On the adsorption of dimethylmethylphosphonate on
self assembled alkanethiolate monolayers: Influence of humidity
Lars Bertilsson , Karin Potje-Kamloth, Hans-Dieter Ließ
Universität der Bundeswehr München, Fakultät für Elektrotechnik, Institut für Physik,
Werner-Heisenberg-Weg 39, D-85577 Neubiberg, Germany
Bo Liedberg *
Molecular Films and Surface Analysis Group, Laboratory of Applied Physics, Linköping University
S-581 83 Linköping, Sweden
Present
address: Tetra Pak GmbH, Untere Waldplätze 27, D-70569 Stuttgart, Germany
e-mail: lars.bertilsson@tetrapak.com
* Corresponding Autor: e-mail: bol@ifm.liu.se
Running Title: Water and DMMP on SAMs
Abstract
Humidity influences the adsorption of dimethylmethylphosphonate (DMMP) on three different
organic model interfaces to a quite different extent. The model interfaces, characterized by the
following tail groups (-OH, -CH3, -COOH), were prepared by solution self assembly of substituted alkanethiols on gold, and the coadsorption of water and DMMP was followed by a
surface acoustic wave sensor and infrared reflection absorption spectroscopy. The surfaces show
both quantitative and qualitative differences concerning the interaction with DMMP. Humidity is
found to influence the hydrogen bonds formed between DMMP and the tail groups, increasing the
strength of interaction. However, the changes in surface coverage of DMMP, as measured by
infrared absorption, is not fully registered by the surface acoustic wave sensor, due to a concomitant
desorption of water. Indeed there is even a loss in sensor signal due to the presence of humidity in
the case of the -COOH terminated surface, where the strongest interaction with DMMP is found.
After compensation for the loss of water the coverage of DMMP increases with relative humidity on
the OH surface and decreases with relative humidity on the COOH surface. The consequences of the
results are discussed in terms of their impact on the design of sensing interfaces based on hydrogen
bonding.
2
Introduction
In earlier work, we have shown that DMMP interacts with hydrogen bond donating groups, where
the P=O group act as hydrogen bond acceptor. The adsorption has been followed by infrared
reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) under
UHV conditions 1,2 and with a surface acoustic wave (SAW) sensor and IRAS under realistic sensor
conditions 2,3, in order to gain detailed knowledge about the interactions involved. It was found that
the SAW-IRAS combination gave relevant and complementary information, and we extend here our
efforts on using self assembled monolayers (SAMs) as artificial sensing interfaces by investigating
how humidity influences the interaction and the sensor response.
Sensors for gases and vapors based on selective absorption all depend on molecular interactions.
Ad/Absorption of an analyte in for instance a polymer layer may involve different types of
interactions and the characteristic behavior of the sensor, concerning sensitivity and selectivity,
relies upon the strength of this interaction. As most laboratory work, for reasons of analytical
clarity, is performed in controlled atmosphere e.g., in some inert gas or even in UHV, the influence
of water vapor is often omitted. This might be a less serious problem in cases where the expected
interaction is with hydrophobic polymers, but will certainly influence the conclusions drawn using
hydrophilic ones. As the main interaction mechanism is due to hydrogen bonding
2,3
the impact of
water vapor on the interaction is expected to be of crucial importance for the sensor performance.
Dimethylmethylphosphonate (DMMP) is used as a model molecule for the highly toxic sarin. We
are using three well characterized organic surfaces prepared by solution self assembly of substituted alkanethiol (SH-CH2)m-X (X:-OH, -COOH, -CH3) on gold surfaces. The surfaces
formed in this way are well characterized and known to form ordered monomolecular layers, with
properties determined primarily by the tail group X 4. This allows adsorption experiments where the
3
molecular interaction with the tail groups can be studied in detail. In this way disturbance from
polymeric matrixes, normally obscuring the results and causing swelling or non equilibrium
conditions due to diffusion can be omitted. Even though, there might be advantageous to use thicker
polymer films in a real sensor application, in order to achieve sufficient selectivity (filtering) and/or
sensitivity (concentrating), it is felt that this type of model study can contribute to the understanding
of the influence of humidity and thus to the development of improved sensing interfaces.
The SAW sensor, here used as a gravimetric sensor, is extremely sensitive to changes in a number
of physical parameters. Mostly, its response is considered to be exclusively due to changes in
adsorbed mass on the surface between the interdigital transducers (IDTs) used to convert the
electrical signal into an acoustic surface wave and vice versa. With this assumption, the used device
has a theoretical detection limit of less than one percent of a DMMP monolayer. It has been shown,
however, that changes of the viscoelastic properties of the surface film can influence the result. This
becomes a serious problem when the film is thick as compared to the wavelength of the acoustic
wave 5. The used device has an acoustic wavelength of 20 m, which is four orders of magnitude
larger than the thickness of the films, and we believe therefore that the viscoelastic effects are of
little importance. There are, however, signs of a possible influence of this kind in literature 6. The
mass sensitivity of the SAW sensor can be used to obtain surface coverages of the adsorbed vapor.
In the case of competing adsorption of more than one type of vapor molecules this is no longer
straight forward and complementary information is needed. The chosen combination of SAW sensor
and IRAS allows the separation of the different contributions. Changes in spectral signature due to
the interaction are furthermore discussed in terms of the coverage dependent structure of the
adsorbed molecules.
4
Experimental
Surface monolayer preparation: The surfaces used for IRAS experiments (10 Å Cr and 2000Å
Au) were sputter coated on 20 x 20 mm2 glass substrates at a background pressure of <10-6 Torr.
The
IRAS
and
the
SAW
devices
were
cleaned
in
piranha
solution
containing
H2O2(30 %):H2SO4(98 %), 1:3 by volume (caution: reacts violently with organic material and
should not be stored!) for one minute, rinsed with copious amounts of water (Millipore) and ethanol
prior to immersion in ethanol solutions (Bundesmonopolverwaltung für Branntwein Sorte 430,
96 %) of thiols with a total concentration of 2 mM. SH-(CH2)15 -CH3 (Aldrich 92 %) was vacuum
destilled (8 Torr 473 K, Ar), SH-(CH2)16 -OH and SH-(CH2)15 -COOH (Generous gifts by Dr.
Löfås, Biacore AB, Uppsala, Sweden) were used as received. After adsorption overnight, the
surfaces where rinsed in ethanol, cleaned in ultrasonic ethanol bath 10 min., dried in a stream of N2
and introduced in the gas cells. The -COOH terminated SAM was shortly rinsed in HCl solution
with pH=3.5 before introduction in the measurement cells, to ensure a fully protonated acid.
SAW Experiment: A SAW delay line working at a frequency of f0=149 MHz was designed by us
and produced on ST-quartz in the Paul Drude Institut für Festkörperphysik, Berlin. The IDT`s and
the delay line between the IDT`s consists of 130 nm Au on Cr. Structures for direct surface
temperature measurements are added on the surface. All experiments were made at room
temperature and temperature drift was compensated by simultaneous temperature measurements.
The device was used in the oscillator configuration with home made electronic circuits and the short
term noise was <2 Hz. The theoretical sensitivity is given by eq.1, where f is the frequency, K a
geometrical constant equal to the fraction of the IDT center to center distance affected by the
adsorption (=1, see discussion), Cm=1.7 cm2(g MHz)-1 and m the change in surface mass per unit
area (g(cm)-2). The sensitivity factor Cm used here is obtained by considering the acoustic properties
of the thin gold layer on top of the quartz.7
5
f/f0=- f0Cmm
(1)
The sensitivity is 27 pg(cm2Hz)-1, which results in a practical detection limit of approximately 100
pg(cm)-2. Saturated DMMP (dimethylmethylphosphonate, Fluka, >97%) vapor was produced using
a gas bubbler, saturated water vapor was produced using two gas washing bottles. The carrier gas
was high purity N2 (Linde, 5.0) and dilution was performed using mass flow controllers (Whiga
GmbH). Flow rates of 50-500 cm3(min)-1 was used. The change of flow rates for dilution purposes
was found not to induce any unwanted effects in the measurements.
IRAS: Vibrational absorption spectra where measured with a Bruker IFS66 FTIR spectrometer
purged with dry air. Infrared reflection absorption spectra where collected using a home built gas
cell, allowing measurements at an angle of incidence of 83°. The cell has a volume of 1.5 cm3 and is
closed by CaF2 windows. The IR-light is traveling through 27 mm of the controlled gas phase.
Before entering the gas cell the light is polarized by a KRS-5 metal grid polarizer (Graseby Specac,
0.12m Al strips) allowing measurements with light polarized either perpendicular (s-) or parallel
(p-) to the plane of incidence. Normally 2000 scans (10 min.) are collected at a resolution of 2 cm-1
using a liquid nitrogen cooled MCT detector. Reference spectra where collected during N2 or
N2/humidity purge (surface without adsorbed DMMP molecules but with the same humidity
0-80 % r.h. as used during DMMP adsorption). Compensation for the gas phase absorption due to
the finite DMMP vapor pressure in the measurement cell at equilibrium was made as elsewhere 2. In
this way only the adsorbed molecules (DMMP) or the changes that are introduced in the SAM or in
the adsorbed water layer, are seen in the resulting spectra.
Contact angle measurements. The static contact angles with water on the IRAS and SAW surfaces
were measured with the sessile drop technique using a modified microscope.
6
Results and discussion
Self assembled alkanethiolate monolayers. The three different SAMs used throughout this study
were prepared from ethanolic solution (2mM) of SH-(CH2)16-OH, SH-(CH2)15-CH3 and
SH-(CH2)15-COOH, in the following called the -OH, -CH3 and -COOH surface, respectively. The
self assembly process of these molecules is well documented and we refer to the extended literature
on this topic 4. The position of the CH2 backbone modes of the SAM can be used as a quality
control of the assembly process and in ref. 2 the positions of the C-H stretching peaks for the
monolayers used also in this work can be found. The position of the CH2 as and s appear at 2919
and 2850 cm-1 , respectively, indicating that well ordered all trans layers are formed 4a.
In the case of the monolayers formed on SAW delay lines, the dimensions of the Au surface
between the IDTs (2 x 4 mm2) exclude direct IRAS measurements. However, the surfaces were
prepared and pretreated in the same way as those used in the IRAS experiments and comparison of
the static contact angles with water showed no discrepancy.
Water coverage at different humidity. The change in surface mass due to introduction of water
vapor in the N2 carrier gas stream was measured with the SAW sensor. The results are presented as
the frequency shift of the used oscillator circuit (f1, as indicated in figure 1A) at equilibrium
adsorption, which was reached after a few seconds exposure. The water coverage due to the
presence of water in the carrier gas was followed at relative humidities between 0 and 99 %.
Typical results for the three different surfaces are shown as isotherms in figure 1B, where the solid
lines are fits using the BET three parameter theory
8,9
. With this extended BET theory the limited
adsorption, observed even at high relative vapor pressures, is handled by a third parameter
restricting the number of monolayers in the model. The coverage, , was calculated from the
theoretical frequency shift of one monolayer of water, giving rise to a frequency shift of 1.2*103 Hz,
7
see experimental for details. As in the case of the adsorption of DMMP on the same surfaces, the
adsorption on the naked quartz between the fingers of the IDTs on the delay line must be
compensated for 2. In the results discussed below this was done by using =1 in the expression used
to calculate the sensitivity of the device for the two hydrophilic surfaces and by subtracting 10 % of
the -OH results from the results obtained by the -CH3 surface. This serves to compensate for the
adsorption on the 10 % of the delay line where the SAW travels on naked (hydrated) quartz.
The isotherms used to fit the results increase exponentially with relative humidity and follow nicely
the equilibrium adsorption measured. For the highest humidity used the adsorption is limited to less
than 6 monolayers, justifying the use of the three parameter BET theory. Comparing the results for
the three surfaces there is an expected difference between the hydrophobic -CH3 surface and the two
hydrophilic ones. For the two hydrophilic surfaces an equilibrium coverage of about one monolayer
is obtained above 50 % r.h., increasing to approximately 3 monolayers at 80 % r.h., and for the -CH3
surface a frequency shift corresponding to one monolayer is reached above 90 % r.h. The similarity
between the two hydrophilic surfaces agree qualitatively with the expected capability of the -COOH
and -OH groups to form strong hydrogen bonds with water.
The adsorption of DMMP in humid N2. The adsorption of DMMP on the same surfaces using dry
N2 as carrier gas was discussed previously 2. A difference in equilibrium coverage between the three
surfaces was observed favoring the adsorption on the -COOH surface, which can be explained in
terms of stronger hydrogen bonding with the P=O part of the DMMP molecules on this surface.
The aim of the present study is to investigate what influence humidity has on the function of a
sensing interface at which hydrogen bonding is used as the main interaction mechanism. This is
performed by analyzing the adsorption of DMMP at different humidities using a SAW device and
IRAS. The change in surface mass recorded by the SAW device is measured by the change in
oscillator frequency as DMMP is introduced in the humid carrier gas stream, keeping the humidity
8
constant, as indicated in figure 1A. In this way DMMP adsorbs on an already water covered surface
and the dependence on humidity (water coverage) as well as DMMP concentration can be studied.
The different amounts of water adsorbed at different humidity, figure 1B, influence the adsorption
of DMMP, as measured by the SAW sensor in a quite different way for the examined surfaces. If
the frequency shifts, obtained upon introducing DMMP at constant humidity, are compared with the
shifts obtained in dry N2 2, this becomes obvious. To illustrate the difference, figure 2A and B also
contains isotherms obtained as the same concentration of DMMP is allowed to interact with the
surfaces in 50 % r.h. and in dry N2. If we start with the -OH surface, figure 2 A, there is an increase
in measured surface mass change as DMMP is allowed to adsorb in the presence of humidity. The
influence of relative humidity on the frequency change due to 26 ppm DMMP (p/p0=0.02, where p0
is the saturated vapor pressure at 295 K) is also shown in figure 2C. There is an approximately
linear increase with the relative humidity, and at 80 % r.h. the frequency shift is approximately 60 %
higher than in dry N2. Note that the frequency change (f2 in figure 1A) corresponds to the
difference in surface mass between a surface with an equilibrium water coverage and a surface
where water and DMMP both are in equilibrium with the vapor phase. Thus, the measured
frequency change may contain contributions from adsorbed DMMP and from changes in the water
coverage due to the DMMP adsorption and we will return to this in the discussion of the IRAS
results below.
In the case of the -COOH surface the same measurements result in a lowering of the frequency
change in the presence of humidity, as shown for the 50 % r.h. isotherm, figure 2B. The influence of
relative humidity on the frequency change due to 26 ppm DMMP (p/p0=0.02) is also shown in
figure 2C. This surface displays an opposite trend, as compared to the -OH surface, and at 80 % r.h.
the frequency shift is approximately 40 % lower than in dry N2.
9
For the -CH3 surface the change due to humidity is of the same order of magnitude as the
uncertainty of the correction for unspecific adsorption on the naked quartz (not shown). An
estimation of the coverage of DMMP on this surface using IRAS 2 showed that even the corrected
SAW result might be an overestimation of the sensor response. If the contact angle with water for
the -CH3 surfaces (110°) is considered it becomes obvious that the surface will not be covered by
water and the measured frequency change must be of different origin. More work must be done in
order to understand the SAW results of this weakly interacting surface.
From the results above it can be concluded that there is a different influence of water on the DMMP
adsorption on the different surfaces. Particularly the opposite trends observed for the two
hydrophilic surfaces attracts our interest and in the following this issue will be discussed in detail
using in situ vibrational spectroscopy.
IRAS investigations of the adsorbed molecules: Vibrational spectroscopy can be used to obtain
information about the nature and strength of the molecular interaction(s) involved. As the molecular
adsorption studied is of reversible character, the gas phase molecules present in the cell at
equilibrium will contribute to the infrared absorption and thus overlap with the absorption of the
adsorbed molecules. It is, however, possible to compensate for gas phase molecules in the cell and
the spectra shown in the following are all obtained as described in ref. 2. The peak positions of
DMMP together with vibrational assignments are reported elsewhere 2. The main features are the
bands due to P=O and a PO-C. The P=O absorption, in particular, is very sensitive to hydrogen
bond interaction and earlier adsorption experiments
10
have shown that the shift due to hydrogen
bonding can be as large as 100 cm-1.
DMMP-OH interaction in humid N2. To study the influence of humidity on the interaction, the
same DMMP concentration as in the discussion of the SAW results above (p/p0=0.02) was used and
the relative humidity of the carrier N2 stream was varied. In each individual experiment the relative
10
humidity was kept constant, figure 1. Thus, the presented difference spectra display only features
caused by DMMP adsorption. These features may originate from adsorption of DMMP as well as
from changes in the preadsorbed water layer and in the SAM.
In figure 3 spectra of the DMMP adsorbed in different humidities are shown in the OH stretching
range (A) and the P=O stretching range (B), together with results from integration of the OH band
(C) and the P=O and CH3 (1315 cm-1) bands (D). Results from adsorption in dry nitrogen at two
different DMMP concentrations (taken from reference 2) are also included in figure 3A and B for
comparison. At the higher concentration (p/p0=0.16, N2) the surface coverage of DMMP (0.7) is
approximately the same as in the 80 % r.h. case at p/p0=0.02 (see discussion below), and at the
lower concentration (p/p0=0.02, N2) the coverage is the same as used in the humid case. As the
relative humidity increases, the equilibrium coverage of water and the DMMP coverage increase,
figure 3A and B. If the DMMP bands are integrated an approximately linear increase in the
coverage with r.h. is obtained as shown for the -CH3 peak at 1315 cm-1 and the P=O peak at
1235 cm-1 in figure 3D. The increase is in agreement with the SAW results. However, the relative
increase is higher when measured by infrared spectroscopy and corresponds to an increase in
DMMP coverage of 100 % in going from pure N2 to 80 % r.h., compared to the increase of 60 %
measured by the SAW sensor, Fig.2C. This indicates that the mass change recorded by the SAW
device contains a loss in mass due to desorption of water, corresponding to approximately a half
monolayer of water at 80 % r.h. or approximately 2 H2O molecules / DMMP molecule. It could be
claimed that it should be possible to see this loss of water as a decrease in infrared intensities
characteristic for water. A comparison with recent low temperature experiments on SAM, i.e. of ice
11,12
, which has a significantly larger extinction coefficient than liquid water, suggests that this
decrease would be of the order of the noise level (10-5) in the present spectra, and thus impossible
to observe. There are also some changes in the spectral signature as the relative humidity is
11
changed. The most pronounced being the changes in the ranges were the hydrogen bonded -OH and
P=O groups are expected to absorb. The position of the P=O peak, found at 1235 cm-1 in N2, is
shifted towards lower wavenumber with increasing humidity. If the spectrum of the same
concentration of DMMP (p/p0=0.02) obtained in dry N2 is subtracted from the spectra measured in
humid N2, the peak position, due to the added DMMP at various relative humidities, can be better
seen, figure 3B. There is a gradual downward shift in the P=O peak position as the humidity
increases and at 80 % r.h. the peak is found at 1222 cm-1. In the OH stretching region the increased
humidity leads to an increasing absorption at 3460 cm-1. If the 80 % spectrum is compared with a
spectrum of the same DMMP surface coverage measured in dry N2 (p/p0=0.16), it becomes obvious
that the presence of water changes this band substantially both in terms of a broadening and an
increased intensity. The similarity, at low r.h., with the bands formed without water present
indicates that the band mainly is due to a reorganization of the surface hydroxyls 2, causing the OH
groups to line up perpendicularly to the surface and appear with enhanced intensity 13,14. No signs of
a reorganization of hydroxyls have been found in the case of exposure to pure water vapor. At high
r.h., however, the presence of water seems to influence the shape of the band. For example, above
50 % r.h. the OH band broadens substantially. Although, we believe that reorganization of the OH is
the main reason for the intensity in the 3400 cm-1 region, the increase must also have another origin.
One plausible explanation is that the DMMP adsorption induces structural changes in the adsorbed
water, leading to new hydrogen bonds of different strength and thereby to a concomitant broadening
of the 3400 cm-1 band. It is for example known that the long range order in water layers can be
extended to a distance of several molecular monolayers, due to interaction with e.g., an ion or a
polar entity, and that the formation of hydrogen bonds increases the polarizabilities up to two orders
of magnitude
15
. The OH peak position found here is also in good agreement with a liquid cell
measurement, where an alcohol interacting with DMMP in a non polar solvent 2 displayed a shift in
the hydrogen bond OH position from 3330 cm-1 (polymeric alcohols) to 3440 cm-1 (DMMP- alcohol
12
dimers) upon increasing DMMP concentration. As water interacts with DMMP the hydrogen
bonded OH peak, found at 3400 cm-1 in pure water, is shifted to 3470 cm-1 as the concentration of
DMMP increases (not shown).
The increase in the 3460 cm-1 absorption at higher relative humidities is accompanied by a decrease
in a sharp peak at 3707 cm-1. The shape and position of this peak indicate that it originates from free
OH modes of water, disappearing as DMMP is adsorbed. That this mode originates from water and
not from the surface hydroxyls can be concluded as the free OH of the surface hydroxyls has been
found at 3660 cm-1
16
. The small intensity of the 3707 cm-1 peak compared to that of the created
hydrogen bonded one can be explained by the known increase in intensity upon hydrogen bonding.
That there are free O-H in the water adsorbed on the surface is most likely due to water molecules at
the gas phase/vacuum interface. There are also reports in the literature indicating that bulk water
contains as much as 13 % free -OH 17. Thus, the increased equilibrium coverage of DMMP at higher
humidity can be explained by an additional interaction between free O-H and the P=O part of
DMMP, leading to a multitude of hydrogen bonds and to a broadening of the infrared absorption in
the OH stretching region. As there is a gradual downward shift of the P=O peak as the equilibrium
water coverage on the surface increases, the water layer has the effect of increasing the strength of
interaction with the P=O. However, the new individual hydrogen bonds created are weaker as
measured by the OH stretching absorption (OH=3460 cm-1), a phenomenon indicating that more
than one hydrogen bond is formed at each P=O. Thus both electron lone pair orbitals and possibly
also the two ether bonds in DMMP are involved in the hydrogen bonding. It is interesting to
compare with the case where a C=O group is hydrogen bonded. For C=O it is known 18 that a single
hydrogen bond formed between one electron lone pair orbital and an OH group is stronger than the
individual hydrogen bonds if both electron lone pairs of the C=O are involved in hydrogen bonding.
The influence on the C=O stretching mode is, however, larger when both lone pairs are involved.
13
Furthermore, if the number of adsorbed water molecules per adsorbed DMMP molecule is
calculated, using the obtained SAW coverages corrected for the loss in water coverage and the
integrated CH3 peak at 1315 cm-1, it becomes obvious that there is plenty of water around each
DMMP molecule. By plotting the P=O position of the increasing amount of DMMP, obtained in
difference spectra, figure 3B, against the molar ratios of water to DMMP, a knee can be seen at a
ratio close to 6 H2O molecules / DMMP molecule , figure 4. A further increase of water above this
ratio only has marginal effect. Interestingly, this value agree with the maximum number of possible
hydrogen bond accepting sites of DMMP including the lone pair orbitals of the two ether links.
An increased negative peak at 2878 cm-1 as compared with the dry N2 case 2 indicates that the SAM
is more affected by the DMMP adsorption when there is water present. The 2878 cm-1 peak has
been assigned to the outermost -CH2- group (-CH2-OH) , and found to be sensitive to
conformational changes of the -OH groups due to adsorption 19,20. The breaking of lateral hydrogen
bonds during the adsorption and the replacement by hydrogen bonds with the adsorbed molecules is
the probable reason for this increase. Thus, the stronger interaction with DMMP due to the precense
of water molecules influences the outermost methylene indicating that new hydrogen bonding
bridges between the surface and DMMP are created by the water molecules. Despite the mentioned
changes in the outermost portion of the SAM there are no signs of disruption of the well ordered
polymethylene backbones, confirmed by the fact that the CH2 vibrations at 2919 and 2850 cm-1 both
are unaffected in any of the adsorption experiments on the -OH surface.
DMMP-COOH interaction in humid N2. Low concentrations of humidity in the carrier gas
(10 % r.h.) increase the amount of adsorbed DMMP, figure 5B and C. This is seen as a slight
increase in the corresponding COOH absorption (OH mode), figure 5A. However, this effect is
turned to a decrease in the amount of adsorbed DMMP as the relative humidity is increased further.
14
The integrated intensities also confirm this trend showing a decrease with relative humidity above
10 % r.h., figure 5C and D. Also in this case the decrease found by the SAW device can not be fully
explained by the change in DMMP coverage, indicating that a mass loss due to desorption of water
occurs. If we subtract the spectrum of DMMP adsorbed in N2 (as for the -OH surface) the effect of
the increasing amount of water on the interaction with DMMP can be made more visible, figure 5A
and B. The difference spectrum (10%-0%) shows a broad absorption for the P=O vibration at
1210-1180 cm-1, with a maximum at 1203 cm-1. The downward shift indicates a strong interaction,
comparable with that found as DMMP was allowed to interact with Cu2+ complexed on the same
surface 6a. However no shift in the position of the corresponding COOH vibration is found, and thus
the increased strength in the interaction probably originates from P=O forming additional hydrogen
bonds. As the relative humidity is increased to 80 % r.h. there is an increasing negative peak at the
P=O positions found in dry N2, figure 5B, indicateing that the type of bonds formed during
adsorption without water present are replaced by the stronger ones. Thus there is a change in the
nature of interaction as the relative humidity is increased. The shoulder at 1250 cm-1 originates most
probably from DMMP adsorbed on top of already adsorbed DMMP(self-association). A relation,
similar to that found for the -OH surface, between the shift in P=O peak position and the molar
fraction of water and DMMP molecules, is impossible to achieve due to the complicated nature of
the spectral changes with humidity. However, changes in the C=O peak of the SAM gives an
indication of the interaction with this surface. The positions found in the monolayer 1741 and
1718 cm-1 have previously been assigned to free and hydrogen bonded C=O, respectively, and
calculations have shown that the terminal -COOH group is oriented leaving the -OH part exposed at
the surface 21. The nature of the hydrogen bond is, however, a matter of discussion and it has been
suggested that short linear polymeric chains 21 or "sideways" dimeric structures 22 are formed. In any
case some of the carbonyls are hydrogen bonded with a neighboring -OH and this vibrational mode
should be affected if the DMMP interacts via hydrogen bonding to that particular -OH. Upon
15
adsorption of DMMP at 10 % r.h. the difference spectra suggest that there is an increase at the
position of the free C=O and a decrease at that of the hydrogen bonded C=O...HO, figure 6. This
behaviour is analogous to that found in the dry case 2 and to findings in recent work by Crooks et al.
who examined diisopropyl methylphosphonate (DIMP) adsorption on SAM (COOH) using a similar
experimental set up.
23
Adsorption of DMMP at 80 % r.h. leaves a complicated pattern of small
perturbations, only suggesting that the changes due to DMMP adsorption are hindered by the water
layer, or that the main change already occurred as the water adsorbed. The results from the
intermediate humidities show a gradual change between the two extremes (not shown). Compared
to the shifts found without humidity 2, there is even an increased effect at 10 % r.h., indicating that a
low equilibrium water coverage tends to help the DMMP molecules to break the lateral hydrogen
bonds, creating free C=O groups. At higher water coverages the C=O and OH groups become
incorporated in a complex hydrogen bonding network with water, which partly hinders the DMMP
molecules to adsorb as in the dry case.
In our previous study using dry N2 as the carrier gas we found that the DMMP molecule bound
more stongly to COOH than to OH groups most likely because of a strong Lewis-acid(COOH)base(DMMP) interaction. In the presence of humidity the preadsorbed water layer influences the
subsequent DMMP adsorption to the COOH and OH surfaces in a quite different manner. For the
OH surface water does not hinder the interaction between DMMP and the surface hydroxyls, but
rather increases the interaction by formation of multiple hydrogen bonds. For the COOH surface,
coverages less than one monolayer of water increases the adsorption of DMMP a phenomenon
which suggests that the lateral hydrogen bonds (COOH…COOH) are still easily replaced by
hydrogen bonds to DMMP (COOH…O=P). At higher water coverages the water layer seems to
interfere in binding to the SAM surface, with a decrease in the DMMP adsorption as result. The
16
strong interaction with water on this surface also leads to a lack of free OH of water (3707 cm -1),
which was observed as a negative peak as DMMP adsorbed on the OH surface, figure 3A. Thus, no
free OH sites in the preadsorbed water layer are available for interaction with DMMP (network
formation) as in the case of the OH surface. The stronger binding of water to the COOH surface
may also be due to stonger Lewis acidity of the COOH group. It should also be remembered that the
number of hydrogen bond accepting sites present on COOH surface is larger than on the OH
surface. However, in both cases there is a loss of water as DMMP adsorbs corresponding to
0.5 ML of water, or approximately 2 H2O/DMMP.
DMMP-CH3 interaction in humid N2. In the case of the methyl surface no signs of adsorbed
DMMP are found in the spectra even above DMMP concentrations
2
of p/p0=0.02. An increased
humidity is not changing this and it can be concluded that the weak interaction expected for this
surface probably results in a too low coverage for detection to be possible with the present
equipment.
The heat of adsorption. The correlation between the shift in the position of the P=O vibration and
the strength of interaction, found in the case of dry N2 2, might be used to estimate the increase in
the strength of interaction due to humidity. This gives an increase in the order of 4 kJ/mol due to
humidity, for both surfaces, which is a small value compared with the heat of one single hydrogen
bond (10-30 kJ/mol). The observation indicates that the heats obtained by analyzing BET curves
and TPD traces represent the differences in energy between the bonds formed with DMMP and
those present on the surface before DMMP was adsorbed. Thus, to use the shift in the P=O position
to conclude about the increased strength of adsorption due to the presence of water only gives part
of the truth. Interaction between an already adsorbed DMMP molecule and excess water can result
in an additional downward shift of the P=O mode without changing the strength of the surface
17
interaction. Even though the exact role of the water molecules in the different behavior of the two
surfaces is not fully understood, the obtained results can be used to make some concluding remarks.
Conclusions
The presence of water on the surface influences the DMMP adsorption on both hydrophilic surfaces.
The increasing adsorption of DMMP found on the -OH surface with increasing humidity is
accompanied by a stronger interaction as measured by the shift in the position of the P=O peak. The
number of water molecules that are needed to saturate this shift suggests that other parts of the
molecule are involved in the interaction and that a complex hydrogen bonding network is formed.
This network involves DMMP, reorganized surface hydroxyls, and free OH group present in the
preadsorbed water layer at this surface. In the case of the -COOH surface the preadsorbed water
molecules seem to interact stronger with the surface, and no free OH groups capable of forming
bonds with DMMP are present in the water layer formed. We believe that the absence of free OH
groups in the water layer is due to the strong Lewis acidity of the COOH groups and its larger
number of hydrogen bond accepting sites as compared to the OH surface. Low water coverage (low
humidity) increases the DMMP adsorption as compared to the dry case. At higher humidity the
adsorbed water occupies the adsorption sites at which DMMP was adsorbed in the dry case and
even though some DMMP interacts stronger with the COOH surface, as can be seen as an additional
IR-shift of the P=O stretching peak, the amount of adsorbed DMMP decreases. Humidity influences
the formation of hydrogen bonds, as expected, and the present results indicate that the function of a
sensor interface, based on formation of hydrogen bonds, should be discussed including the effect of
humidity variations. Due to desorption of water, induced by the adsorption of DMMP, the response
of the mass sensitive SAW sensor did not correspond to the real changes of equilibrium DMMP
coverage. This is an effect relevant for all types of mass sensitive devices and corrections for
18
competing adsorption must be considered. Altogether, the use of hydrogen bonds to increase the
molecular interaction should be carefully considered if the device is giong to be used in humid
environment.
Acknowledgment
Part of the project was supported by the European Commission (Contract No:ERBCHBICT930832)
and the Swedish Natural Science Research Council (Dnr. Ö-HC 10593-300) by a HCM grant and
part from BWB (contract E/B31E/I0095/G5210/). The project was also supported by a grant from
the Swedish Research Council for Engineering Science (TFR). The professional assistance from Dr.
Fröhlich, Dr. Jungnickel and Dr. Enderlein (Paul Drude Institut für Festkörperphysik, Berlin) during
the design and production of the SAW devices is gratefully acknowledged. We also would like to
express our sincere thanks to Prof. Ingemar Lundström for critical reading of the manuscript.
19
References
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Martin S.J., Frye G.C., Senturia S.D., Anal. Chem., 1994, 66, 2201
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a) Kepley L.K., Crooks R.M., Ricco A.J., Anal. Chem. 1992, 64, 3191.
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7
Theoretical calculation made by Dr. Fröhlich, Dr. Jungnickel and Dr. Enderlein (Paul Drude
Institut für Festkörperphysik, Berlin), private comunication.
8
Brunauer S., Emmett P.H., Teller E., J. Am. Chem. Soc., 1938,60 , 309.
9
Steele W.A., The interaction of gases with solid surfaces, Pergamon Press, Oxford, 1974.
10
Henderson, M.A.; White, J.M., J.Am.Chem.Soc., 1988, 110, 6939.
20
11
Engquist, I., Lundström, I., Liedberg, B., Parikh, A.N., Allara, D.L., J. Chem. Phys., 1997,
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12
Engquist I. ,Liedberg B. J. Phys. Chem., 1996, 100, 20089.
13
Greenler, R.G. J. Chem. Phys. 1966, 44, 310.
14
Francis, S.A.; Ellison, A.H. J.Opt. Soc. Am. 1959, 49, 131.
15
Gutman V. "The Donor-Acceptor Approach to Molecular Interactions", Plenum Press, New
York, 1978.
16
Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141.
17
Luck W. A. P., Borgholte H., Habermehl T., J. Mol. Struct. 1988.177, 523.
18
Bellamy, L.J. The infrared spectra of complex molecules, John Whiley & Sons: NewYork,
1975.
19
Nuzzo R. G., Dubois L. H., Allara D. L., J. Am. Chem. Soc., 1990, 112, 558.
20
Atre S. V., Liedberg B,. Allara D. L., Langmuir, 1995, 11, 3882.
21
Nuzzo R. G., Dubois L. H., Allara D. L., J. Am. Chem. Soc. 1987, 109, 3559.
22
Smith E.L., Alves C. A., Anderegg J. W., Porter M. D., Siperko L. M., Langmuir, 1992, 8,
2707.
23
Crooks, R.M., Yang, H.C., McEllistrem, L.J., Thomas, R.C., Ricco, A.J., Faraday Discuss.,
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21
Figure captions
Figure 1. An example of a frequency trace measured by the SAW device is shown in (A), with the
used definitions for frequency shifts due to humidity, f1, and DMMP, f2. Indicated are also the
time windows used to collect the IRAS spectra, where R0 is the reference spectrum measured in the
same humidity as the sample spectrum (R), which is measured at equilibrium DMMP adsorption.
The frequency changes, f1, due to humidity are shown (B) for the COOH (), OH () and CH3 ()
surfaces, after correction for adsorption on the naked quartz, as described in the text (for the CH3
surface the uncorrected values are also included ()). The lines are fits using a three parameter BET
theory 8,9. On the right axis the corresponding water coverage, , is given in monolayers (ML).
Figure 2. The frequency change, f2, due to DMMP concentrations are shown for the -OH and -CH3
surfaces in A, and for the -COOH surface in B. Isotherms measured in dry N2 and in 50 % r.h. are
shown for the -OH and -COOH surfaces. In C the dependence on humidity is shown for the
frequency change due to DMMP (p/p0=0.02) for the -OH and -COOH surfaces.
Figure 3. IRAS spectra of DMMP adsorbed on the -OH surface (A and B). The relative
concentration of DMMP was 0.02 and the humidity of the carrier gas changed. The difference
spectra were obtained by subtracting the 0 % r.h. spectrum. Shown are also two spectra of DMMP
in N2. Below, the integrated peak intensity in C from 3660 to 3100 cm-1 (OH 3400 cm-1), and from
3723 to 3683 cm-1 (OH 3707 cm-1), and in D from 1340 to 1300 cm-1 (CH3 1315 cm-1) and from
22
1276 to 1150 cm-1 (P=O 1230 cm-1) of the spectra measured at different relative humidity, are
shown.
Figure 4. The position of the P=O vibration of the DMMP molecule, as a function of the ratio of the
number of adsorbed water to DMMP molecules (In the case of 0 % r.h.( n(H2O/n(DMMP)=0) the
original peak position is shown). The molar ratios used have been corrected for the loss of water due
to the DMMP adsorption, using the integrated infrared intensities of the CH3 peak, figure 3.
Figure 5. IRAS spectra of DMMP adsorbed on the -COOH surface (A and B) . The relative
concentration of DMMP was 0.02 and the humidity of the carrier was varied. The difference spectra
were obtained by subtracting the 0 % r.h. spectrum. Below, the integrated peak intensity in C from
3400 to 2400 cm-1(OH 3000 cm-1), and in D from 1340 to 1300 cm-1 (CH3 1315 cm-1) and from
1276 to 1150 cm-1 (P=O 1230 cm-1) of the spectra measured in different humidity, are shown.
Figure 6. IRAS spectra of the C=O peak region of the -COOH SAM. A spectrum of the -COOH
monolayer (bottom) is shown together with the changes induced as DMMP(p/p0=0.02) was
adsorbed at different relative humidities, as indicated in the figure.
23
(Fig. 1)
6
humid N2
DMMP (in humid N2)
N2
6
f1
-f1 [kHz]
4
4
f2
R0
R
-OH

SAW-oscillator frequency
-COOH
B
A
2
2
-CH3
0
Time
0
20
40
60
Relative humidity [%]
80
0
100
24
(Fig. 2)
50 % r.h.
A
2
-OH
-f2 [kHz]
N2
-CH3
0
0.00
0.04
B
2
0.08
p/p0 (DMMP)
0.12
0.16
N2
-f2 [kHz]
-COOH
50 % r.h.
0
0.00
1300
0.04
0.08
p/p0 (DMMP)
0.12
0.16
C
-f2 [Hz]
-OH
800
-COOH
0
20
40
60
Relative humidity [%]
80
100
25
(Fig.3)
-OH
A
p/p0=0.02 (humid N2)
p/p0=0.02 (humid N2) B
80% r.h.
66% r.h.
50% r.h.
33% r.h.
10% r.h.
-OH
80% r.h.
66% r.h.
50% r.h.
33% r.h.
-log(R/R0)
-log(R/R0)
10% r.h.
p/p0=0.16 (N2)
0.0002
p/p0=0.02 (N2)
p/p0=0.16 (N2)
0.0004
p/p0=0.02 (N2)
difference spectra
difference spectra
3800
3600
3400
3200
3000
-1
Wavenumber [cm ]
1400
2800
1300
1200
-1
Wavenumber [cm ]
1100
6
16
0
-1
6
O-H 3707cm
-2
-2
-1
Integrated intensity [10 cm ]
-1
D
-1
P=O 1235cm
-1
CH3 1315cm
-3
-3
Integrated intensity [10 cm ]
Integrated intensity [10 cm ]
C
Integrated intensity [10 cm ]
O-H 3400cm
-1
-1
-1
-4
0
0
20
40
60
Relative humidity [%]
80
100
0
0
20
40
60
80
0
100
Relative humidity [%]
26
(fig. 4)
1236
-1
 P=O [cm ]
1232
1228
1224
1220
0
2
4
6
8
n(H2O)/n(DMMP)
10
12
27
(fig. 5)
A
-COOH
50 %, 0 % r.h.
0.0005
-log(R/R0)
-log(R/R0)
0.0005
p/p0=0.02 (humid N2)
10 % r.h.
50 % , 0 % r.h.
80 % r.h.
B
p/p0=0.02 (humid N2)
10 % r.h.
-COOH
80 % r.h.
difference spectra
10% - 0%
difference spectra
10% - 0%
80% - 0%
80% - 0%
3600
3400
3200
3000
2800
-1
Wavenumber [cm ]
2600
2400
1400
0.1
1300
1200
-1
Wavenumber [cm ]
1100
8
8
D
-1
C
-1
-1
-3
OH 3000 cm
Integrated intensity [10 cm ]
-1
CH3 1315cm
-3
Integrated intensity [cm ]
-1
Integrated intensity [10 cm ]
P=O 1220cm
-1
0.0
0
20
40
60
Relative humidity [%]
80
100
0
0
20
40
60
Relative humidity [%]
80
0
100
28
(fig. 6)
p/p0=0.02 (humid N2)
10 % r.h.
-log(R/R0)
80 % r.h.
0.0005
1741
1718
-COOH SAM
1900
1850
1800
1750 1700 1650
-1
Wavenumber [cm ]
1600
1550
29