Molecular Dynamic Studies on Langmuir Monolayers of Stearic Acid

Compression isotherm for stearic acid was obtained by means of molecular dynamic simulation and compared to experimentally measured values for the Langmuir monolayers. Compared to the previous simulation, the present simulation has provided a method to reproduce the compression of the monolayer. The result is consistent with other experimental results. By analyzing the alkyl tails, the configuration of stearic acid molecules during the compression process was studied and a uniform monolayer was obtained after compression. Stearic acid molecules were observed to form fine organized monolayer from completely random structure. Hexatic order of the arrangement has been identified for the distribution of stearic acid molecules in the monolayer. At the end of the compression, the stearic acid molecules were tightly packed in the gap of two other molecules. At last, the hydrogen bonds in the system were analyzed. The main hydrogen bonds were from stearic acid-water interaction and their intensities constantly decreased with the decreased of surface area per molecule. The weak hydrogen bond interaction between stearic acid molecules may be the reason of easy collapse.