Entropic contribution to the lateral pressure in Langmuir monolayers

VILLARREAL MA; MONTICH GG

Bariloche, Rio Negro

Congreso; XXXII Reunión Anual de la Sociedad Argentina de Biofísica; 2003

Sociedad Argentina de Biofísica

We present a series of molecular dynamics simulations of tetradecanol monolayers at the air-water interfase. The compression isotherm was obtained in two ways. In the first method the alcohol molecules (a total of 1800) were arranged in a hexagonal lattice at the desired lateral area per molecule. In this way we constructed systems at 40, 35, 30, 27, 24, 22 and 20 Å2 /molecule, which were simulated for 1 ns in the NVT ensemble. In the second method, the equilibrated system at 27 Å2 /molecule was allowed to compress by coupling the system to a pressure bath. In all cases, the calculated isotherm was in qualitative agreement with the experimental results. Some discrepancies were observed in the region of low molecular area, where the simulations are probably trapped in a metastable state. As the molecules compress, the fraction of dihedrals in the trans state increases, following the lateral pressure-area curve. Also, the potential energy decreases continuously as the lateral pressure increases. This indicates that the increment in the larteral pressure is not due to the van der Waals repulsion between the chains, but to the appearance of an entropic pressure. We propose that this entropic pressure is a consequence of the redistribution of velocities along the aliphatic chain in the condensed state as compared with the expanded state.