J. Phys. Chem. B 1997, 101, 6021-6027
6021
Interaction of Dimethyl Methylphosphonate with Alkanethiolate Monolayers Studied by Temperature-Programmed Desorption and Infrared Spectroscopy Lars Bertilsson, Isak Engquist, and Bo Liedberg* Molecular Films and Surface Analysis Group, Laboratory of Applied Physics, Linko¨ ping UniVersity, S-581 83 Linko¨ ping, Sweden ReceiVed: February 28, 1997; In Final Form: May 20, 1997X
The adsorption of dimethyl methylphosphonate (DMMP) on well-defined organic surfaces consisting of selfassembled monolayers (SAMs) of ω-substituted alkanethiolates on gold has been studied. Three different surfaces were examined: one terminated with -OH groups (Au/S-(CH2)16-OH), one with -CH3 (Au/S(CH2)15-CH3), and one mixed surface with approximately equal amounts of -OH and -CH3 terminated thiols. Detailed information about the nature and strength of the interaction was gathered by infrared reflectionabsorption spectroscopy and temperature-programmed desorption under ultrahigh-vacuum conditions. It is found that the outermost functional groups of the thiol monolayer have a pronounced impact on the interaction with DMMP at low coverage. The -OH surface, allowing for hydrogen bonds with the PdO part of the DMMP molecule, increases the strength of interaction by approximately 3.8 kJ/mol as compared to the -CH3 surface. A preadsorbed layer of D2O leads to stronger interaction on all surfaces. This is explained by additional hydrogen bond formation between free O-D at the ice-vacuum interface and DMMP.
Introduction Sensors for gases and vapors at low concentrations are often based on the idea of collecting the molecules in a medium until the concentration reaches a level where it can be quantitatively analyzed. For increased sensitivity the medium should have high affinity to the analyte molecules. It should at the same time display reversibility with reasonable dissociation time constants. A suitable medium can thus be found by considering the strength of the molecular interactions involved during the absorption of the analyte in the medium. One family of gas sensors that has shown promising results is the mass-sensitive devices. Here the mass change, caused by adsorption of the vapor of interest in a layer, is transformed into an electrical signal by a suitable transducer. There are several different types of transducers that could be used, the most common ones being the surface acoustic wave (SAW) and the quartz crystal microbalance (QCM) devices. Extreme sensitivity to mass changes can be obtained with SAW devices at high frequencies, and only very thin sensing layers are needed, which improves the time constants. The sensitivity as well as the selectivity of the sensor relies on the molecular interactions between the vapor molecule and the thin layer into/onto which it absorbs/adsorbs. Different types of polymers can be used as sensitive layers, and the sorption of vapor varies with the physical/chemical properties of the used polymer. Several attempts have been made to improve the mechanistic understanding of the interaction involved. In some cases it is claimed that the gas molecules adsorb selectively in cavities where the chemical as well as the geometrical properties play a rule (host-guest chemistry).1 Other groups argue that the physical properties play a minor role compared to general solvation effects.2 In any case we expect that both the chemical and the physical properties of the polymer influence the adsorption and that it might be difficult to distinguish between these two contributions to the sensor signal. Dimethyl methylphosphonate (DMMP) is often used as a less toxic model molecule for sarin in the development of sensors * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 1, 1997.
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for war-fare agents and in experiments where the catalytic decomposition of such agents is studied. To learn more about the molecular interaction involved during absorption of DMMP in organic layers, we have chosen to study the adsorption of the molecule on a few well-characterized organic surfaces prepared by solution self-assembly of ω-substituted alkenethiols (SH-(CH2)m-X) on gold. The use of self-assembly to produce ordered organic surfaces is well documented, and we refer to the extensive literature for a detailed discussion on their molecular properties.3 We are in this work using SAMs to produce homogeneous interfaces with a distinct molecular signature given by the tail group X. A combination of two different thiols, one alcohol (SH(CH2)16-OH) and one alkane (SH-(CH2)15-CH3), allowing the surface tension to be changed over a wide range was used. It has been shown in earlier work4 that a mixture of these two thiols forms mixed monolayers with a surface concentration of alcohols approximately equal to the concentration of alcohols in the solution used to prepare the monolayer. The composition of the SAMs has a pronounced influence on the adsorption of DMMP under conditions normal for sensor development. This was found previously,5 where a combination of SAW and in situ infrared reflection-absorption spectroscopy (IRAS) was used to follow the DMMP adsorption at room temperature using low concentrations of the vapor in N2. Under those conditions the amount of adsorbed DMMP was proportional to the number of -OH terminated thiols on the surface. This was explained by the increasing heat of adsorption originating from hydrogen bond formation between surface -OH groups and the PdO group in DMMP. In this paper we show results obtained by using a combination of infrared reflection-absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD), which gives us additional information about the interaction under ultrahighvacuum (UHV) conditions. Experimental Section Monolayer Preparation. The surfaces used for IRAS experiments (25 Å Ti and 2000 Å Au electron beam evaporated on 20 × 20 mm2 Si substrates at a background pressure of © 1997 American Chemical Society
6022 J. Phys. Chem. B, Vol. 101, No. 31, 1997
Bertilsson et al.
TABLE 1: Self-Assembled Monolayer Characteristicsa SAM
d+ (cm-1)
d- (cm-1)
-OH mixture -CH3
2848 2848 2848
2916 2916 2916
r+ (cm-1) 2876 2876
ra- (cm-1) 2963 2963
rb- (cm-1) 2956 2956
νC-H (cm-1)
t (Å)
CA(H2O) (deg)
2876 2876
21 ( 1 20 ( 1 19 ( 1
