PHYSISORPTION OF CYCLOHEXANE ON A SIO2/SI SUBSTRATE - EVIDENCE OF A WETTING TRANSITION ABOVE THE TRIPLE POINT

The physisorption of cyclohexane vapor onto silicon wafers with grossly different oxide layer thicknesses delta (25 and 570 nm) was studied in the temperature range 15-40 degrees C by ellipsometry. Isotherms of film thickness d exhibit an extended linear region up to a relative pressure p/p(0) approximate to 0.7, with a small and nearly temperature independent slope partial derivative d/partial derivative(p/p(0)), indicating weak adsorbate-substrate interactions. Above p/p(0) = 0.7 the isotherms show a pronounced temperature dependence and reveal a wetting transition with a wetting temperature T-w near 20 degrees C, i.e., ca. 14 K above the triple point of the adsorptive. Above T-w the film thickness increases steeply, with values exceeding 20 nm near the saturation pressure p(0), as expected for complete wetting. Below T-w the film thickness reaches only a few monolayers at p(0). In the temperature range just above T-w a pronounced hysteresis is observed in pressure quenches near saturation, which is explained by temperature excursions on crossing the prewetting line. The multilayer regime of the-isotherms above T-w was analyzed in terms of the Frenkel-Halsey-Hill (FHH) equation, ln(p/p(0)) = -(alpha/kT)d(-n), where the exponent n is expected to have a value of 3 for nonretarded dispersion force interactions and for sufficiently large d. The present data for the range from above one monolayer up to p/p(0) = 0.95 yield much lower values (n = 1.65 +/- 0.2 and 1.0 +/- 0.2 for wafers with delta = 25 nm and 570 nm, respectively). These low values of n are consistent with results of theoretical physisorption isotherms obtained by the Born-Green-Yvon theory when the interaction parameters are chosen in a way to reproduce the observed wetting transition. From these results it is concluded that the low values of n found in this study are characteristic for wetting systems just above the wetting transition.