MOLECULAR-DYNAMICS SIMULATION OF TETRADECYLTRIMETHYLAMMONIUM BROMIDE MONOLAYERS AT THE AIR-WATER-INTERFACE

Molecular dynamics simulations of tetradecyltrimethylammomium bromide (C(14)TAB) monolayers at the air/water interface were performed at sur face areas of 45 and 67 Angstrom(2)/molecule. The computed density pro files normal to the interface for the headgroups and hydrocarbon tails are in good agreement with those derived from neutron reflectivity da ta. The simulations predict a decrease in the widths of the headgroup and the hydrocarbon chain distributions with increasing surface area/m olecule. Analysis of the distributions of the nitrogen atoms within th e monolayers by radial and three-dimensional distribution functions sh owed that the headgroups form a disordered layer. The large widths of the headgroup density profiles seem to be related to an intrinsic roug hness of the monolayers and not to a long-wavelength periodic structur e. The headgroup radial distribution functions display split first pea ks whose inner portion corresponds to about one neighbor at a distance of 6.5 Angstrom. The hydrocarbon chain structure was analyzed by taki ng into account the headgroup out-of-plane position. This analysis sho wed that the chains are highly disordered (2-3 gauche conformations/ch ain) and that their internal structure depends on the vertical positio n with respect to the interface: A slight difference observed between the chain conformations in the two systems is related to the different width's of the headgroup distributions. The analysis of the distribut ion functions of water molecules and bromide counterions with respect to the headgroups indicated that the coordination shell structures aro und the headgroups are similar in the: two systems. The in-plane diffu sion constants for the centers-of-mass of the C(14)TAB molecules were of the same order of magnitude as those in other lipid/water systems.