A lattice dynamics study of a Langmuir monolayer of monounsaturated fatty acids

A Monte Carlo dynamics (MCD) scheme has been applied in a study of the effects of unsaturated double bonds on the internal conformational dynamics and orientational order of hydrocarbon chains arranged in a monolayer on the surface of an impenetrable interface. The MCD algorithm makes use of the high coordination {2 1 0) lattice for the representation of both sp3 and sp2 valence states of the carbon atoms. The chain dynamics are considered to arise from a superposition of local conformational rearrangements. The simulations reproduced the principal features of the experimentally observed order parameter profiles of the C-H bonds on taking into account the intramolecular conformational energy of the molecules and excluded volume effects. The results show that the introduction of a rigid, planar, unsaturated segment enhances the orientational order in the monolayer. The extent of the enhancement is larger for the tram unsaturated chains than for the cis unsaturated ones. The increase in orientational order is accompanied by a marked increase in the effective rotational correlation times, indicating that the unsaturated segments undergo slow and restricted motion. In addition, the C-H bonds of the saturated chain segment between the cis double bond and the headgroup of the chain undergo slower motions than the corresponding vectors in the saturated and trans unsaturated chains. This arises from the anchoring of the headgroup at the impenetrable monolayer interface.