Combined Small-Angle Neutron Scattering, Diffusion NMR, and Molecular Dynamics Study of a Eutectogel: Illuminating the Dynamical Behavior of Glyceline Confined in Bacterial Cellulose Gels

Abstract

A deep eutectic solvent (DES) entrapped in a bacterial cellulose (BC) network gives rise to a gelatin-like, selfsupported material termed a bacterial cellulose eutectogel (BCEG).Although this novel material holds potential for numerous industrial, environmental, energy, or medical applications, little is known about the structural features or dynamical behavior within a eutectogel. In this work, we employ X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and small-angle neutron scattering (SANS) to probe the structural and diffusive behavior of the prevailing DES glyceline (1:2 molar ratio of choline chloride:glycerol) confined within bacterial cellulose. XRD investigations demonstrate that the bacterial cellulose maintains its crystallinity even as the glyceline content approaches 95 wt % in the BCEG, an outcome corroborated by molecular dynamics (MD) simulations, which suggest minimal changes in the structural features of the cellulose chains due to the presence of glyceline. SANS measurements reveal a significant reduction in the radius of gyration (R g ) for BC in a BCEG compared to its hydrogel analogue, indicating a collapse in the microfibrillar structure that we attribute to removal of waters from the interfibrillar space due to a higher affinity of DES for water than for cellulose. Furthermore, SANS experiments suggest that the vast majority of DES is hosted within large micropores in the BCEG (i.e., mesoscopic confinement). Interestingly, proton NMR experiments disclose faster diffusional rates for choline and glycerol entrapped in a BCEG compared to neat glyceline. MD simulations offer the possible explanation that this diffusional acceleration results from significant migration of chloride from the bulk to cellulose microfibrillar surfaces, thereby reducing hydrogen bonding with choline and glycerol partners. This study provides the first comprehensive investigation into the structure and diffusional dynamics of glyceline within a eutectogel, offering insights into mass transport that should be useful for tailoring these novel materials to potential applications.

Computer Simulations of Deep Eutectic Solvents: Challenges, Solutions, and Perspectives

“…Although charge scaling helps, at least in some cases, to achieve agreement with experiments, it can also lead to artificial structural and dielectric properties, such as an excessive decrease in density [ 125 ], less intense peaks in the radial distribution functions, and an artificial dielectric response [ 126 ]. In addition, changes in the atomic charges can affect the parameterization of intermolecular interactions leading to artificial structural characteristics [ 122 , 127 ]. One particular problem is the incapacity of the scaled-charge models to fix the artificially enhanced long-range ion–ion correlations present in non-polarizable models, as discussed by McDaniel and Yethiraj [ 128 ].…”

“…It is worth mentioning that the cellulose-based products obtained from biomass could be used to develop novel DES-containing materials. For example, Smith et al [ 127 ] reported experimental and simulation results on bacterial cellulose gels containing DES glyceline. Both X-ray diffraction analyses and MD simulations confirmed that the DESs has almost no effect on the crystalline structure of cellulose.…”

Computer Simulations of Deep Eutectic Solvents. Challenges, Solutions, and Perspectives

“…Although charge scaling helps, at least in some cases, to achieve agreement with experiments, it can also lead to artificial structural and dielectric properties such as an excessive decrease in density [108], less intense peaks in the radial distribution functions, and an artificial dielectric response [109]. In addition, changes in the atomic charges can affect the parameterization of intermolecular interactions leading to artificial structural characteristics [105,110]. One particular problem is the incapacity of the scaled-charge models to fix the artificially enhanced long-range ion-ion correlations present in non-polarizable models, as discussed by McDaniel and Yethiraj [111].…”

“…It is worth mentioning that the cellulose-based products obtained from biomass could be used to develop novel DES-containing materials. For example, Smith et al [110] reported experimental and simulation results on bacterial cellulose gels containing DES glyceline. Both X-ray diffraction analyses and MD simulations confirmed that the DESs has almost no effect on the crystalline structure of cellulose.…”

Deep Eutectic Solvents: A New Class of Versatile Liquids

“…A number of studies report results from theory (Hayyan, Tuinier) , as well as classical and ab initio molecular dynamics simulations (Atilhan, Aparicio, Kashyap, Hung, Chaumont, Banerjee, Pádua, Kirchner, Zahn, Paul, and Balasubramanian) − to shed light on the structure, thermodynamics, and dynamics of DESs. X-ray and neutron scattering studies, sometimes in conjunction with molecular simulations, are reported to better understand DES structure (Triolo, Russina, Lewandowski, Brown, Atkin, Maginn, and Edler). − Various spectroscopic techniques are used, sometimes in conjunction with simulations, to study solvation structure, dynamics, and diffusion (Mitra, Sen, Biswas, Pandey, Kuroda, Gurkan, and Baker). − Other studies have focused on catalysis (Liu, Zhang), the effect of water on properties (Gutiérrez, Panzer), , electrodeposition (Magagnin), and fundamental thermodynamics and phase behavior (Coutinho, Marrucho). , …”