Molecular aggregation state and molecular motion of organosilane monolayers prepared at the air/water interface

Relationships between the molecular aggregation state and the molecular motion of organosilane monolayers with various chain lengths prepared by the Langmuir-Blodgett (LB) method were investigated by lateral force microscopic, Fourier transform-infrared (FT-IR) spectroscopic, and electron diffraction (ED) measurements. The lateral force of the organosilane monolayers increased with increasing their chain length. This is mainly because the intermolecular cohesive energy is proportional to the number of carbons in a chain. The lateral force of the alkylsilane monolayer was compared with that of the fluoroalkylsilane monolayer for the corresponding alkyl chain length. Consequently, the fluoroalkylsilane monolayer showed higher lateral force than the alkylsilane one. Temperature-dependent ED and FT-IR measurements revealed that the molecular aggregation state of the n-octadecyltrichlorosilane (OTS) monolayer changed from the rectangular lattice to the amorphous state via the hexagonal lattice with increasing temperature. The transition temperatures, rectangular to hexagonal and hexagonal to amorphous, were found to be 240 and 333 K, respectively. The distinct lateral force decrease was concurrently observed for the OTS monolayer at 240 and 333 K. The remarkable decrement of the lateral force at 240 K might be attributed to the decreasing molecular density and the activation of molecular motion (rotational motion along the molecular axis) owing to the rectangular-hexagonal crystal transition. And, the second remarkable depression of the lateral force observed at 333 K might come from the activation of thermal molecular motion due to the melting of alkyl chains.