MOLECULAR-DYNAMICS SIMULATIONS OF LONG-CHAIN AMPHIPHILIC MOLECULES IN LANGMUIR MONOLAYERS

Molecular dynamics simulations have been used to investigate phase transitions in a monolayer model of long-chain amphiphilic molecules (20 pseudoatoms) at the air-water interface. Several simulations were performed in the density range near close packing in which a number of phases have been identified experimentally (18.5-25 angstrom2/molecule). Between 24 and 25 angstrom2/molecule the monolayer undergoes a first-order phase transition exhibited by a break-up of lattice structure, a strong decrease in trans fraction and end-to-end distance, and an onset of center of mass translational displacement. Between 18.5 and 21 angstrom2 the relaxation time for molecules rotating around their major axes decreases significantly, the average chain conformation in the molecule midsections changes from a mostly trans to a gauche defected conformation, and the frequency of the head group movements normal to the monolayer plane changes noticeably. Simulations of monolayers of short-chain amphiphilic molecules (16 pseudoatoms) indicate that the first-order phase transition does not have the same characteristics as that in the long-chain monolayer.