Molecular dynamic simulations of eicosanoic acid and 18-methyleicosanoic acid langmuir monolayers

The conformational and dynamical properties of Langmuir monolayers of 18-methyleicosanoic acid (18-MEA) and the parent material, eicosanoic acid (EA), are compared using molecular dynamics simulations. The effects on various properties, including film thickness, tilt angle, and order parameter, of the methyl group at the 18 position in 18-MEA were investigated as a function of film-packing density. NVT simulations were run as a function of decreasing areal-packing density similar to experimental Langmuir-Blodgett film compressions and expansions. We find that the order parameters and film thickness for 18-MEA monolayers were markedly different from those of EA. The order parameters for methylene groups for both 18-MEA and EA are greater in the middle region of the chain than at the ends in high-density films. This trend becomes reversed in lower density films. Significantly, our simulations show that the order parameters for methylene groups near the CH3 and carboxyl termini in 18-MEA are comparatively independent of film density in contrast with those of EA. Our findings show that the presence of the methyl group at the 18-position in 18-MEA induces unique intermolecular structural correlations compared to EA.