Biaxiality ordering of molecular orientation in a monolayer at the liquid-air interface

The biaxiality ordering of molecular orientation of a monolayer at the liquid-air interface is theoretically analyzed assuming that the molecular dipole of the constituent rodlike molecules is not parallel to the molecular long axis. Two independent order parameters, biaxiality parameter ξ and the orientational order parameter Szz[=〈P₂(cosθ)〉], exhibit a smooth change from zero at a critical molecular area A₀ by monolayer compression. ξ is found to be not so weak in monolayers on a water surface, due to the dipole-medium interaction. This prediction may be confirmed by nuclear magnetic resonance and nonlinear optical techniques.