On the Adsorption of Dimethyl Methylphosphonate on Self-Assembled Alkanethiolate Monolayers:  Influence of Humidity

Humidity influences the adsorption of dimethyl methylphosphonate (DMMP) on three different organic model interfaces to quite different extents. The model interfaces, characterized by the following tail groups (−OH, −CH3, −COOH), were prepared by solution self-assembly of ω-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, are not fully registered by the surface acoustic wave sensor, because of 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 the relative humidity on the OH surface and decreases with the 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.