Molecular Dynamics Simulation of A Langmuir Monolayer
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MOLECULAR DYNAMICS SIMULATION OF A LANGMUIR MONOLAYER
James P. Baremant and Michael L. Klein Department of Chemistry and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104 -6323
Abstract Molecular dynamics calculations were carried out for a Langmuir monolayer at high density. A model was employed in which the motions of both the amphiphiles and the water are allowed, enabling investigation of the details of the water-monolayer interaction and the structure of the interface. Results are presented for the density profiles of the water and the amphiphiles and for the distribution of gauche defects.
Introduction The study of monolayers at the air/water interface dates from before the turn of the century, when the first conjectures of the nature of these films were made.[l] Subsequently, detailed studies of amphiphile monolayers were reported by Irving Langmuir in 1917,[21 in recognition of which these monolayers are usually referred to as Langmuir monolayers. Already as early as 1920 Langmuir, working in conjunction with Katharine Blodgett, had discovered that amphiphilic monolayers on water surfaces could be transferred to solid surfaces.[3] Blodgett later published an account describing the sequential transfer of Langmuir monolayers to form multilayer films on solid substrates.[4] Films which have been built up in this way are now referred to as Langmuir-Blodgett (L-B) films. The potential ability to construct thin films with very specific properties using the L-B film technique has resulted in a great deal of interest in these films from a technological viewpoint.[5,6] As well, Langmuir and L-B films can be thought of as primitive models of biological membranes, and are therefore of biophysical interest.[7] The combination of these two factors and an increased interest in interfacial phenomena in general has generated a great deal of research on these films in the past several years.[8] Much of this work has involved the application of new experimental techniques especially suited to the study of interfaces, such as second harmonic generation (SHG)[9] and sum-frequency generation (SFG)[101 spectroscopy, fluorescence microscopy,[8,11,12] synchrotron X-ray reflectivity[ 13-151 and grazing incidence diffraction (GID),[16-18] scanning tunneling microscopy (STM)[19,20] and low energy electron diffraction (LEED).[211 These techniques have the advantage of being able to probe the structure of the adsorbed layer(s), whereas earlier work generally investigated thermodynamics of the system without providing direct structural information. tPresent address: Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1
Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
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Arguably the most powerful and successful of these techniques as applied to the study of Langmuir and L-B films are synchrotron X-ray reflectivity and GID. The first application of GID in this field involved the study of an L-B film.[ 161 However, most o
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