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A new coarse-grained (CG) model of chitosan has been developed for predicting solution behavior as a function of degree of acetylation (DA). A multiscale modeling approach was used to derive the energetic and geometric parameters of this implicit-solvent, CG model from all-atom simulations of chitosan and chitin molecules in explicit water. The model includes representations of both protonated d-glucosamine (GlcN(+)) and N-acetyl-d-glucosamine (GlcNAc) monomers, where each monomer consists of three CG sites. Chitosan molecules of any molecular weight, DA, and monomer sequence can be built using this new CG model. Discontinuous molecular dynamics simulations of chitosan solutions show increased self-assembly in solution with increasing DA and chitosan concentration. The chitosan solutions form larger percolated networks earlier in time as DA and concentration increase, indicating "gel-like" behavior, which qualitatively matches experimental studies of chitosan gel formation. Increasing DA also results in a greater number of monomer-monomer associations, which has been predicted experimentally based on an increase in the storage modulus of chitosan gels with increasing DA. Our model also gives insight into how the monomer sequence affects self-assembly and the frequency of interaction between different pairs of monomers.

Hydrophobically modified chitosan (HMC) is being considered as a possible oil dispersant additive to reduce the volume of dispersant required in oil spill remediation. We present the results of discontinuous molecular dynamics (DMD) simulations intended to determine how the HMC architecture affects its ability to prevent oil aggregation. The HMCs have a comb copolymer architecture with hydrophobic side chains (modification chains) of various lengths (5-15 spheres) to represent alkane chains that are attached to the chitosan backbone. We calculated the oil's solvent accessible surface area (SASA), aggregate size distribution, and aggregate asymmetry at various values of the HMC modification chain length, modification density, and concentration to determine HMC efficacy. HMCs with long modification chains result in larger oil SASA than HMCs with short modification chains. For long modification chains, there is no increase in oil SASA with increasing modification density above a saturation value. The size distribution of the oil aggregates depends on the modification chain length; systems with long modification chains lead to large aspherical aggregates, while systems with short modification chains lead to small tightly packed aggregates. A parametric analysis reveals that the most important factor in determining the ability of HMCs to prevent oil aggregation is the interaction between the HMC's modification chains and the oil molecules, even when using short modification chains. We conclude that HMCs with long modification chains are likely to be more effective at preventing oil aggregation than HMCs with short modification chains, and that long modification chains impede spherical oil droplet formation.

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Chitosan is a versatile biopolymer that can self-assemble in solution to form hydrogels and nanoparticles. It consists of two types of monomers: glucosamine (GlcN) and N-acetylglucosamine (GlcNAc). Chitosan self-assembly is controlled by a balance of interactions between these two types of monomers: GlcN which gets protonated in solution leading to electrostatic repulsion and GlcNAc which contains an acetyl group, leading to hydrophobic and hydrogen bonding interactions. We present the results of discontinuous molecular dynamics (DMD) simulations aimed at understanding how the degree of acetylation (DA) and monomer sequence affect network formation in solution. Chitosans with DAs ranging from 10% to 50% and three different monomer sequencesrandom, evenly spaced, and blockyare studied. We show that chitosans with blocky sequences of GlcNAc monomers form percolated networks earlier in time than random and evenly spaced sequences for all DAs tested. Analysis of the pore size distributions of the resulting chitosan networks shows that blocky sequences of GlcNAc monomers lead to larger pores than random and evenly spaced sequences for DAs less than or equal to 30%. The monomer sequence has little impact on pore size distribution when the DA is 40% or higher. Finally, we show that at low DA, chitosan networks allow free diffusion of small molecules through the network but slow the diffusion of large molecules. At high DA, chitosan networks allow free diffusion of both large and small molecules. We conclude that controlling the monomer sequence of chitosan could be effective at controlling the structure of the resulting network.