Dipole Surface Orientational Order

We present ellipsometric observations of the orientational order a₂ of highly polar molecules at the noncritical liquid/vapor surface of critical highly polar + nonpolar mixtures. The dipoles, which are repelled from the interface via interactions with their image dipoles, are preferentially oriented with their axes parallel to the surface and possess an orientational order which is well described by a₂~ - t ^2bY(z/x) where t=|T-T_c|/T_c is the reduced temperature, b= 0.328 is a critical exponent, and Y is a universal function of the dimensionless depth z/x with x a surface correlation length.

Further readings:  J. –H. J. Cho, Ph.D. dissertation, Physics Dept., Kansas State Univ. (2002).
                           J. -H. J. Cho and B. M. Law, to be published.
 
 

Comparison of various adsorption / orientational order models with the highly polar + non-polar critical mixture 2-nitroanisole + cyclohexane 2NC (symbols): rP model with xo+=0.27 nm (blue solid line) or  xo+s=0.42 nm (blue dashed line), ra model with xo+s=0.42 nm and MD=-15.1 (red solid line) or MD=+15.1 (red dotted line).

Comparison of the ellipticity r for the 2NC and 4NC mixtures. Experiment: one- (squares) and two-phase (circles) data. Theory: a2  model (solid lines).

Comparison of the local density v2(z) =(1-v(z))a2(z)/4p derived from the a2(z) model for the mixture 2NC (solid line) with the computer simulation data (symbols) from Eggebrecht et al. The calculation was done at a reduced temperature of t = 0.05, which is similar to the effective reduced temperature of the simulations.