Topological Confinement and Energetic-Configurational Compensation at Nanoporous Silica Interfaces

Conventional descriptions of adsorption stability emphasize direct adsorbate-surface interactions, leaving the configurational consequences of three-dimensional nanoconfinement insufficiently resolved. Here, simulations reveal how nanoporous topology reorganizes energetic stabilization, configurational cost, and directional interfacial loading during dibutyl phthalate (DBP) adsorption on amorphous fused silica. Molecular dynamics, directional-force decomposition, and laterally restrained umbrella sampling across a 0-50% porosity series identify an accessibility crossover at 30-40% porosity, where adsorption shifts from two-dimensional surface accommodation to three-dimensional topological envelopment. On flat surfaces, molecules undergo 2D interfacial flattening, adopting extended surface-parallel conformations dominated by direct energetic stabilization and surface-normal loading. In contrast, nanoporous confinement imposes 3D topological clamping, progressively redistributing interfacial loading toward lateral components and establishing multidirectional mechanical confinement. A dimensionless topological-mechanical descriptor, S, quantifies this coupling between molecular envelopment and directional force response. Critically, confinement decouples energetic stabilization from free-energy stability: within 50% porous silica, a deep-pore state with a 3.52 kcal mol-1 energetic disadvantage exhibits a 4.39 kcal mol-1 lower effective configurational residual than its near-surface counterpart, producing a 0.87 kcal mol-1 point-estimate energy-free-energy rank reversal. The accompanying reduction in torsional penalty identifies internal conformational accessibility as one contributor to this compensation. These results establish a topology-induced energetic-configurational compensation mechanism, whereby three-dimensional confinement can generate strongly stabilized adsorption states without stronger direct adsorbate-surface attraction, providing a molecular framework for topology-guided regulation of porous interfaces.