DEM-Based Approach for the Modeling of Gelation and Its Application to Alginate

Abstract

The gelation of biopolymers is of great interest in the material science community and has gained increasing relevance in the past few decades, especially in the context of aerogels lightweight open nanoporous materials. Understanding the underlying gel structure and influence of process parameters is of great importance to predict material properties such as mechanical strength. In order to improve understanding of the gelation mechanism in aqueous solution, this work presents a novel approach based on the discrete element method for the mesoscale for modeling gelation of hydrogels, similarly to an extremely coarsegrained molecular dynamics (MD) approach. For this, polymer chains are abstracted as dimer units connected by flexible bonds and interactions between units and with the environment, that is, diffusion in implicit water, are described. The model is based on Langevin dynamics and includes an implicit probabilistic ion model to capture the effects of ion availability during ion-mediated gelation. The model components are fully derived and parameterized using literature data and theoretical considerations based on a simplified representation of atomistic processes. The presented model enables investigations of the higher-scale network formation during gelation on the micrometer and millisecond scale, which are beyond classical modeling approaches such as MD. As a model system, calcium-mediated alginate gelation is investigated including the influence of ion concentration, polymer composition, polymer concentration, and molecular weight. The model is verified against numerous literature data as well as own experimental results for the corresponding Ca-alginate hydrogels using nitrogen porosimetry, NMR cryoporometry, and small-angle neutron scattering. The model reproduces both bundle size and pore size distribution in a reasonable agreement with the experiments. Overall, the modeling approach paves the way to physically motivated design of alginate gels.

Wetting of alginate aerogels, from mesoporous solids to hydrogels: a small-angle scattering analysis

“…This qualitatively points to a mesoporous structure comprising elongated structures. This is typical of the nanostructured skeleton of Ca alginate aerogels, which is built from overlapping dense polymer fibrils that are a few nanometres wide (Depta et al, 2022). Upon hydration, this structure progressively disappears.…”

“…Alginate is a linear binary copolymer of mannuronic (M) and guluronic (G) acids assembling in around 285-571 dimeric units (Depta et al, 2022). The sodium alginate used for preparing the present calcium alginate aerogel is from a commercial source (Sigma Life Science, catalogue No.…”

“…Their polyanionic chains can be ionotropically cross-linked and gelled, which is most frequently realized using Ca II cations. The molecular structures of the resulting supramolecular assemblies have been characterized by the egg-box model for the coordination of the cations, and by physical models detailing the quaternary structures of the interlinked polysaccharide chains (Depta et al, 2022). The resulting hydrogels can be converted into aerogels in a classical way, through solvent exchange with acetone or ethanol followed by its extraction using supercritical CO 2 .…”

A Perspective on Methods to Computationally Design the Morphology of Aerogels

“…Adapted with permission. [ 85 ] Copyright 2021, The Authors. Published by American Chemical Society.…”

“…There has been a strong need for a model capable of adequately describing the fibrillar morphology of aerogels. Recently, Depta et al [ 85 ] proposed a mesoscale model under a DEM‐based Langevin dynamics framework for modeling the gelation in Ca‐alginate hydrogels. As an example model system, the Ca‐alginate system was chosen, thus, the modeling of alginate polymer systems using Ca + in aqueous media.…”