Interfaces That Never Settle: Reconceptualizing Micro- and Nanoplastic Interaction Coefficients as Evolving Descriptors

interaction coefficients, micro-and nanoplastics, physicochemical and biological processes I nteraction coefficients for micro-and nanoplastics (MPs/ NPs) are widely used to parametrize exposure, fate, and risk assessments.These descriptors�including sorption and partition coefficients (e.g., K d ) and uptake and desorption rate constants�are often treated as transferable material properties.Here, we argue that this interpretation is conceptually incomplete and can obscure the processes governing MP/NP interactions.Rather than reflecting intrinsic material constants, interaction coefficients represent interface-resolved descriptors of behavior on interfaces that evolve through physicochemical and biological processes.Most coefficients used in environmental modeling originate from laboratory sorption, desorption, or uptake experiments designed to probe specific interfacial conditions.While informative within their experimental context, reuse as static descriptors limits environmental relevance.Figure 1 situates commonly reported coefficients along interfacial transformation regimes, clarifying what each measurement represents.Across these regimes, we propose a shift in interpretation: from early sorption and uptake descriptors (K d , k f , and k d ), to exchange-focused parameters for dynamic coronas (K d,app , k in , and k out ), and finally to biofilm-governed affinity and release descriptors (K b and k des ).These coefficients are framed as indices of interfacial state rather than material constants.■ WHY INTERACTION COEFFICIENTS FAIL AS CONSTANTS From initial contact with aqueous environments, MPs/NPs undergo rapid interfacial reorganization.Hydration-driven restructuring, ionic screening, polymer relaxation, and molecular adsorption-desorption immediately modify surface energetics, while oxidation, hydrolysis, and abrasion further alter surface