Visualization of Ion|Surface Binding and In Situ Evaluation of Surface Interaction Free Energies via Competitive Adsorption Isotherms

Function and propertiesat biologic as well as technological interfacesare controlled by a complex and concerted competition of specificand unspecific binding with ions and water in the electrolyte. Itis not possible to date to directly estimate by experiment the interfacialbinding energies of involved species in a consistent approach, thuslimiting our understanding of how interactions in complex (physiologic)media are moderated. Here, we employ a model system utilizing polymerswith end grafted amines interacting with a negatively charged micasurface. We measure interaction forces as a function of the moleculedensity and ion concentration in NaCl solutions. The measured adhesiondecreases by about 90%, from 0.01 to 1 M electrolyte concentration.We further demonstrate by molecular resolution imaging how ions increasinglypopulate the binding surface at elevated concentrations, and are effectivelycompeting with the functional group for a binding site. We demonstratethat a competing Langmuir isotherm model can describe this concentration-dependentcompetition. Further, based on this model we can quantitatively estimateion binding energies, as well as binding energy relationships at acomplex solid|liquid interface. Our approach enables the extractionof thermodynamic interaction energies and kinetic parameters of ionicspecies during monolayer level interactions at a solid|liquid interface,which to-date is impossible with other techniques.

Automated summary

The study investigates the interaction forces at interfaces by using a model system involving polymers interacting with a negatively charged mica surface. It is found that the measured adhesion decreases by around 90% as the electrolyte concentration increases from 0.01 to 1M. Furthermore, molecular resolution imaging demonstrates the increasing population of ions competing with functional groups for binding sites at higher concentrations.