Analysis of solvation and gelation behavior of methylcellulose using atomistic molecular dynamics simulations.

Abstract

We adopt atomistic molecular dynamics (MD) simulations to study the solvation and gelation behavior of homogeneous methylcellulose (MC) and model random oligomers that represent the commercial cellulosic polymer product METHOCEL A in water and acetone solvents. We demonstrate that the two carbohydrate-specific GROMOS force fields, GROMOS 45A4 and 56Acarbo, are capable of reproducing characteristic angle distributions and the persistence length of MC chains reported in the literature. We then use the GROMOS 56Acarbo force field in both single-chain and multiple-chain simulations to characterize their solvation behavior through radial distribution functions, hydrogen-bond counts, and contact map analyses. We find that the un-methylated O6 position on the cellulose ring forms the most hydrogen bonds, followed by O2 and O3, implying that methylation at the 6 position reduces hydrogen bonding more than does methylation at other positions. O6-O6 is the most probable intermolecular hydrogen bond between different MC molecules. Dimethylated and trimethylated MCs form aggregated structures at low temperatures and precipitate-like structures at high temperatures in water but disperse randomly in acetone. This is consistent with experimental observations of gelation at elevated temperatures in water. The heterogeneous METHOCEL A model shows increased aggregation of trimethylated monomer units at elevated temperatures, suggesting that hydrophobic interaction is the main factor that induces the gelation, rather than hydrogen bonding.