Styliani Consta

A 118 nm vacuum ultraviolet laser/time-of-flight mass spectroscopic study of methanol and ethanol clusters in the vapor phase

Clusters of methanol and ethanol formed above neat liquid samples were entrained in a supersonic jet of helium and probed in the expansion using 118 nm vacuum ultraviolet laser single-photon ionization/time-of-flight (TOF) mass spectrometry. Almost every cluster ion observed in the TOF mass spectra could be represented by the formula H(ROH)n+, where R=CH3 or C2H5, and n=1–5. Formation of these species is attributed to a well-established ionization pathway where each protonated (n−1)-mer originates from its n-mer neutral parent. Signals in the TOF mass spectra due to the protonated trimers H(CH3OH)3+ and H(CH3CH2OH)3+ were found to be the most intense and provides direct evidence that these particular cluster ions are “magic-number” structures. The possible relationships between the observed ion data and the neutral cluster vapor phase distributions are discussed. In this context, methanol and ethanol vapor cluster distributions at 298.15 K and at several pressures⩾the equilibrium vapor pressure were compu...

Recoil growth: An efficient simulation method for multi-polymer systems

We present a new Monte Carlo scheme for the efficient simulation of multi-polymer systems. The method permits chains to be inserted into the system using a biased growth technique. The growth proceeds via the use of a retractable feeler, which probes possible pathways ahead of the growing chain. By recoiling from traps and excessively dense regions, the growth process yields high success rates for both chain construction and acceptance. Extensive tests of the method using self-avoiding walks on a cubic lattice show that for long chains and at high densities it is considerably more efficient than configurational bias Monte Carlo, of which it may be considered a generalization.

Manifestation of Rayleigh instability in droplets containing multiply charged macroions.

The Rayleigh limit and manifestations of instability in liquid droplets containing charged macroions are examined by molecular simulations. It is found that beyond the Rayleigh limit, the spherical droplets become unstable and form structures with distinct features. Regardless of the nature of the charged macroion, an assembly of spines of highly ordered polar solvent molecules form on the droplet surface. The surface charge distribution of the spiny droplet is highly nonuniform, and the macroscopic description of the droplet energy as a sum of electrostatic and surface terms is no longer valid. When the macroion is a charge-saturated polyhistidine chain, it is shown that the changes in the structure of the droplet are accompanied by the chain extension. Contrary to the conventional point of view, it is found that single ions present in droplets containing a highly charged macroion do not escape spontaneously but rather form complexes stable on the nanosecond time scale, depending on the degree of deviation from the Rayleigh limit and the nature of the ion. The effect of the instability in the disintegration mechanism of charged droplets in electrospray mass spectrometry experiments is discussed.

Fragmentation mechanisms of aqueous clusters charged with ions

Fragmentation processes of mesoscopic aqueous clusters charged with ions of similar sign are studied by computer simulations. In order to examine differences in the fragmentation that depend on the nature of the ions and the charge distribution, the clusters contain positive (Na+ and Ca2+) or negative (Cl−) ions. Insight into the fragmentation mechanism is obtained by theories of activated processes. Critical to this approach is the use of a new reaction coordinate that captures the shape fluctuations of the droplet that are responsible for the reaction. Reversible work profiles for the reaction are constructed along the reaction coordinate, and dynamics is performed. The dynamics validates the use of the reaction coordinate, and shows diffusive barrier crossing. It is found that clusters with even number of charges fragment unevenly in contrast to analytical theories that predict even fission by considering only the energetic factors that determine the stability of charged droplets.

Recoil growth algorithm for chain molecules with continuous interactions

The recoil growth (RG) scheme is a dynamic Monte Carlo algorithm that has been suggested as an improvement over the configurational bias Monte Carlo (CBMC) method (Consta, S., Wilding, N. B., Frenkel, D. and Alexandrowicz, Z., 1999, J. chem. Phys., 110, 3220). The RG method had originally been tested for hard core polymers on a lattice, and it was found that RG outperforms CBMC for dense systems and long chain molecules. In the present paper, the RG scheme is extended to the practically more relevant case of off-lattice chain molecules with continuous interactions. It is found that for longer chain molecules RG becomes over an order of magnitude more efficient than CBMC. However, other schemes are better suited to the computation of the excess chemical potential. Moreover, it is more difficult to parallelize RG than CBMC.

Fragmentation reactions of charged aqueous clusters

Fragmentation reaction of mesoscopic aqueous clusters loaded with several sodium ions is investigated by computer simulations. The ratio of the number of ions over that of the solvent molecules is such that the clusters are stable for several nanoseconds. Fragmentation is caused by infrequent shape fluctuations. A new reaction coordinate is introduced that captures the shape changes that determine the reaction mechanism and allows for the study of the reaction using theories of activated processes. The nature of the shape fluctuations responsible for the fragmentation is characterized by free energy profiles computed as function of the new reaction coordinate. Dynamics of the fragmentation shows that the barrier crossing is diffusive so that the dynamical corrections to the transition state are large. The structure of the decay of the time-dependent rate constant reflects the diffusive character of the recrossing dynamics so that a plateau is established after a long transient time of 5 ps. The free energy and dynamics of the reaction demonstrate that clusters fragment unevenly in contrast to predictions of analytical theories.

A spectroscopic and computer simulation study of butanol vapors

Clusters of butanol formed above neat liquid samples were entrained in a supersonic jet and probed using 10.5 eV vacuum ultraviolet laser single-photon ionization/time-of-flight mass spectrometry. The four different isomers of butanol (n-butanol, sec-butanol, iso-butanol, and tert-butanol) were studied separately to assess the influence of the structure of the alkyl chain on the formation and stability of the hydrogen bonded clusters. Most of the higher mass features observed in the mass spectra could be assigned to protonated alcohol clusters, H(ROH)n+, n⩽3; R=C4H9, that arise from facile proton-alkoxy radical/alkoxide anion dissociation. Signals due to protonated trimers were only evident in the spectra of tert- and sec-butanol. Empirical force fields, density functional theory and ab initio methods were used to identify the geometries of all clusters up to the pentamers for the different isomers. Monte Carlo simulations established vapor-phase cluster distributions, while molecular dynamics was used to...

Disintegration mechanisms of charged nanodroplets: novel systems for applying methods of activated processes

We review recent advances in the understanding of ejection mechanisms of solvated ions and charged macromolecules from highly charged nanodroplets. While the physical basis for the instability leading to droplet fragmentation is relatively well understood, a description of molecular mechanism of the fragmentation in complex systems is still missing. Development of a comprehensive model for the droplet fragmentation is further complicated by chemical modifications of the charged macromolecules (macroions) in a changing droplet environment. We highlight several different molecular simulation techniques used to study fragmentation of charged droplets with different solutes. Ejection of simple ions is analysed using theory of activated processes and transfer reaction coordinate (TRC). The TRC was shown to adequately represent complex rearrangement of solvent molecules in the course of evaporation. The critical value of the square of the charge to volume ratio for spontaneous ejection of simple solvated ions from aqueous droplets is found to be very close to that predicted by Rayleighs model. On the contrary, the presence of macromolecules adds a level of complexity into the system where the charge-induced instabilities cannot be described by a conventional theory such as Rayleigh or ion-evaporation mechanism. Additional charge–charge interactions between charged sites on a macromolecule dramatically change the macroion ejection mechanism. Molecular dynamics simulations reveal a number of distinct scenarios: contiguous extrusion, drying-out, star-like formation of solvent surrounding a macroion and pearl formation along the macromolecular chain.

Effect of counterions on the charging mechanisms of a macromolecule in aqueous nanodrops

We report the first molecular dynamics study of the effect of counterions on the charging mechanisms of a macromolecule found in an aqueous droplet that contains excess charge. To investigate the principles of the charging mechanisms of a macromolecule in a droplet, we simulate aqueous droplets that contain a poly(ethylene glycol) (PEG) molecule, sodium, and chloride ions. We study the effect of counterions by varying the concentration of the chloride ions and the temperature of the droplets. We find that the size of the droplet from which the macromolecule is released is determined by the competition between the counterions and the macromolecule for capturing the sodium ions. In droplets with radii in the range of 4 nm and smaller, [Na2Cl](+) ion complexes and sodium chloride aggregates are formed. The smaller the droplet the more pronounced is the formation of the NaCl aggregates. At very high temperature, in the larger droplets the Na(+) ions are distributed throughout the entire droplet. Therefore, the sodiated PEG is released with a higher average charge than from droplets with no counterions because it has access to a higher concentration of Na(+) ions. At moderately high temperature, the NaCl aggregates do not affect the final charge state of the macromolecule relative to the no-counterion droplets. We also report that regardless of the concentration of the counterions, the temperature plays a critical role in determining the nature of the droplet shape fluctuations that are responsible for the charging of a macromolecule and its extrusion from a droplet. At high temperature the macromolecule is released by the formation of a Taylor cone that transports ions onto the macromolecule. Differently, at lower temperature the Taylor cones are absent or have subsided. These findings provide insight into the mechanisms that macromolecules acquire their charge in droplets produced in electrospray ionization experiments.

Ionization reactions of ion complexes in mesoscopic water clusters

The free energy and dynamics of the dissociation reactions of the [Na+(Cl−)2] ion complex in mesoscopic water clusters are examined. The free energy surface shows the existence of stable single and double solvent-separated complex species formed from ionization of the stable double-contact ion complex. The reaction occurs on the cluster surface for mesoscale clusters composed of tens of water molecules. Passage between stable species is an activated process but barrier crossing has a large diffusive component so that dynamical corrections to transition state theory are large. The structure of the decay of the time-dependent rate constant reflects the diffusive character of the recrossing dynamics so that a plateau is not established on a 10 ps time scale in contrast to ionization dynamics in bulk fluids.