Effect of Humidity on the Interaction of Dimethyl Methylphosphonate (DMMP) Vapor with SiO2 and Al2O3 Surfaces, Studied Using Infrared Attenuated Total Reflection Spectroscopy

Infrared attenuated total reflection spectroscopy has been used to study the interaction of DMMP vapor with SiO2, Al2O3, and AlO(OH) vs relative humidity (RH) and DMMP partial pressure (P/P-0). For SiO2 the growth with increasing RH once-like and liquid-like layers is seen in agreement with previous work. H <-> D exchange during exposure to H2O and D2O indicates that the ice-like layer is more resistant to exchange, consistent with stronger H-bonding than in the liquid-like layer. Exposure of nominally dry SiO2 to D2O indicates the existence of adsorbed H2O that does not exhibit an spectrum. The ice-like layer appears only at a finite RH. Exposure of SiO2 to DMMP in the absence of intentionally added H2O shows the formation of a strongly bound molecular species followed by a liquid-like layer. The strong interaction involves SiO-H center dot center dot center dot O=P bonds to surface silanols and/or HO-H center dot center dot center dot O=P bonds to preadsorbed molecular H2O. At a finite RH the ice-like layer forms on SiO2 even in the presence of DMMP up to P/P-0 = 0.30. DMMP does no: appear to penetrate the ice-like layer under these conditions, and the tendency to form a such a layer drives the displacement of DM MP. Amorphous Al2O3 and AlO(OH) do not exhibit an ice-like H2O layer. Both have a higher surface OH content than does SiO2, which leads to higher coverages of H2O or DM MP at equivalent RH or P/P-0. At low P/P-0, for which adsorption is dominated by Al-OH center dot center dot center dot O=P bonding, a-Al2O3 interacts with DM MP more strongly than does AlO(OH) as a result of the higher acidity of OH sites on the former. Up to RH = 0.30 and P/P-0 = 0.30, DMMP appears to remain bonded to the surface rather than being displaced by H2O. H2O appears to have little or no effect on the total amount of DMMP adsorbed on any of these surfaces, up to an RH of 0.30 and a P/P-0 of 0.30. The results have implications for the transport of DMMP and related molecules on oxide surfaces in the environment.