On the formation and structure of self-assembling monolayers. I. A comparative atr-wettability study of Langmuir—Blodgett and adsorbed films on flat substrates and glass microbeads

Organized oleophobic monolayers of several long chain compounds and steroid derivatives produced on flat solid substrates by spontaneous adsorption from organic solutions are compared with Langmuir—Blodgett (LB) monolayers transferred on identical substrates from the water-air interface. Quantitative infrared ATR and polarized ATR spectroscopy, and wettability measurements are used to correlate the various films and to determine their molecular density and orientation, mode of film-to-surface binding, and other structural characteristics. Formation of oleophobic adsorbed monolayers on a model powder substrate—smooth glass microbeads—is also investigated. It is concluded that, irrespective of the mode of film-to-surface binding (ionic, covalent, or hydrogen bonding), and the nature of the substrate (Ge, Si, ZnSe, glass slides, glass microbeads), saturation of the adsorption leads in all studied systems to the formation of tightly packed and highly oriented monolayers, structurally equivalent to LB monolayers of same or similar compounds deposited on the bare surfaces of the respective substrates. These findings are interpreted in terms of a cooperative surface process leading to aggregation of molecules into a characteristic “monolayer phase.” Significant structural differences may develop in LB built-up films thicker than one monolayer. A mechanism for the formation of covalently bonded silane monolayers is proposed.