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 corre lation shows that the arrays approach the melting behavior of molecules lac king internal rotors; i.e., liquid n-alkanes bind into monolayers with slig ht conformational entropy losses. Therefore, organized surface phases persi st above TF because their end-chain torsional barriers are lowered from sol id-phase values at such modest energy cost that net stabilization accrues f rom extra conformational disorder. The reported Delta T vs n diagram is con sistent with about n/2 end-chain monolayer internal rotors gaining the Delt a 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, infini te 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).