Atomistic free-energy analysis of polydopamine adhesion at hydrated cellulose interfaces

Polydopamine (PDA) adhesion to hydrated cellulose is governed by cellulose surface geometry, multivalent contact formation, and interfacial water dynamics. We further hypothesize that rate-dependent nonequilibrium pulling combined with temperature-dependent interfacial relaxation can constrain the low-rate adsorption limit. Simulations: Atomistic molecular dynamics simulations were used to study dopamine monomer and a 10-mer PDA model on cellulose Iβ. Dopamine adsorption was benchmarked by umbrella sampling and alchemical double decoupling, while PDA detachment from cellulose Iβ (100) and (110) was evaluated using multi-rate steered molecular dynamics, Jarzynski-based estimates, and empirical time-temperature superposition.

Dopamine benchmarks yielded consistent adsorption free energies of -11.56±0.76 and -10.45±1.51 kJ mol^-1, establishing the monomer reference scale. PDA adsorption was stronger on the flatter cellulose Iβ (100) surface than on the rougher (110) surface, with a low-rate estimate of -74.93±5.48 kJ mol^-1. This difference was associated with a smaller surface-normal PDA-cellulose separation, a larger grouped contact footprint, and lower PDA positional fluctuations on the (100) face. Hydrogen-bond analysis showed more PDA-cellulose and PDA-water hydrogen bonds on (110), indicating that stronger adsorption on (100) is not driven simply by hydrogen-bond number, but by surface-specific contact organization. Temperature-dependent pulling showed less favorable apparent adsorption at elevated temperature, consistent with faster hydrated-interface relaxation during detachment. These results identify surface accommodation, multivalent contacts, adsorbate conformation, and hydration as coupled variables controlling wet adhesion at cellulose-based interfaces.