Infrared reflection absorption spectroscopic study on self-assembling processes of (methacryloyloxy)alkyl dihydrogen phosphates on evaporated silver films

Infrared reflection absorption (IRA) spectroscopy was applied to study the adsorption process and structural change of(methacryloyloxy)alkyl dihydrogen phosphates, M10P and M2P (see Figure 1), during their self-assembling process at a silver substrate from a methylmethacrylate solution (5 x 10(-5) mol/L). The saturation coverage of M10P was completed after about 50 min, while that of M2P was completed after about 25 min. The self-assembled monolayers of both M10P and M2P gave the v(s)(PO32-) band around 980 cm(-1), indicating that the dissociation of the dihydrogen phosphate groups takes place on adsorption to the silver substrate. At an initial stage of the self-assembling process of M10P the IRA spectra gave only the v(s)(PO32-) band. After several minutes of incubation there arose the v(as)(PO32-) band near 1080 cm(-1) and the intensity ratio of v(as)(PO32-)/v(s)(PO32-) increased with time, indicating that the axis of the PO32- group of the adsorbate changes from a perpendicular orientation to a tilted one. On the other hand, the IRA spectra of the self-assembled monolayer of M2P gave both the v(s)(PO32-) and v(as)(PO32-) bands, with the onset of adsorption and the intensity ratio of v(as)(PO32-)/v(s)(PO32-) virtually constant. Thus, in contrast to the case of M10P, the PO32- groups of M2P take an almost random orientation during the self-assembling process. The explicit differences in the kinetics of adsorption and structural change between M10P and M2P were explained in terms of a larger mobility of M2P at the substrate surface and/or an interaction between the alkyl moieties of M10P.