Investigating the structural and chemical properties of self -assembled monolayers as a function of hydrocarbon chain length and terminal functional group

In this research we have investigated how the intermolecular interactions, mostly van der Waals interchain interactions, depend on the hydrocarbon chain length and terminal functional group, and how these interactions in turn determine the structure, mechanism of formation, and also packing density of the monolayers. We have considered octadecyltrichlorosilane, undecyltrichlorosilane, 11-cyano undecyltrimethoxysilane, and aminopropyltrimethoxysilane varying in chain length and terminal functional groups. FTIR results showed that molecules with long hydrocarbon chain formed densely packed, ordered crystalline structures and the order in the structure decreased with decrease in the hydrocarbon chain length. The packing density decreased with decrease in hydrocarbon chain length, and also with incorporation of hydrophilic surface group for the same chain length. FTIR and ellipsometer results showed that low chain molecules with hydrophilic surface group, like APS, have tendency of forming multilayers.;We investigated the mechanism of formation of monolayers as a function of temperature. Self-assembled monolayers formation is initiated by three distinct growth regimes: (i) island-like growth regime below phase transition temperature; (ii) homogeneous growth regime at high temperatures (above phase transition) and (iii) mixed regime at intermediate regimes.;We successfully demonstrated the modification of cyano terminal group of CUTMS monolayer into carboxylic group using a simple in situ hydrolysis reaction approach for the surface transformation. We have investigated the reaction kinetics and conversion both by contact angle measurements and FTIR.;We also successfully demonstrated the use of SAMs as model membranes to study the sorption properties of various volatile organic vapors, and in turn understand the prominent interactions that govern the absorption process. We studied the absorption of polar acetone vapors and non-polar hexane vapors.;Interactions of hexane vapors also depended on the properties of the underlying monolayers. Moreover, alignment of the absorbed hexane vapors exhibited a strong correlation with the structure of the underlying monolayer. Absorbed hexane structure varied from all-trans on OTS monolayer to liquid like structure on clean silicon wafer. We also modeled the alignment of hexane molecules on the OTS (methyl terminated) monolayer using interfacial energy arguments. (Abstract shortened by UMI.).