BINARY SELF-ASSEMBLED MONOLAYERS AS PREPARED BY SUCCESSIVE ADSORPTIONOF 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 t he other component. The comparison with coadsorbed monolayers shows th at both procedures lead to the same kind of binary monolayers. The rat io of the UV-absorption bands at 224 nm (B(b)) and 280 nm (L(a)) of th e naphthalene chromophore in UV spectra of binary monolayers is indepe ndent of composition, indicating constant orientation of the chromopho re. Thus, the UV absorbance of the naphthalene tag in backfilled monol ayers could be used to determine the surface composition and thus the original coverage of the partial monolayers. The transient growth proc ess monitored in this fashion does not follow irreversible first-order kinetics. This is proposed to be the result of the rinsing process th at is carried out after removing the substrates from the adsorption so lution. Part of the monolayers are removed by this process and the por tion of removable material depends on surface coverage. Partial monola yers exhibit a liquid character as shown by orientational disorder (UV ), reduced lateral interaction of alkyl chains (FTIR), and the occurre nce of the full-sandwich excimer in partial monolayers of 2-Np. Backfi lling solidifies the monolayers. In such systems the naphthalene chrom ophores become oriented, the alkyl chains show higher lateral interact ion and the full-sandwich excimer disappears in favor of the partial-o verlap excimer. Backfilled monolayers show less defects with lower sur face density. This is proposed to be the result of the second immersio n in a fresh alkyltrichlorosilane solution that contains more monomeri c silanetriols to fill vacancies in the monolayer.