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.
There are less than a handful of papers in the literature on the phase measurement of surface Second Harmonic Generation (SHG) from liquid interfaces, in sharp contrast to tens of the works on such measurement of films on solid substrate surfaces. Even though the SHG phase measurement is very important for obtaining structural and spectroscopic information, such as molecular orientation, at the liquid interfaces, experimentally there are many difficulties due to the intrinsically weak SH signal of liquid interfaces. Using a femtosecond pulsed laser and photon counting system, we have demonstrate the ability to measure the SH phase for the non-resonant neat air/water interface, which is clearly among the systems with the smallest surface SH susceptibility, and is well below the detection limit of experimental setups in most laboratories. After clarifying some confusions about the reference standards for absolute SH phase measurement using the α-quartz crystal, we used air/water interface as an intrinsic SH phase reference standard to measure the SH phases of some air/pure-liquid interfaces, and adsorbed molecular layers at the air/water interface. These results demonstrate the effectiveness of this very sensitive technique in obtaining molecular structural and spectroscopic information of liquid interfaces.
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