PHASE-TRANSITION IN LANGMUIR FILMS OF OCTADECYLMALONIC ACID

The phase transitions observed in the surface pressure-molecular area (pi-A) isotherms of monolayers of the dicarboxylic compound octadecylm alonic acid(OMA) at the air/water interface were investigated using mo lecular dynamics simulation in the molecular area ranging from 20 to 5 0 A(2)/molecule in a triangular lattice. Potential energy, torsion ang les, and hydration of the head groups under periodic boundary conditio ns have been used for the study. Hydration was done by a stacked array of water molecules relaxing under these boundary conditions. Twelve s imulations were performed at T = 300 K to study the conformation of th e packed molecular system. Bonded and nonbonded interaction potentials were taken from Biosym force fields. The transitions seen in the pi-A curves are explained from the breaks in the plots of potential energy at various concentrations, related to a liquid expanded-to-liquid con densed (LE/LC) transition, and classified under the first order type. Electron diffraction patterns of these films transferred onto transmis sion electron microscopy (TEM) grids were studied by high-resolution i maging and showed a clear transition from the LE to the LC phase, (C) 1998 Academic Press.