Conformational Disorder Binds n-Alkanes into Surface Monolayers above the Normal Freezing Point

Abstract

The monolayer arrays formed on liquid n-alkanes within ΔT ≈ 3 K of the normal freezing point T_F 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 T_F 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 ΔT vs n diagram is consistent with about n/2 end-chain monolayer internal rotors gaining the ΔS = 5.3 J K^(-1) mol^(-1) entropy increase associated with a reduction of the gauche/trans energy barrier from V_o = 17 kJ mol^(-1) (as in packed, infinite polyethylene chains) to V_o < 6 kJ mol^(-1) (cf. with V_o = 3.4 kJ mol^(-1) in liquid n-alkanes) at a cost of about 1.6 kJ mol^(-1).