arXiv subjects

Styliani Consta

Publications and source records attributed to Styliani Consta.

5 recordsLinked to original sources

arXiv2025

From Spheres to Cones: Structural Instabilities and Acidity at Conical Regions in Trivalent Metal Ion Nano-clusters

Sub-nanometer aqueous clusters containing a single trivalent metal cation can exhibit charge-induced structural instabilities. Here, we present computational evidence that clusters containing a single \ce{Fe^{3+}}, \ce{Lu^{3+}}, or \ce{La^{3+}} ion undergo continuous geometric transformations as a consequence of this instability. These clusters dynamically evolve across their potential energy landscape, adopting triangular, elongated two-point, single-point, and more spherical configurations often with distinct conical surface protrusions. The manifestation of this instability differs from that observed in mesoscopic and microscopic droplets containing macroions, where stable ``star-like'' structures form, characterized by a specific number of conical protrusions that varies with the droplet size. In the present study, we find that the orientation of the \ce{H2O} molecules surrounding the metal ion is influenced not only by the electric field of the trivalent ion but also by the local conical protrusions. To further investigate the local acidity in the conical protrusions, we employ a proxy model system consisting of an aqueous nano-cluster containing three \ce{H3O+} ions, simulated using ab initio molecular dynamics. Within the conical regions of the cluster, protons exhibit diffusion across several water molecules, in contrast to the more localized proton delocalization observed in the compact body of the cluster. These findings suggest that local geometry can significantly modulate acidity in highly charged nano-clusters, with potential implications for understanding charge-transfer and ionization mechanisms in techniques such as electrospray ionization mass spectrometry and complement interpretations from infrared spectroscopic data.

cond-mat.soft↗

arXiv2023

Limitations of atomistic modeling to reveal ejection of proteins from charged nanodroplets

Molecular dynamics using atomistic modeling is frequently used to unravel the mechanisms of macroion release from electrosprayed droplets. However, atomistic modeling is currently feasible for only the smallest window of droplet sizes appearing at the end of a disintegrating droplet's lifetime. The relevance of the observations made to the actual droplet evolution, which is much longer that the simulated sizes, has not been addressed. Here, we perform a systematic study of desolvation mechanisms of poly(ethylene glycol) (PEG), protonated peptides of different compositions and proteins in order to (a) examine whether atomistic modeling can establish the extrusion mechanism of proteins from droplets and (b) obtain insight for the charging mechanism in larger droplets than those simulated. Atomistic modeling of PEG charging shows that above a critical droplet size charging occurs transiently by transfer of ions from the solvent to the macroion, while below the critical size, the capture of the ion from PEG has a lifetime sufficiently long for extrusion of the charged PEG from an aqueous droplet. This is the first report of the role of droplet curvature in the conformations and charging of macroions. Simulations with highly hydrophobic peptides show that partial extrusion of a peptide from the droplet surface is rare relative to desolvation by drying-out. Differently from what has been presented in the literature we argue that atomistic simulations have not sufficiently established extrusion mechanism of proteins from droplets and their charging mechanism. Moreover, we argue that release of highly charged proteins can occur at an earlier stage of a droplet's lifetime than predicted by atomistic modeling. In this earlier stage, we emphasize the key role of jets emanating from a droplet at the point of charge-induced instability in the release of proteins.

cond-mat.soft↗

arXiv2021

Direct evidence of jets emanating from droplets at the Rayleigh charge-induced instability point

Highly charged liquid droplets are unstable above the critical charge squared-to-volume ratio given by the Rayleigh limit. The instability leads to ion ejection from jets formed on the droplet's surface. Despite the many experiments that have been performed to capture the jet formation the precise fission mechanism has not yet been observed because of its brief transient nature. Here, we present the first atomistic simulations that reveal the mechanism of Rayleigh fission. We demonstrate that ion ejection takes place through a drop's deformation from a spherical into a distinct shape that contains a conical protrusion. We assert that the latter state is a free energy minimum along an order parameter that measures the degree of droplet asphericity. The charged droplet's long-time evolution proceeds by alternating between the two minima above and below the critical value that are reached through solvent evaporation and ion ejection, respectively. For the first time, this mechanism allows one to explain the nature of the progeny droplets and the percentage of charge lost during fission. We determine that the cone half-angle is close to the value predicted from the solution of the electrostatic equation for the dielectric liquid. It is found that the conical deformation is independent of the effect of electrohydrodynamic forces reported in experiments. Contrary to the experimental observations of two diametrically opposite jets for droplets suspended in the electric field, we found that a single jet is formed at the Rayleigh limit. This indicates that super-charged droplet states may have been detected in the experiments.

physics.flu-dyn↗

arXiv2021

Low density interior in supercooled aqueous nanodroplets expels ions to the subsurface

The interaction between water and ions within droplets plays a key role in the chemical reactivity of atmospheric and man-made aerosols. Here we report direct computational evidence that in supercooled aqueous nanodroplets a lower density core of tetrahedrally coordinated water expels the cosmotropic ions to the denser and more disordered subsurface. In contrast, at room temperature, depending on the nature of the ion the radial distribution in the droplet core is nearly uniform or elevated towards the center. We analyze the spatial distribution of a single ion in terms of a reference electrostatic model. The energy of the system in the analytical model is expressed as the sum of the electrostatic and surface energy of a deformable droplet. The model predicts that the ion is subject to a harmonic potential centered at the droplet's center of mass. We name this effect "electrostatic confinement". The model's predictions are consistent with the simulation findings for a single ion at room temperature but not at supercooling. We anticipate this study to be the starting point for investigating the structure of supercooled (electro)sprayed droplets that are used to preserve the conformations of macromolecules originating from the bulk solution.

cond-mat.soft↗

arXiv1996

Dynamics of Proton Transfer in Mesoscopic Clusters

Proton transfer rates and mechanisms are studied in mesoscopic, liquid-state, molecular clusters. The proton transfer occurs in a proton-ion complex solvated by polar molecules comprising the cluster environment. The rates and mechanisms of the reaction are studied using both adiabatic and non-adiabatic molecular dynamics. For large molecular clusters, the proton-ion complex resides primarily on the surface of the cluster or one layer of solvent molecules inside the surface. The proton transfer occurs as the complex undergoes orientational fluctuations on the cluster surface or penetrates one solvent layer into the cluster leading to solvent configurations that favor the transfer. For smaller clusters the complex resides mostly on the surface of the cluster and proton transfer is observed only when the complex penetrates the cluster and solvent configurations that favor the proton transfer are achieved. Quantitative information on the cluster reaction rate constants is also presented.

chem-ph↗