A NEW SURFACE PHASE IN LIQUID NORMAL-ALKANES

A new surface phase is found in liquid normal-alkanes. X-ray and surface tension measurements show the formation of a crystalline monolayer on the surface of the liquid at temperatures up to 3-degrees-C above the bulk solidification temperature, T(f). The molecules in the monolayer are hexagonally packed and oriented normal to the surface. A single solid monolayer persists down to T(f), thus exhibiting a very limited partial wetting. The new surface phase is obtained only for chain lengths 14 < n less-than-or-equal-to 50. Its vanishing for n less-than-or-equal-to 14 is interpreted as a possible transition from surface freezing to surface melting behaviour. The measurements are satisfactorily accounted for within a simple theory based on surface energy considerations.