MOLECULAR-DYNAMICS SIMULATION OF A LANGMUIR-BLODGETT PATCH

The molecular dynamics technique has been used to investigate the stru cture and stability of an isolated Langmuir-Blodgett patch. The patch consists of 144 stearic acid molecules, modelled using methylene group s with explicit hydrogen atoms and an all-atom representation of the c arboxylic acid head group. The molecules are physically adsorbed on a smooth Lennard-Jones substrate with the head groups down. Our results show that the stearic acid patch remains stable when the molecular dyn amics simulation at 298 K starts from an energy-minimised structure at a head group area of A(m) = 20.6 Angstrom(2) molec(-1). Under these c onditions the patch contracts and the stearic acid molecules in the ce ntre of the system form a translational ordered structure at head grou p area of A(m) = 19.9 Angstrom(2) molec(-1). At this system size the h ead-group area of the molecule is 4% lower than that observed experime ntally for much larger films. The long axes of the molecules are align ed perpendicular to the surface: on average 97% of molecules have tilt angles of less than 5 degrees. As the patch contracts from its starti ng configuration there is a slight change in the unit cell. The initia l hexagonal structure distorts slightly to form an oblique unit cell w ith two cell sides differing by 0.35 Angstrom. The vertical orientatio n of molecules within the patch centre contrasts with significant tilt angles measured by X-ray diffraction for the bulk stearic acid crysta ls but is supported by recent experimental FTIR observations of the ti lt of stearic acid molecules close to a germanium surface.