<title>New phenomena in phase transition and accurate determination of orientational order in Langmuir monolayer</title>

Abstract

Langmuir monolayer at the air/water interface is considered ideal model for studying 2-dimensional phase behaviors. In this work, the molecular detail on phase transition and orientational order in Langmuir monolayer is studied with simultaneous measurements using surface Second Harmonic Generation (SHG) and surface pressure. With the general methodology for SHG orientational analysis developed recently in our group, we studied the 4'-n-octyl-4-cyanobiphenyl (8CB) monolayer at the air/water interface. We were able to determine both the orientational angle and angular distribution, and also to determine the orientational angular changes in the liquid phase of the 8CB monolayer. These new data pointed to a novel phenomenon which can be attribute to the domain interaction driven orientation distribution narrowing (DIDODIN) mechanism during the orientational phase transition, which implies the phase transition is second-order. The same phenomenon is also observed for a quite different molecular system, Parc18. It implies this phenomenon can be general for Langmuir monolayers. Our findings demonstrated that quantitative orientational analysis is capable of determining the molecular interactions at interfaces in surprising details, which are responsible for the behaviors of the phase transition and orientational order in the Langmuir monolayer and molecular films.