Molecular Insights into Hierarchical Zn²⁺ Hydration States Driving Energy-Efficient Cellulose Dissolution

Developing efficient and sustainable solvents for cellulose dissolution at ambient conditions is pivotal for advancing green chemistry and reducing reliance on petroleum-derived materials. Although ZnCl₂-based solvents, such as the typical ZnCl₂/H₂O systems, demonstrate remarkable room-temperature dissolution capabilities, their molecular-level mechanisms remain poorly understood. Traditional experimental approaches face limitations in resolving dynamic interactions and thermodynamic drivers, hindering solvent optimization. Herein, molecular dynamics (MD) simulations elucidate the mechanistic synergy of hydrated Zn²⁺ ions, Cl^- ions, and H₂O molecules in disrupting cellulose's hydrogen-bonding network and hydrophobic interactions. Results reveal that [Zn(H₂O)₃]²⁺ hydrate preferentially penetrates cellulose chains via O3 hydroxyl sites, forming O(cellulose)···HO(H₂O) hydrogen bonds, while [Zn(H₂O)₄]²⁺ hydrates stabilize dispersed chains. Cl^- ions reduce hydrophobicity by interacting with C-H groups, complementing Zn²⁺-driven hydrogen bond cleavage. Thermodynamically, entropy compensation from released H₂O molecules and Cl^- ions offsets Zn²⁺ ions adsorption, yielding a minimal entropy change (-498.00 J/mol/K). Enthalpy (-40.63 kJ/mol) dominates the favorable Gibbs free energy (-177.56 kJ/mol), underscoring the system's efficiency. This work provides atomistic insights into solvent-cellulose interactions, guiding the design of eco-friendly, energy-efficient cellulose solvents aligned with sustainable development goals.