The ground-state phase behavior of model Langmuir monolayers

A coarse-grained model for surfactant molecules adsorbed at a water surface is studied at zero temperature to elucidate ground-state tilt ordering. The surfactants are modeled as rigid rods composed of head and tail segments, where the tails consist of effective monomers representing methylene CH2 groups. These rigid rods interact via site-site Lennard-Jones potentials with different interaction parameters for the tail-tail, head-tail, and head-head interactions. In this work, we study the effects due to variations in both the head diameter and bond length on transitions from untilted to tilted structures and from nearest-neighbor (NN) to next-nearest-neighbor (NNN) tilting. Coupling between tilt ordering and lattice distortion is also considered. We provide a molecular derivation of a scaling relation between tilt angles and distortion obtained previously by phenomenological arguments. Due to the discrete site-site nature of the model interactions, the predicted ground-state phase behavior is much richer than evidenced by models employing cylindrical rods. In particular, we have found transitions between different phases (i.e., NN-NN' and NNN-NNN') of similar symmetry, which may have experimental support. We have also examined the sensitivity of the transitions to details of the model, such as replacing Lennard-Jones head-head and head-tail potentials by purely repulsive interactions. (C) 2000 American Institute of Physics. [S0021-9606(00)70525-3].PT: J; CR: BALASHOV SM, 1994, THIN SOLID FILMS, V239, P127 BAREMAN JP, 1990, J PHYS CHEM-US, V94, P5202 BIBO AM, 1990, ADV MATER, V2, P309 BOMMARITO GM, 1996, J CHEM PHYS, V105, P5265 FISCHER TM, 1999, COMMUNICATION HAAS FM, 1995, J CHEM PHYS, V102, P2960 HAAS FM, 1996, J PHYS CHEM-US, V100, P15290 KAGANER VM, 1993, J CHEM PHYS, V98, P3512 KAGANER VM, 1993, J PHYS II, V3, P813 KAGANER VM, 1995, J CHEM PHYS, V102, P9412 KAGANER VM, 1999, REV MOD PHYS, V71, P779 KNOBLER CM, 1992, ANNU REV PHYS CHEM, V43, P207 LAUTZ C, 1997, J CHEM PHYS, V106, P7448 OPPS SB, UNPUB OPPS SB, 1999, THESIS U GUELPH SCHERINGER M, 1992, J CHEM PHYS, V96, P2269 SCHMID F, 1997, J CHEM PHYS, V106, P3757 SCHMID F, 1997, PHYS REV E B, V55, P5774 SHIH MC, 1994, J CHEM PHYS, V101, P9132 STADLER C, 1999, J CHEM PHYS, V110, P9697 STADLER C, 1999, PHYS REV E, V59, P4248 SWANSON DR, 1996, J CHEM PHYS, V105, P673 TEER E, 1997, J CHEM PHYS, V106, P1913 TIPPMANNKRAYER P, 1991, LANGMUIR, V7, P2303; NR: 24; TC: 4; J9: J CHEM PHYS; PG: 10; GA: 326TESource type: Electronic(1