Structure and dynamics of water at carbon-based interfaces

Water structure and dynamics are affected by the presence of a nearby interface. Here, first we review recent results by molecular dynamics simulations about the effect of different carbon-based materials, including armchair carbon nanotubes and a variety of graphene sheets—flat and with corrugation—on water structure and dynamics. We discuss the calculations of binding energies, hydrogen bond distributions, water’s diffusion coefficients and their relation with surface’s geometries at different thermodynamical conditions. Next, we present new results of the crystallization and dynamics of water in a rigid graphene sieve. In particular, we show that the diffusion of water confined between parallel walls depends on the plate distance in a non-monotonic way and is related to the water structuring, crystallization, re-melting and evaporation for decreasing inter-plate distance. Our results could be relevant in those applications where water is in contact with nanostructured carbon materials at ambient or cryogenic temperatures, as in man-made superhydrophobic materials or filtration membranes, or in techniques that take advantage of hydrated graphene interfaces, as in aqueous electron cryomicroscopy for the analysis of proteins adsorbed on graphene.Postprint (author's final draft

Membranes with different hydration levels: the interface between bound and unbound hydration water

The interaction of water with membranes is fundamental in many biological processes. Recently we found that, upon increasing hydration, water molecules first fill completely the interior of the membrane, next accumulate in layers in the exterior region. Here, we show by all-atom simulations that the translational and rotational dynamics of water molecules is strongly determined by their local distance to the membrane so that we can identify the existence of an interface between the first hydration shell, partially made of hydra- tion water bound to the membrane, and the next shells entirely made of unbound hydration water. Bound hydration water has a possible structural role and an extremely slow dynamics, while unbound hydration water, with no water-lipids hydrogen bonds, has a dynamics ten time faster than bound water but still one order of magnitude slower than bulk water. Our results could be relevant to understand the slowdown of biological activity upon dehydration

Modeling and simulation of lipid membranes

Cell membranes separate the interior of cells and the exterior environment, providing protection, controlling the passage of substances, and governing the interaction with other biomolecules and signalling processes. They are complex structures that, mainly driven by the hydrophobic effect [1], are based upon phospholipid bilayer assemblies containing sterols, glycolipids, and a wide variety of proteins located both at the exterior surface and spanning the membrane [2,3]. There exist a large number of different types of phospholipids, each with a given function, although we understand only a small fraction of them [4]. Recently, studies of the physical and biochemical characteristics of lipid molecules as been referred to as lipidomics [5] in recognition of their fundamental importance for the understanding of cell biology.Postprint (author's final draft

Molecular Dynamics simulations of concentrated aqueous electrolyte solutions

Transport properties of concentrated electrolytes have been analyzed using classical molecular dynamics simulations with the algorithms and parameters typical of simulations describing complex electrokinetic phenomena. The electrical conductivity and transport numbers of electrolytes containing monovalent (KCl), divalent (MgCl$_2$), a mixture of both (KCl + MgCl$_2$), and trivalent (LaCl$_3$) cations have been obtained from simulations of the electrolytes in electric fields of different magnitude. The results obtained for different simulation parameters have been discussed and compared with experimental measurements of our own and from the literature. The electroosmotic flow of water molecules induced by the ionic current in the different cases has been calculated and interpreted with the help of the hydration properties extracted from the simulations

Self-thermophoresis at the nanoscale using light induced solvation dynamics

Downsizing microswimmers to the nanoscale, and using light as an externally controlled fuel, are two important goals within the field of active matter. Here we demonstrate using all-atom molecular dynamics simulations that solvation relaxation, the solvent dynamics induced after visible light electronic excitation of a fluorophore, can be used to propel nanoparticles immersed in polar solvents. We show that fullerenes functionalized with fluorophore molecules in liquid water exhibit substantial enhanced mobility under external excitation, with a propulsion speed proportional to the power dissipated into the system. We show that the propulsion mechanism is quantitatively consistent with a molecular scale instance of self-thermophoresis. Strategies to direct the motion of functionalized fullerenes in a given direction using confined environments are also discussed.Postprint (author's final draft

Self-assembly of microscopic rods due to depletion interaction

In this article, using numerical simulations we investigate the self-assembly of rod-like particles in suspension due to depletion forces which naturally emerge due to the presence of smaller spherical depletant particles. We characterize the type of clusters that are formed and the evolution of aggregation departing from a random initial configuration. We show that eventually the system reaches a thermodynamic equilibrium state in which the aggregates break and reform dynamically. We investigate the equilibrium state of aggregation, which exhibits a strong dependence on depletant concentration. In addition, we provide a simple thermodynamic model inspired on the theory of self-assembly of amphiphilic molecules which allows us to understand qualitatively the equilibrium aggregate size distributions that we obtain in simulation

Redefining the concept of hydration water near soft interfaces

Water determines the properties of biological systems. Therefore, understanding the nature of the mutual interaction between water and biosystems is of primary importance for a proper assessment of any biological activity, e.g., the efficacy of new drugs or vaccines. A conve- nient way to characterize the interactions between biosystems and water is to analyze their impact on water density and dynamics in the proximity of the interfaces. It is commonly accepted that water bulk density and dynamical properties are recovered at distances of the order of 1 nm away from the surface of biological systems. This notion leads to the definition of hydration or biological water as the nano- scopic layer of water covering the surface of biosystems and to the expectation that all the effects of the water-interface interaction are limited to this thin region. Here, we review some of our latest contributions, showing that phospholipid membranes affect the water dynam- ics, structural properties, and hydrogen bond network at a distance that is more than twice as large as the commonly evoked 1 nm thick layer and of the order of 2.4 nm. Furthermore, we unveil that at a shorter distance 0:5 nm from the membrane, instead, there is an addi- tional interface between lipid-bound and unbound water. Bound water has a structural role in the stability of the membrane. Our results imply that the concept of hydration water should be revised or extended and pave the way to a deeper understanding of the mutual interac- tions between water and biological systems

Pair interactions among ternary DPPC/POPC/cholesterol mixtures in liquid-ordered and liquid-disordered phases

Saturated phospholipids, unsaturated phospholipids, and cholesterol are essential components of cell membranes, making the understanding of their mutual interactions of great significance. We have performed microsecond molecular dynamics simulations on the ternary mixtures of DPPC/POPC/cholesterol to systematically examine lipid-lipid and cholesterol-lipid interactions in the liquid-ordered and the liquid-disordered phases. The results show that there exists a competition between the tighter packing of cholesterol-lipid and the looser packing of lipid-lipid as the membrane changes from the liquid-disordered phase to the liquid-ordered phase. Depending on the lipid saturation, the favor of lipid-lipid interactions is in the order of saturated-saturated > monounsaturated-monounsaturated > saturated-monounsaturated. Cholesterol-saturated lipid interactions are more favorable than cholesterol-monounsaturated lipid ones. The results are consistent with the push-pull forces derived from experiments and give general insights into the interactions among membrane components.Postprint (author's final draft