Hydraulic Conductivity Prediction of Pristine and Polymer‐Modified Bentonite‐Rich Porous Media via Molecular Dynamics

Abstract Bentonite and polymer‐modified bentonites, as ubiquitous clay minerals in geological formations and widely utilized barrier materials in engineered systems, significantly influence the hydraulic properties of porous media due to their high swelling capacity and ultralow hydraulic conductivity ( k ). Accurate prediction of k is crucial not only for critical natural processes, such as rainfall infiltration, groundwater flow, and solute transport in subsurface aquifers, but also for engineering applications, such as contaminant containment, nuclear waste disposal, and CO 2 geological storage. However, existing predictive models often rely on empirical assumptions and non‐physical fitting parameters. This study employs large‐scale molecular dynamics simulations integrated with the original Kozeny‐Carman (K‐C) equation to predict k across a wide range of dry densities for both pure montmorillonite (MMT, = 134.6–1759.4 kg/m 3 ) and carboxymethyl cellulose‐modified MMT (CMC‐MMT, = 147.5–1550.7 kg/m 3 ). Our approach fundamentally challenges the conventional understanding that original K‐C equation is unsuitable for clayey soils: molecular‐scale parameterization of free water porosity (), tortuosity (), and water‐accessible surface area ( S 0 ) yields predictions within a factor of five of experimental data. Critically, k reduction was proven to stem from bound water immobilization, where polymer modification amplifies this effect through three synergistic mechanisms: (a) enhanced water adsorption thickens bound layers (reducing by 18%–76%), (b) pore filling increases by up to 6 times, and (c) elevated S 0 further restricts flow pathways. This work establishes a physically rigorous framework for hydraulic conductivity prediction in clay‐rich porous media, resolving long‐standing controversies in clay hydraulics and offering insights applicable to both natural and engineered systems.