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Applied Surface Science 252 (2005) 1941–1946
Characterisation and stability of hydrophobic surfaces in water
M. Maccarini, M. Himmelhaus *, S. Stoycheva, M. Grunze
Angewandte Physikalische Chemie, Ruprecht Karls Universitat Heidelberg, Im Neunheimer Feld 253, 69120 Heidelberg, Germany
Received 1 February 2005; received in revised form 20 March 2005; accepted 20 March 2005
Available online 18 April 2005
Abstract
The stability of four different hydrophobic surfaces in contact with water is assessed and discussed: H-terminated silicon,
hexamethyldisilazane (HMDS) coated silicon, silicon surfaces covered with self-assembled monolayers (SAMs) of octadecyl-
trichlorosilane (OTS) and gold surfaces modified with SAMs of alkanethiols. Changes in hydrophobicity and surface oxidation
were determined by contact angle measurements, X-ray photoelectron spectroscopy and AFM.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Hydrophobic surfaces; Stability; Surface modification; Self-assembling monolayers
1. Introduction
The preparation of homogeneous and molecularly
flat stable hydrophobic surfaces plays an important
role in technological applications, but is also of
interest in studies of the dynamical and structural
aspects of the solid/liquid interface. The interaction
between solid surfaces separated by a thin layer of
water is a central issue in many technological fields
such as the study of friction and wear, and lubrication
in an humid environment. Many experimental efforts
have been made to measure the forces between
hydrophobic surfaces in water by surface force
apparatus and atomic force microscopes [1]. One
* Corresponding author. Tel.: +49 6221 545065;
fax: +49 6221 545060.
E-mail address: michael.himmelhaus@urz.uni-heidelberg.de
(M. Himmelhaus).
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved
doi:10.1016/j.apsusc.2005.03.157
important conclusion derived in review by Christen-
son and Claesson [1] is that a number of controversial
results have been generated due to changes in the
surfaces properties during the experiments and due to
a lack of characterisation before, during, and after the
experiments. Indeed, it was found that surface forces
between more robust and stable hydrophobic surfaces
had a significant shorter range with respect to less
stable surfaces [2,3].
Interfaces between water and hydrophobic surfaces
have been widely studied in the last decades also in
view of the fundamental role in molecular self-
assembly processes governing the behaviour of
biological membranes, micellar formation, and the
stability of supramolecular structures. The properties
of a liquid in the bulk differs quite markedly from the
properties of the same liquid in the vicinity of a solid
surface. The attractive/repulsive interactions between
water molecules and the wall as well as the
.
M. Maccarini et al. / Applied Surface Science 252 (2005) 1941–19461942
geometrical constraints induce structural rearrange-
ments that extend over several molecular layers in
the bulk liquid. These structural changes are
reflected in the hydration forces between the two
surfaces immersed in the fluid. Among the few
methods available to probe experimentally such a
thin interfacial liquid layer are neutron and X-ray
reflectometry. However, both methods require long
acquisition times, up to several hours in the case of
neutron reflectometry. During this time, the surface
must be stable, otherwise the results are question-
able. Furthermore, surface roughness tends to
increase the noise in the reflectivity curves, thereby
limiting the observable range of momentum transfer
[4] and consequently the resolution of the experi-
ment [5]. It is therefore advisable to perform
reflectivity measurements with surfaces as flat as
possible. In addition, theoretical studies showed that
hydrophobic hydration depends significantly on the
radius of curvature of the hydrophobic body [6,7].
While water molecules can rearrange around small
apolar molecules in order to maintain the network of
hydrogen bonds, large objects tend to disrupt this
network, thus affecting the density of water
molecules in their vicinity. For flat surfaces, this
means that the character of surface roughness in
terms of corrugation and waviness will affect the
hydration at the water/solid interface and care must
be taken when results of different experiments are
discussed and compared with each other.
In this work, four commonly used hydrophobic
surfaces have been characterised in terms of their
hydrophobicity, stability in water and surface rough-
ness by means of contact angle measurements, X-ray
photoelectron spectroscopy (XPS), and atomic force
microscopy, thus exploring their potential use for the
study of interfacial water layers.
2. Experimental
2.1. Chemicals and materials
Dodecanethiol (DDT, 98%+), hexamethyldisila-
zane (HMDS), octadecyltrichlorosilane (OTS, 90%+)
were purchased from Sigma–Aldrich and used without
further purification. Also ethanol, methanol, chloro-
form, dimethylformamide (DMF), hydrogen peroxide
(H2O2), concentrated sulfuric acid (all analytical
grade) were used as received. Toluene absolute (H2O
< 0.005%) was purchased from Fluka. The water
used was Baker HPLC analysed purchased from J.T.
Baker. (1 0 0)-Silicon substrates were purchased from
Crystec GmbH, Germany.
2.2. Surface modification
2.2.1. Silicon oxide hydrogenised by HF etching
New (1 0 0)-silicon wafers were first immersed in
piranha solution (1/3 hydrogen peroxide + 2/3
sulfuric acid) for 10 min, and then immersed in
0.5% hydrofluoric acid for 10 min in order to remove
the oxidation layer and replace it with a hydrogen
termination. The HF etching was performed in pre-
cleaned plastic containers (sonication in a 3%
aqueous solution of Decon 90 detergent, and rinsed
with copious amount of Baker HPLC analysed
water) since glass-ware is attacked by HF and could
not be used. The samples were blown dry and stored
under nitrogen for a maximum time of few hours
before exposure to water. The measurement of
contact angle and XPS were performed immediately
after the samples were removed from water and
blown dry with nitrogen.
2.2.2. Silicon oxide modified with
hexamethyldisilazane
New (1 0 0)-silicon wafers were precleaned in
piranha solution for 10 min, and sonicated with
methanol, mixtures of methanol and chloroform and
pure chloroform. Subsequently, they were placed
under nitrogen and HMDS vapour overnight and
cleaned in an ultrasonic bath in mixtures of chloro-
form and methanol.
2.2.3. Self-assembled monolayer of
octadecyltrichlorosilane on silicon
New Si wafers (1 0 0) were immersed for 30 min
in piranha solution, then rinsed with millipore water
and ethanol. Subsequently, they were placed for
30 min in vacuum. Then the incubating solution,
prepared with OTS in toluene at a concentration of
25 mM, was introduced under nitrogen overpressure,
and left for 5 days. The samples were then placed in an
ultrasonic bath of toluene and ethanol and blown dry
with and stored under nitrogen.
M. Maccarini et al. / Applied Surface Science 252 (2005) 1941–1946 1943
Fig. 1. The advancing contact angles of Si(1 0 0)–H (full circles);
SiO2–HMDS (empty circles); OTS terminated SiO2 (empty squares)
and SAM of dodecanethiole on gold (triangles) measured as a
function of the immersion time in water. The initial values of the
dry samples are denoted with the dashed line for the dodecanethiol
SAM, dotted line for the OTS, and with the solid line for the
Si(1 0 0)–H and SiO2–HMDS.
2.2.4. Self-assembled monolayers of dodecanethiol
on gold
Thin films of polycrystalline gold were prepared by
thermal evaporation of 2 nm of titanium as adhesion
promoter and subsequent deposition of 50 nm of gold
of 99.99% purity onto polished single-crystal silicon
wafers (Crystec). Evaporation was performed at a
pressure of 2� 10�7 Torr and a deposition rate of
0.5 nm/s. The freshly evaporated gold substrates were
sonicated in dimethylformamide for 5 min prior to
UV cleaning with a UV lamp for 1 h. Immediately
after the UV irradiation, the substrates was immersed
in ethanol for at least 10 min in order to reduce the
oxidation layer which might result in the formation of
isolated multilayer islands as reported by Woodward
et al. [8]. The gold substrates were then immersed in a
1 mM solution of dodecanethiol in ethanol overnight.
2.3. Measurements
The advancing contact angles were measured with
a Kruss G1 goniometer using purified deionized water
(Milli-Q plus system, Millipore, Eschborn, Germany).
The AFM measurements were performed with a
Park Autoprobe (Veeco Instruments) operated in
contact mode and equipped with a 5mm� 5mm
scanner. Roughness values were calculated as rms
deviations from the mean using built-in software.
X-ray photoelectron spectra were obtained with a
Leybold Max 200 Analysis System using a Al K a
source operated at 200 W and a hemispherical energy
analyzer. The pass energy of the analyzer was set to
96 eV for survey scans and to 48 eV for recording
specific core levels. The electron binding energies
were referenced to the Si 2p peak at 99.15 eV for the
silicon substrates and to the Au 4f7/2 peak at 84.0 eV in
case of the gold-coated substrates.
3. Results and discussion
The advancing contact angles of the four hydro-
phobic surfaces as a function of the immersion time in
water are presented in Fig. 1. The contact angles of the
substrates modified with self-assembled monolayers
are 107� for dodecanethiol and 110� for OTS,
respectively. They prove to be very stable since they
are substantially unaffected by the exposure to water
even for long periods of time. The surfaces coated with
HMDS have a contact angle of 90�. This value remains
stable in contact with water for about 100 min, and
then slightly decreases to 80�. Hydrogen terminated
silicon on the contrary is not stable in contact with
water. The contact angle of the freshly prepared
surface is around 90�, but decreases significantly evenafter few minutes of exposure to water.
In order to track the changes in the properties of our
films, we performed XPS measurements on the
samples as a function of the contact time with water.
In Fig. 2a, we report the XPS survey spectra of
Si(1 0 0)–H before immersion in water, after 5 h in
water and, for comparison, also an XPS spectrum of
the native oxide on Si(1 0 0). In all spectra, the Si 2p
and 2s peaks are clearly discernible. The presence of
an oxygen peak can be observed in the SiO2 sample as
well as in the Si(1 0 0)–H sample which had been
immersed in water.
For the latter, XPS detail spectra of the Si 2p region
are depicted in Fig. 3 for different immersion times of
the samples in water. The energy scale was calibrated
by using the literature value of the Si 2p bulk peak at
99.15 eV. A second peak corresponding to SiO2 is
present at 102.9 eV. As expected, the SiO2 peak is
absent in the spectrum of the freshly prepared sample
due to the HF-etching of the native oxide layer.
However, an oxygen XPS peak is observed for the
M. Maccarini et al. / Applied Surface Science 252 (2005) 1941–19461944
Fig. 2. (a) XPS survey spectra of H–Si(1 0 0), of H–Si(1 0 0) after
5 h immersed in water. For comparison the spectra of native oxide
on Si(1 0 0) are shown. (b) XPS survey spectra of HMDS–SiO2 as
prepared, and after 24 h immersed in water. The spectra are verti-
cally displaced for clarity.
Fig. 3. Si 2p narrow XPS spectra of H–Si(1 0 0) at different
immersion times in water.
Si(1 0 0)–H sample before exposure to water, together
with a carbon peak, yet the intensity of the SiO2
emission is zero. We cannot exclude a residual
oxidation layer, since the sensitivity factor of the O1s
peak is almost four times larger than the one of the Si
2p peak. Hence, the presence of a small amount of
silicon oxide could generate a peak in the O1s region
without a corresponding feature in the Si 2p region.
However, due to the presence of a visible C1s peak
(with a sensitivity factor very close to that of silicon),
it is plausible that unavoidable contamination of the
plastic containers used in the HF etching stage, which
cannot be cleaned with the aggressive methods used
for glass-ware (e.g. piranha solution), are the most
likely origin of the O1s and C1s peaks. The area of the
SiO2 band increases with immersion time in water, and
correspondingly the area of the Si 2p bulk peak
decreases due to the oxide overlayer growth.
Obviously, the Si(1 0 0)–H surfaces are oxidized in
Fig. 4. The normalized area of the SiO2 and O XPS peaks on the
Si(1 0 0)–H and on the SiO2–HMDS, vs. contact time with water.
M. Maccarini et al. / Applied Surface Science 252 (2005) 1941–1946 1945
Table 1
XPS peaks of OTS on SiO2 and DDT on Au
OTS on SiO2 DDT on Au
Binding energy (eV) Peak assignation Binding energy (eV) Peak assignation
99.15 Si 2p 84.0 Au 4f7/2
150.3 Si 2s 87.6 Au 4f5/2
285 C1 164.2 S2 p3/2
532.4 O1s 284.8 C
water. The evolution of the area of the SiO2 and of the
oxygen peaks normalised to the area of the bulk Si 2p
peak is shown in Fig. 4a and b (full symbols). The area
of the SiO2 peak remains substantially zero for 10 min,
and then it growths until it saturates after about
1000 min. The induction period indicates an oxide
nucleation process. The characteristic time scale of this
process te, i.e. the time atwhich the area of the peaks are
1/e their saturation value, is of the order of 120 min.
The survey XPS spectra of SiO2–HMDS are
displayed in Fig. 2b besides the Si 2p and 2s peaks,
a significant O1s peak can be observed due to the
native oxide layer. The C1s peak originates from the
carbon in hexamethyldisilazane. Contact with water
affects the chemical properties of the surface only
marginally, as can be seen from the evolution of the
SiO2 and O peak areas in Fig. 4 (open circles).
However, for long immersion times in water, this
surface shows a decline of hydophobicity (Fig. 1),
which corresponds to a slight increase of the area of
the corresponding Si 2p and O1s peaks.
The XPS peaks of silicon coated with a monolayer
of OTS are summarised in Table 1. Again, the Si 2s
and Si 2p, and the O1s peaks are the dominating
features of the spectra. In addition, the C1s peak at
285 eV is is present due to the presence of the OTS.
The spectra of the dry sample and of the sample after
immersion in water do not show any appreciable
difference: for example, the difference in the carbon
peak area before and after immersion is less than 2%.
Furthermore, no peak other than the components of the
substrate and those of the adsorbed organic layer are
present.
The XPS peaks of gold coated with a monolayer of
DDT are also listed in Table 1. The gold peaks are
discernable over the entire binding energy range with
the 4f5/2 and 4f7/2 at 87.7 and 84.0 eV the most
prominent ones. The peak at 284.8 eV corresponds to
the carbon peak of the aliphatic chains of the
alkanethiols. No other components but those of the
substrate and the organic overlayer are present in the
spectra indicating a film free of contamination. The
spectrum obtained from the sample immersed in water
for 24 h does not show any appreciable change as
compared to the original state.
It follows that only surfaces hydrophobised with
long chain aliphaticmoieties satisfy the requirements of
stability inwater necessary to performmeasurements at
the water/hydrophobic interface. The roughness of
these surfaces, which is assumed to play an important
role in the hydration of hydrophobic surfaces [7], was
studied by atomic force microscopy in contact mode.
The root mean square roughness of the unloaded Au
substrate is 2.3 A, and 3.5 A for theAu substrate coated
with the SAM. This means that the dodecanethiols
adsorb on the gold surface following its irregular profile
only slightly affecting its topography. Also after the
adsorption of the organic monolayer, the roughness
remains small. Also in the case of silicon coated with
the OTS SAM, we found a very small roughness of
about 1 A, slightly smaller than the roughness of bare
silicon which was around 1.6 A.
4. Conclusions
The stability of four hydrophobic surfaces in contact
withwaterwas assessedby contact anglemeasurements
and XPS. Hydrogen-terminated silicon surfaces turned
out to have insufficient stability with significant
changes in the contact angle already after one minute
of exposure to water due to oxidation. HDMS-
terminated silicon exhibits better performance with a
drop in the contact angle from90� to 80� aftermore than
two hours of immersion in water. Only the surfaces
coated with long-chain aliphatic self-assembled mono-
layers, such asOTS on silicon andDDTon gold, did not
exhibit any appreciable changes even after 24 h of
M. Maccarini et al. / Applied Surface Science 252 (2005) 1941–19461946
exposure to water. In view of their stability, these latter
surfaces are proper candidates for studies on water/
hydrophobic surface effects bymeans of neutron,X-ray
reflectometry, surface force apparatus and atomic force
microscopy.
Acknowledgments
We gratefully acknowledge Georg Albert for
preparation of the gold-coated silicon substrates,
Roland Steitz and Jorg Fick for helpful discussions.
This work was funded by the Bundesministerium fur
Bildung und Forschung under grant no. GRE1HD and
in parts by the Deutsche Forschungsgemeinschaft
under grant no. Hi 693/2-1.
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