Specific Ion Effects in Cholesterol Monolayers.

Teresa Del Castillo-Santaella,Julia Maldonado-Valderrama,Jordi Faraudo,Alberto Martín-Molina

doi:10.3390/ma9050340

Teresa Del Castillo-Santaella, Julia Maldonado-Valderrama + Show 2 more

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https://doi.org/10.3390/ma9050340

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Abstract

The interaction of ions with interfaces and, in particular, the high specificity of these interactions to the particular ions considered, are central questions in the field of surface forces. Here we study the effect of different salts (NaI, NaCl, CaCl2 and MgCl2) on monolayers made of cholesterol molecules, both experimentally (surface area vs. lateral pressure isotherms measured by a Langmuir Film Balance) and theoretically (molecular dynamics (MD) all-atomic simulations). We found that surface isotherms depend, both quantitatively and qualitatively, on the nature of the ions by altering the shape and features of the isotherm. In line with the experiments, MD simulations show clear evidences of specific ionic effects and also provide molecular level details on ion specific interactions with cholesterol. More importantly, MD simulations show that the interaction of a particular ion with the surface depends strongly on its counterion, a feature ignored so far in most theories of specific ionic effects in surface forces.

Highlights

The effect of ions is of key importance in determining physicochemical properties and functionality of systems as diverse as macromolecules, colloids, membranes, or microfluidic devices [1]
We show that specific ion effects in cholesterol films show atypical and interesting features, which challenge our current understanding of specific ionic effects in surface forces
Before discussing the results for different ions, we will first discuss the case of NaCl experimentally and using molecular dynamics (MD) simulations

Summary

Introduction

The effect of ions is of key importance in determining physicochemical properties and functionality of systems as diverse as macromolecules, colloids, membranes, or microfluidic devices [1]. By ionic specificity we mean that in a wide range of phenomena (surface forces, surface tensions, colloidal stability, protein stability, etc.) one finds that different ions, which have the same electrical charge, induce different behavior. These effects are important in surface forces acting in biological systems [3]. These effects are important for monovalent ions and for biologically relevant divalent ions such as Mg2+ or Ca2+ [4] for which electrostatics are usually assumed to be dominant

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