.3. Preparation of films
NMR (500 MHz, CDCl3):
2H, CH2CF2), 1.93 (m, 2H, CH2CH2), 1.38 (t, J = 8.02 Hz, 1H, CH2SH);
19F NMR (471 MHz, CDCl3):
-80.69 (3F, CF3), À113.95 (2F,
d 2.63 (q, J = 7.25 Hz, 2H, CH2S), 2.23 (m,
Gold substrates were prepared by thermal evaporation of the
metals (chromium and then gold) onto Si(1 0 0) wafers under
d
CF2CH2), À121.63 (2F, CF2), À121.87 (4F), À122.65 (2F, CF2),
vacuum at a pressure ꢂ6 Â 10À5 Torr. The chromium layer of 100 A
˚
À123.37 (2F, CF2), À126.05 (2F, CF2); 13C NMR (125 MHz, CDCl3):
d
was deposited on the silicon surface to aid in the adhesion of a
˚
29.54 (t, JCF = 22.04 Hz, C-3), 24.69 (s, C-2), 23.98 (s, C-1). Broad
subsequent 1000 A layer of gold. To optimize film formation, the
˚
peaks at
d
108.19–120.38 are characteristic of a long perfluoro-
gold was deposited at a rate of 1 A/s. The substrates were rinsed
carbon chain [22]. GC–MS, m/z: 494 (C11H7F17SH+), 119 (C2F5+), 69
(CF3+), 61 (C2H5S+).
with absolute ethanol, dried with ultra-pure nitrogen gas, and used
promptly after cleaning. Thiol solutions at 1 mM concentration in
absolute ethanol were prepared in glass vials that had been
previously cleaned with piranha solution and rinsed thoroughly
with deionized water, followed by absolute ethanol. [Caution:
Piranha solution is highly corrosive, should never be stored, and should
be handled with extreme care.]. Two freshly cut and cleaned gold
slides (3 cm  1 cm) were inserted into each of the solutions. The
thin film samples were allowed to equilibrate 48 h, after which
they were rinsed with absolute ethanol and dried with ultra-pure
nitrogen gas before characterization.
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,13-Henicosafluoro-
tridecane-1-thiol (F10H3) was obtained as a white solid in 86% yield
(mp: 66 8C). 1H NMR (500 MHz, CDCl3 at 40 8C):
J = 7.22 Hz, 2H, CH2S), 2.23 (m, 2H, CH2CF2): 1.93 (m, 2H, CH2CH2),
1.37 (t, J = 8.08 Hz, 1H, SH); 19F NMR (471 MHz, CDCl3 at 40 8C):
d 2.62 (q,
d
-80.77 (3F, CF3), À113.76 (2F, CF2CH2), À121.52 to À121.69 (10F),
À122.54 (2F, CF2), À123.29 (2F, CF2), À125.95 (2F, CF2); 13C NMR
(125 MHz, CDCl3 at 40 8C):
d 29.63 (t, JCF = 23.05 Hz, C-3), 24.70 (s,
C-2), 23.89 (s, C-1). Broad peaks at
d 107.62–120.41 are
characteristic of a long perfluorocarbon chain [22]. GC–MS, m/z:
594 (C13H7F21SH+), 119 (C2F5+), 69 (CF3+), 61 (C2H5S+).
4.4. Characterization of SAMs
4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,15,15,15-Pen-
tacosafluoropenta-decane-1-thiol (F12H3) was obtained as a white
solid in 90% yield (mp: 101.6 8C). 1H NMR (500 MHz, CDCl3 at
4.4.1. Ellipsometric thickness measurements
The thicknesses of the monolayers were measured using a
Rudolph Research Auto EL III ellipsometer equipped with a He–Ne
laser (632.8 nm). The incident angle was fixed at 708. The refractive
index (RI) of the sublayer was set to 1.45, in accordance with
the established protocol [23]. Thickness measurements for the
fluorinated films were also acquired with optical constants
40 8C):
(m, 2H, CH2CH2), 1.37 (t, J = 8.08 Hz, 1H, SH); 19F NMR (471 MHz,
CDCl3):
-80.62 (3F, CF3), À113.92 (2F, CF2CH2), À121.60 to
À121.76 (14F), À122.59 (2F, CF2), À123.34 (2F, CF2), À126.01 (2F,
CF2); 13C NMR (125 MHz, CDCl3 at 40 8C):
29.63 (t, JCF = 22.25 Hz,
d 2.62 (q, J = 7.22 Hz, 2H, CH2S), 2.23 (m, 2H, CH2CF2), 1.92
d
d

136
O. Zenasni et al. / Journal of Fluorine Chemistry 168 (2014) 128–136
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employed to measure the contact angles of water (H2O) and n-
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Appendix A. Supplementary data
Supplementary data associated with this article can be found at
http://dx.doi.org/10.1016/j.jfluchem.2014.09.003.
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