Interaction of Dimethyl Methylphosphonate with Alkanethiolate Monolayers Studied by Temperature-Programmed Desorption and Infrared Spectroscopy

The adsorption of dimethyl methylphosphonate (DMMP) on well-defined organic surfaces consisting of self-assembled monolayers (SAMs) of ω-substituted alkanethiolates on gold has been studied. Three different surfaces were examined: one terminated with −OH groups (Au/S−(CH 2 ) 16 −OH), one with −CH 3 (Au/S−(CH 2 ) 15 −CH 3 ), and one mixed surface with approximately equal amounts of −OH and −CH 3 terminated thiols. Detailed information about the nature and strength of the interaction was gathered by infrared reflection−absorption 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 P O part of the DMMP molecule, increases the strength of interaction by approximately 3.8 kJ/mol as compared to the −CH 3 surface. A preadsorbed layer of D 2 O 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.