First-order thermal roughening of normal alkane crystals

In this paper a study of the critical behavior of the thermal-roughening transition occurring at the {110} faces of odd-numbered n-paraffin crystals in n-hexane solution systems is presented. It follows from our measurements that the edge free energy, the order parameter of the roughening phase transition, remains constant when the temperature T is lower than the roughening temperature Tᵣ, and abruptly vanishes at T=Tᵣ. (Tᵣ=273.260 K.) This critical behavior of the order parameter shows the character of a first-order phase transition, rather than the conventional infinite-order phase transition of the Kosterlitz-Thouless type. An explanation of this observation is suggested in terms of the change of the motional state of solid molecules in the interfacial phase.