posted on 2024-02-28, 20:09authored byZijie Wu, Audrey M. Collins, Arthi Jayaraman
We present a multiscale molecular
dynamics (MD) simulation study
on self-assembly in methylcellulose (MC) aqueous solutions. First,
using MD simulations with a new coarse-grained (CG) model of MC chains
in implicit water, we establish how the MC chains self-assemble to
form fibrils and fibrillar networks and elucidate the MC chains’
packing within the assembled fibrils. The CG model for MC is extended
from a previously developed model for unsubstituted cellulose and
captures the directionality of H-bonding interactions between the
–OH groups. The choice and placement of the CG beads within
each monomer facilitates explicit modeling of the exact degree and
position of methoxy substitutions in the monomers along the MC chain.
CG MD simulations show that with increasing hydrophobic effect and/or
increasing H-bonding strength, the commercial MC chains (with degree
of methoxy substitution, DS, ∼1.8) assemble from a random dispersed
configuration into fibrils. The assembled fibrils exhibit consistent
fibril diameters regardless of the molecular weight and concentration
of MC chains, in agreement with past experiments. Most MC chains’
axes are aligned with the fibril axis, and some MC chains exhibit
twisted conformations in the fibril. To understand the molecular driving
force for the twist, we conduct atomistic simulations of MC chains
preassembled in fibrils (without any chain twists) in explicit water
at 300 and 348 K. These atomistic simulations also show that at DS
= 1.8, MC chains adopt twisted conformations, with these twists being
more prominent at higher temperatures, likely as a result of shielding
of hydrophobic methyl groups from water. For MC chains with varying
DS, at 348 K, atomistic simulations show a nonmonotonic effect of
DS on water-monomer contacts. For 0.0 < DS < 0.6, the MC monomers
have more water contacts than at DS = 0.0 or DS > 0.6, suggesting
that with few methoxy substitutions, the MC chains are effectively
hydrophilic, letting the water molecules diffuse into the fibril to
participate in H-bonds with the MC chains’ remaining –OH
groups. At DS > 0.6, the MC monomers become increasingly hydrophobic,
as seen by decreasing water contacts around each monomer. We conclude
based on the atomistic observations that MC chains with lower degrees
of substitutions (DS ≤ 0.6) should exhibit solubility in water
over broader temperature ranges than DS ∼ 1.8 chains.