Measurement of Dielectric Polarizations for Analyzing the Orientational Order of Langmuir Monolayers

The dielectric polarizations, i.e. spontaneous, linear, and second order non-linear polarizations, generated from Langmuir monolayers comprised of rod-shaped molecules were analyzed, and represented as functions of the orientational order parameters S 1 , S 2 , and S 3 ( S n = 〈 P n ( cos ⁡ θ ) 〉 , θ : molecular tilt angle, 〈 〉 : thermodynamic average, P n ( cos ⁡ θ ) : Legendre polynomial of n th order). It has been shown that the Maxwell displacement current (MDC), Brewster angle reflectometry (BAR), and optical second harmonic generation (SHG), which are attributed to the spontaneous, linear, and non-linear polarization, respectively, are capable of detecting the orientational order parameter S 1 , S 2 , and S 3 of Langmuir monolayers. An experiment using the MDC-BAR-SHG was conducted on Langmuir monolayers comprised of 4-octyl-4′-cyanobiphenyl rod-shaped molecules. It is shown that the MDC-BAR-SHG measurements are useful to investigate the orientational structure of Langmuir monolayers.