Conformational disorder binds n-alkanes into surface monolayers above the normal freezing point

The monolayer arrays formed on liquid n-alkanes within Delta T approximate to 3 K of the normal freezing point TF are but the two-dimensional termini of a series of disordered solid phases. An enthalpy/entropy of fusion correlation shows that the arrays approach the melting behavior of molecules lacking internal rotors; i.e., liquid n-alkanes bind into monolayers with slight conformational entropy losses. Therefore, organized surface phases persist above TF because their end-chain torsional barriers are lowered from solid-phase values at such modest energy cost that net stabilization accrues from extra conformational disorder. The reported Delta T vs n diagram is consistent with about n/2 end-chain monolayer internal rotors gaining the Delta S = 5.3 J K-1 mol(-1) entropy increase associated with a reduction of the gauche/trans energy barrier from V-0 = 17 kJ mol(-1) las in packed, infinite polyethylene chains) to V-0 < 6 kJ mol(-1) (cf. with V-0 = 3.4 kJ mol(-1) in liquid n-alkanes) at a cost of about 1.6 kJ mol(-1).