Positional order and thermal expansion of surface crystalline N-alkane monolayers - art. no. 032602

We report a high-resolution synchrotron grazing incidence x-ray diffraction measurement of a surface crystalline monolayer at the Liquid-vapor interfa ce of the n-alkane eicosane (C20H42) just above its melting temperature. Th e peak width of the surface monolayer rotator phase is shown to be resoluti on limited and implies positional correlations of at least similar to1 mum. The high resolution allowed determination of the temperature dependence of the peak position over the narrow (3 degreesC) temperature range of the su rface crystal phase. The two-dimensional thermal expansion was determined t o be (dA/dT)/A = 1.8(+/-0.1) x 10(-3)degreesC(-1), which is comparable to t he expansion in similar chain length bulk n-alkane rotator phases. Our data are consistent with the power-law shaped scattering tails expected from qu asi-long-range order in two dimensions.