Wetting at the nanoscale

Wetting represents one of the most fundamental features of water interactions with a surface, resulting from interplay between cohesive and adhesive molecular forces. The wettability of a surface by a liquid is characterized by the contact angle of the liquid on that surface, which is also given by the well-known Young equation. At nanoscale, however, additional contributions enter the play and challenge the validity of the Young equation. We perform all-atom simulations of small water droplets and water films on flat surfaces with tunable polarities. The outcomes provide important insight into the relation between the surface chemistry and wettability, and imply the concept of generalized line tension. The latter stems from the three-phase contact line and represents a critical player in the stability of small droplets and liquid deposits in various contexts.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Vicerrectorado de Investigación de la UMA

Attraction between Neutral Dielectrics Mediated by Multivalent Ions in an Asymmetric Ionic Fluid

We study the interaction between two neutral plane-parallel dielectric bodies in the presence of a highly asymmetric ionic fluid, containing multivalent as well as monovalent (salt) ions. Image charge interactions, due to dielectric discontinuities at the boundaries, as well as effects from ion confinement in the slit region between the surfaces are taken fully into account, leading to image-generated depletion attraction, ion correlation attraction and steric-like repulsive interactions. We investigate these effects by employing a combination of methods including explicit-ion and implicit-ion Monte-Carlo simulations, as well as an effective interaction potential analytical theory. The latter incorporates strong ion-image charge correlations, which develop in the presence of high valency ions in the mixture. We show that the implicit-ion simulations and the proposed analytical theory can describe the explicit simulation results on a qualitative level, while excellent quantitative agreement can be obtained for sufficiently large monovalent salt concentrations. The resultant attractive interaction between the neutral surfaces is shown to be significant, as compared with the usual van der Waals interactions between semi-infinite dielectrics, and can thus play a significant role at the nano scale.Comment: 9 pages, 4 figure

Perspective: Coulomb fluids - Weak coupling, strong coupling, in between and beyond

We present a personal view on the current state of statistical mechanics of Coulomb fluids with special emphasis on the interactions between macromolecular surfaces, concentrating on the weak and the strong coupling limits. Both are introduced for a (primitive) counterion-only system in the presence of macroscopic, uniformly charged boundaries, where they can be derived systematically. Later we show how this formalism can be generalized to the cases with additional characteristic length scales that introduce new coupling parameters into the problem. These cases most notably include asymmetric ionic mixtures with mono- and multivalent ions that couple differently to charged surfaces, ions with internal charge (multipolar) structure and finite static polarizability, where weak and strong coupling limits can be constructed by analogy with the counterion-only case and lead to important new insights into their properties that cannot be derived by any other means

Conditions for the stable adsorption of lipid monolayers to solid surfaces

Lipid monolayers are ubiquitous in biological systems and have multiple roles in biotechnological applications, such as lipid coatings that enhance colloidal stability or prevent surface fouling. Despite the great technological importance of surface-adsorbed lipid monolayers, the connection between their formation and the chemical characteristics of the underlying surfaces has remained poorly understood. Here, we elucidate the conditions required for stable lipid monolayers nonspecifically adsorbed on solid surfaces in aqueous solutions and water/alcohol mixtures. We use a framework that combines the general thermodynamic principles of monolayer adsorption with fully atomistic molecular dynamics simulations. We find that, very universally, the chief descriptor of adsorption free energy is the wetting contact angle of the solvent on the surface. It turns out that monolayers can form and remain thermodynamically stable only on substrates with contact angles above the adsorption contact angle, θads⁠. Our analysis establishes that θads falls into a narrow range of around 60∘–70∘ in aqueous media and is only weakly dependent on the surface chemistry. Moreover, to a good approximation, θads is roughly determined by the ratio between the surface tensions of hydrocarbons and the solvent. Adding small amounts of alcohol to the aqueous medium lowers θads and thereby facilitates monolayer formation on hydrophilic solid surfaces. At the same time, alcohol addition weakens the adsorption strength on hydrophobic surfaces and results in a slowdown of the adsorption kinetics, which can be useful for the preparation of defect-free monolayers

Counterion-Mediated Weak and Strong Coupling Electrostatic Interaction between Like-Charged Cylindrical Dielectrics

We examine the effective counterion-mediated electrostatic interaction between two like-charged dielectric cylinders immersed in a continuous dielectric medium containing neutralizing mobile counterions. We focus on the effects of image charges induced as a result of the dielectric mismatch between the cylindrical cores and the surrounding dielectric medium and investigate the counterion-mediated electrostatic interaction between the cylinders in both limits of weak and strong electrostatic couplings (corresponding, e.g., to systems with monovalent and multivalent counterions, respectively). The results are compared with extensive Monte-Carlo simulations exhibiting good agreement with the limiting weak and strong coupling results in their respective regime of validity.Comment: 19 pages, 10 figure