BINARY SELF-ASSEMBLED MONOLAYERS AS PREPARED BY SUCCESSIVE ADSORPTION OF ALKYLTRICHLOROSILANES

Binary monolayers of octadecyltrichlorosilane (OTS) and 11-(2-naphthyl)undecyltrichlorosilane (2-Np) were prepared by backfilling of partial monolayers of one component as a result of exposure to solutions of the other component. The comparison with coadsorbed monolayers shows that both procedures lead to the same kind of binary monolayers. The ratio of the UV-absorption bands at 224 nm (B(b)) and 280 nm (L(a)) of the naphthalene chromophore in UV spectra of binary monolayers is independent of composition, indicating constant orientation of the chromophore. Thus, the UV absorbance of the naphthalene tag in backfilled monolayers could be used to determine the surface composition and thus the original coverage of the partial monolayers. The transient growth process monitored in this fashion does not follow irreversible first-order kinetics. This is proposed to be the result of the rinsing process that is carried out after removing the substrates from the adsorption solution. Part of the monolayers are removed by this process and the portion of removable material depends on surface coverage. Partial monolayers exhibit a liquid character as shown by orientational disorder (UV), reduced lateral interaction of alkyl chains (FTIR), and the occurrence of the full-sandwich excimer in partial monolayers of 2-Np. Backfilling solidifies the monolayers. In such systems the naphthalene chromophores become oriented, the alkyl chains show higher lateral interaction and the full-sandwich excimer disappears in favor of the partial-overlap excimer. Backfilled monolayers show less defects with lower surface density. This is proposed to be the result of the second immersion in a fresh alkyltrichlorosilane solution that contains more monomeric silanetriols to fill vacancies in the monolayer.