Abstract
Function and properties at biologic as well as technological interfaces are controlled by a complex and concerted competition of specific and unspecific binding with ions and water in the electrolyte. It is not possible to date to directly estimate by experiment the interfacial binding energies of involved species in a consistent approach, thus limiting our understanding of how interactions in complex (physiologic) media are moderated. Here, we employ a model system utilizing polymers with end grafted amines interacting with a negatively charged mica surface. We measure interaction forces as a function of the molecule density and ion concentration in NaCl solutions. The measured adhesion decreases by about 90%, from 0.01 to 1 M electrolyte concentration. We further demonstrate by molecular resolution imaging how ions increasingly populate the binding surface at elevated concentrations, and are effectively competing with the functional group for a binding site. We demonstrate that a competing Langmuir isotherm model can describe this concentration-dependent competition. Further, based on this model we can quantitatively estimate ion binding energies, as well as binding energy relationships at a complex solid|liquid interface. Our approach enables the extraction of thermodynamic interaction energies and kinetic parameters of ionic species during monolayer level interactions at a solid|liquid interface, which to-date is impossible with other techniques.
Highlights
All active systems that are subject to change, motion, or flow of matter are governed by molecular level interactions that drive and steer the way in which macroscopic structures develop, evolve, adapt, and age
Many studies on specific binding systems have been performed in order to establish an understanding of interfacial interactions across the full range of length and energy scales, bridging the gaps between molecular scale interactions and macroscopic properties.[15−17] Still, we lack a detailed understanding of how molecular level competition and interplay impact macroscopic interactions in complex media such as physiological solutions containing a complex mixture of ions and water,[18] as well as functional molecules.[19,20]
The jump into contact is mediated by the specific intermolecular interaction between the positively charged amines and the negatively charged mica binding sites, which indicate a shorter range at higher ion concentrations due to the expected screening effect
Summary
All active systems that are subject to change, motion, or flow of matter (i.e., all biologic systems, and all mechanical systems) are governed by molecular level interactions that drive and steer the way in which macroscopic structures develop, evolve, adapt, and age. The study of molecular interactions is a shared and fundamental interest in seemingly unrelated fields, such as biophysics[1] and adhesion,[2] corrosion science,[3] and stem cell research,[4] or electro-osmosis in ion channels.[5] In essence, competing molecular interactions, such as competitions of different specific and unspecific bonds, drive subtle molecular balances and equilibria in the complex machinery of life and in technology. This includes studies on biofouling of marine fauna,[6−8] receptor−ligand interactions,[9,10] engineered lipid bilayer membranes,[11,12] and polymers investigated for different density, electrolyte, or pH conditions.[13,14] Further, many studies on specific binding systems have been performed in order to establish an understanding of interfacial interactions across the full range of length and energy scales, bridging the gaps between molecular scale interactions and macroscopic properties.[15−17] Still, we lack a detailed understanding of how molecular level competition and interplay impact macroscopic interactions in complex media such as physiological solutions containing a complex mixture of ions and water,[18] as well as functional molecules.[19,20] Generating a detailed molecular understanding of complex, simultaneous interactions at reactive and/or dynamic solid| fluid interfaces is a challenge across disciplines, and has intrigued researchers for decades.[21−25] Whether it is, for example, in medical adhesives, friction of articular cartilage,[26] or the adhesion of organisms in seawater,[24] complex macroscopic properties at crowded biologic solid|liquid
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