Formation of Self-Assembled Octadecylsiloxane Monolayers on Mica and Silicon Surfaces Studied by Atomic Force Microscopy and Infrared Spectroscopy

Abstract

The formation and growth of self-assembled octadecylsiloxane monolayers on native silicon and mica substrates have been studied using atomic force microscopy, ellipsometry, and infrared spectroscopy. Submonolayer ODS films of varying surface coverages were prepared by immersing the substrates into dilute solutions of octadecyltrichlorosilane in toluene for different periods of time, and the submonolayer film structures were compared between mica and silicon substrates for different water contents of the adsorbate solutions and for different time delays between solution preparation and substrate immersion (solution age). It was found that, in general, both a continuous growth (formation of disordered, liquidlike submonolayers) and an island-type growth (formation of organized assemblies with vertically aligned hydrocarbon chains) are involved in the formation of ODS monolayers, whereby the relative contributions depend strongly on the solution properties. With increasing water content or increasing age of the adsorbate solution, island-type growth is strongly favored on both silicon and mica surfaces, which indicates the kinetically controlled formation of larger, preordered aggregates of silanol molecules as the primary hydrolysis products in solution. For identical conditions of film preparation, both the degree of structural order in the submonolayer films and the overall adsorption rate was found to be higher on mica in comparison to silicon. The higher structural order was interpreted as a consequence of the lower hydroxyl group concentration and a correspondingly enhanced surface diffusion rate of weakly bound film molecules on a mica substrate. The enhanced adsorption rate, on the other hand, points to some additional activation of a mica surface with respect to silanol adsorption, which might be related to its ionic composition containing mobile surface charges in contrast to the covalent, neutral character of a native silicon surface.

Effects of silane coupling agents and solutions of different polarity on PMMA bonding to alumina

“…Three solutions, ETH, ISP, and TOL, were selected in the present study based on their polarity and widely use in previous studies 9,18,20) . It is essential to hydrolyze silane coupling agents in silanization, therefore, 70% ethanol aqueous solution was used.…”

“…Additionally, moisture or water content is needed for initiating hydrolysis and condensation reaction of silane 7) . Water also affected the formation of monolayer and complete siloxane formation 20,21) . To assess the good and durable bonding between PMMA and the silanized materials, the shear bond test in combination with thermocycling challenge has been often used 22,23) .…”

The molecular level modification of surfaces: from self-assembled monolayers to complex molecular assemblies

“…28,36 More recent studies showed that with increasing water content, or increasing age of the adsorbate solution, island type growth is strongly favoured. 37 To further complicate the debate, the film growth seems to proceed by different mechanisms under ''dry'' or ''wet'' conditions. 34 Most importantly, Wang and Lieberman have reported the preparation of ultra smooth octadecyltrichlorosilane (OTS) monolayers by exposing clean native SiO 2 surfaces to a dry solution of OTS.…”

The effect of chain density on the frictional behavior of surfaces modified with alkylsiloxanes and immersed in n-alcohols

“…With OTS, all of the chlorine groups can hydrolyze, permitting the formation of both surface-chain and interchain siloxane linkages. 19,20,[31][32][33][34] When the oxide layer has an adsorbed layer of water as it does after piranha treatment, Si-Cl bonds may undergo hydrolysis to form a hydrogen bond with the adsorbed water layer on the surface or to react with the silanol groups on the oxide surface to form a siloxane bond. For very dilute silanizing solutions prepared from anhydrous solvents, deposition of oligomeric species from solution is negligible.…”

“…Interchain siloxane linkages and van der Waals forces between the long alkyl chains cause these islands to coalesce into a homogeneous ordered monolayer. 19,31,34 The van der Waals forces between the chains act to orient the chains so that they are normal to the surface of the substrate. The characteristic area per chain approaches 0.21-25 nm 2 /chain [35][36][37] for these films.…”