(552j) Investigation on Competition at Confined Interfaces: The Interplay between Polymers and Ions for a Binding Site.

Generating a detailed molecular understanding of complex, simultaneous inter actions at reactive and/or dynamic solid|fluid interfaces is a challenge across disciplines, and has intrigued researchers for decades.[1, 2] Whether it is, for example, in medical adhesives, friction of articular cartilage,[3] or the adhesion of organisms in seawater,[2] complex macroscopic properties at crowded biologic solid|liquid interfaces are mediated by large numbers of individual nanoscale interactions;[4] namely similar or dissimilar molecule/molecule and molecule/surface interactions, surface-dipole interactions[5] or the competing interactions with cations and water.[6]

Exactly this complex competition and molecular structuring at interfaces are central to a multitude of interfacial phenomena, such as membrane transport,[7] membrane conductance, [8,9] cellular adhesion [10] and adhesion regulation in the marine environment. [11]

In our previous work, we characterised a lipid-based model system (LMS) in terms of its stability and bending properties. [12] Here, we further modify its outer face with amine-terminating polymers (varied in density during the experiments) to investigate the specific electrostatic interaction between the amine and a negatively charged mica surface. We utilised a surface forces apparatus (SFA) and an atomic force microscope (AFM) to appreciate the force-distance interaction profile and the sample topography. Moreover, we examine how interaction forces are affected by the electrolyte concentration.[13]

In details, at the mica interface, we observe that cations and polymers begin a nanoscopic competition for the available binding site. We support this observation with experimental data showing the correlation between electrolyte concentration and the measured work of adhesion for LMS on mica. Specifically, we observe a loss in adhesion of 90% when the electrolyte concentration is increased to 1M. Visualisation of super resolved ionic layers onto the mica lattice further confirms the cations presence increasing with concentration. Finally, based on a kinetic model using two competing Langmuir adsorption isotherms we can estimate ion/surface interaction energies from the experimentally recorded interaction force measurements, demonstrating a path for a comprehensive combined experimental and modelling approach.[13]

The authors acknowledge support by the European Research Council (ERC-StG Grant No. 677663). The authors acknowledge the TU Wien Bibliothek for financial support through its Open Access Funding Program.

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(13) Bilotto, P.; Imre, A. M.; Dworschak, D.; Mears, L. L. E.; Valtiner, M. ACS Physical Chemistry Au 2021, 1, 45-53.