Molecular Adsorption on Self Assembled Monolayers studied by Surface Acoustic Waves and Infrared Spectroscopy

Sammanfattning: Chemical sensors based on the selective absorption of gas/vapors in polymer layers rely on molecular interactions. Details of the mechanisms involved can be obtained by studying the adsorption on well defined model organic surfaces. In this thesis, the adsorption of dimethylmethylphosphonate (DMMP), a model molecule for the nerve gas, sarin, and the influence of humidity on the interaction, have been studied. Well defined organic interfaces were obtained by solution self assembly of long chain co-substituted thiol molecules, SH-(CH2)m-X, on gold surfaces. The interfacial properties are given by the tail group X and the following three different functional groups, -CH3, -OH, -COOH, have been analyzed.In the first paper, the properties of the self assembled monolayers (SAMs) were examined. The monolayers form ordered semi-crystalline layers with the tail groups defining the chemical properties of the interfaces. In mixed monolayers the surface concentration of the OH and CH3 terminated thiols corresponds to that of the preparation solution and no signs of macroscopic phase separation could be observed.The equilibrium adsorption of DMMP was analyzed under sensor conditions by a surface acoustic wave sensor (SAW), sensitive to changes in surface mass, and infrared reflection absorption spectroscopy (IRAS). It was found that DMMP interacts with hydrogen bond donating groups and that the lone pair electrons of the P=O oxygen is the main interacting part of the molecule. Both the temperature programmed desorption of DMMP adsorbed at 100 K and the SAW results from room temperature measurements indicate that the strongest interaction is on the -COOH surface followedby -OH and -CH3. For the -OH surface there is also a surface coverage dependent in the strength of interaction.The influence of humidity on the interaction was studied as DMMP was allowed to adsorb at different levels of relative humidity. For the -OH surface, an increase in the DMMP coverage proportional to the relative humidity was observed both with the SAW and with IRAS. A closer examination, however, revealed that a loss of water occurred during adsorption of DMMP. The conclusion from this experiment is that SAW results and other mass sensitive devices must be corrected if competing adsorption takes place. For the -COOH surface, low humidities tend to increase the DMMP coverage but at higher humidities a clear loss was registered by IRAS and the SAW sensor even after compensation for changes in water coverage. As water is present, the increased interaction can be explained by hydrogen bonding to free OH of the water. On the -COOH surface, the adsorption sites become blocked by water, which is more tightly bound on this surface. In the mixed (OH / CH3) monolayers water vapor starts to influence the DMMP adsorption above a critical concentration of OH groups of 0.6. This is also the OH concentration above which total wetting with DMMP occurs.The well defined organic monolayers formed by solution self-assembly was found to be suitable for interaction studies of the hydrogen bond accepting DMMP molecule and the influence of humidity on the interaction. The complementary information resulting from the chosen analytical techniques can be used for both qualitative and quantitative evaluation of the adsorption. For example, an optimal composition for the mixed (OH/CH3) SAM has been identified, where the number of adsorption sites and the interaction is sufficiently high, and the influence from humidity still low.