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[1] Myong In Oh and Styliani Consta. What factors determine the stability of a weak protein-protein interaction in a charged aqueous droplet? PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 19(47):31965--31981, DEC 21 2017.

Maintaining the interface of a weak transient protein complex transferred from bulk solution to the gaseous state via evaporating droplets is a critical question in the detection of the complex association (dissociation) constant by using electrospray ionization mass spectrometry (ESI-MS). Here we explore the factors that may affect the stability of a protein-protein interaction (PPI) using atomistic molecular dynamics (MD) modelling of a complex of ubiquitin (Ub) and the ubiquitin-associated domain (UbA) (RCSB PDB code 2MRO) and a non-covalent complex of diubiquitin (RCSB PDB code 2PEA) in aqueous droplets. A general method is presented to determine the protonation states of the complexes we investigate in particular, and that of a protein in general, under various pH conditions that an evaporating droplet acquires due to its change in size. We find that the combination of high temperature and high charge states of the protein complexes may destabilize the interface by creating new interfaces instead of a direct rupture of the initial stable interface. We provide evidence that highly charged protein complexes are found in droplets that form conical extrusions of the solvent on the surface due to charge-induced instability. This distinct droplet morphology leads to a higher solvent evaporation rate that assists in transferring the complex in the gaseous state without dissociation. The conical solvent protrusions expose on the droplet surface certain amino acids that otherwise would be solvated in a droplet with the protein complex of low charge states. The new vapor-protein interface does not have a direct effect on the stability of the PPI. A common way in experiments to stabilize the protein complexes in droplets is to reduce the protonation state of the proteins. Here we find that weakly bound protein complexes even at high protonation states can be stabilized by the presence of a small number of counterions, without affecting the protonation state of the protein. Our findings may provide guiding principles in ESI-MS experiments to stabilize weak transient PPIs.

[2] Myong In Oh, Anatoly Malevanets, Maxim Paliy, Daan Frenkel, and Styliani Consta. When droplets become stars: charged dielectric droplets beyond the Rayleigh limit. SOFT MATTER, 13(46):8781--8795, DEC 14 2017.

When a nano-drop comprising a single spherical central ion and dielectric solvent is charged above a well-defined threshold, it acquires a stable star morphology. In contrast, conducting droplets, will undergo fission. Here we report combined atomistic molecular dynamics and continuum modelling study of star formation of droplets that contain a highly charged ion. We assume that in the continuum model the dielectric response is linear. In this linear continuum model, which is an extension of Rayleigh model, the energy of the drop is comprised of terms analogous to those in Rayleigh model, which are surface energy and electrostatic energy of dielectric droplet charged by a central point charge. We present the stability analysis of the continuum model to determine the threshold of instability. Indeed we find that the model accounts well for the onset of the instabilities. Molecular dynamics show that the number of points of the star-shaped nano-drops depends only on the surface tension, dielectric constant and size of the droplet, and on the magnitude of the charge of the central ion, but not on its sign. Intuitively, it is expected that when a spherical dielectric drop becomes unstable it would transform into a non-spherical finite shape of the same volume as the initial spherical shape with the point charge located in the drop interior. To test whether the extended Rayleigh model can account for the observed droplet shapes, we performed numerical simulations of the linear continuum model. Contrary to the expectations, the simulations of the extended Rayleigh model does not reproduce the stable star shapes found in the atomistic simulations, not even when we account for the bending rigidity and spontaneous curvature of the surface. We argue that the assumption that the dielectric response is linear breaks down if the droplet surface approaches the central macro-ion, where the electric field strength is such that dielectric saturation sets in. We envisage that for certain solvents, these stars could be made permanent by cross-linking, opening the way to the production of a novel class of highly-non-convex colloids.

[3] Myong In Oh and Styliani Consta. Charging and Release Mechanisms of Flexible Macromolecules in Droplets. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY, 28(11):2262--2279, NOV 2017.

We study systematically the charging and release mechanisms of a flexible macromolecule, modeled by poly(ethylene glycol) (PEG), in a droplet by using molecular dynamics simulations. We compare how PEG is solvated and charged by sodium Na+ ions in a droplet of water (H2O), acetonitrile (MeCN), and their mixtures. Initially, we examine the location and the conformation of the macromolecule in a droplet bearing no net charge. It is revealed that the presence of charge carriers do not affect the location of PEG in aqueous and MeCN droplets compared with that in the neutral droplets, but the location of the macromolecule and the droplet size do affect the PEG conformation. PEG is charged on the surface of a sodiated aqueous droplet that is found close to the Rayleigh limit. Its charging is coupled to the extrusion mechanism, where PEG segments leave the droplet once they coordinate a Na+ ion or in a correlated motion with Na+ ions. In contrast, as PEG resides in the interior of a MeCN droplet, it is sodiated inside the droplet. The compact macro-ion transitions through partially unwound states to an extended conformation, a process occurring during the final stage of desolvation and in the presence of only a handful of MeCN molecules. For charged H2O/MeCN droplets, the sodiation of PEG is determined by the H2O component, reflecting its slower evaporation and preference over MeCN for solvating Na+ ions. We use the simulation data to construct an analytical model that suggests that the droplet surface electric field may play a role in the macro-ion-droplet interactions that lead to the extrusion of the macro-ion. This study provides the first evidence of the effect of the surface electric field by using atomistic simulations.

[4] Styliani Consta, Mahmoud Sharawy, Myong In Oh, and Anatoly Malevanets. Advances in Modeling the Stability of Noncovalent Complexes in Charged Droplets with Applications in Electrospray Ionization-MS Experiments. ANALYTICAL CHEMISTRY, 89(16):8192--8202, AUG 15 2017.

Electrospray ionization mass spectrometry is used extensively to measure the equilibrium constant of noncovalent complexes. In this Perspective, we attempt to present an accessible introduction to computational methodologies that can be applied to determine the stability of weak noncovalent complexes in their journey from bulk solution into the gaseous state. We demonstrate the usage of the methods on two typical examples of noncovalent complexes drawn from a broad class of nucleic acids and transient protein complexes found in aqueous droplets. We conclude that this new emerging direction in the use of simulations can lead to estimates of equilibrium constant corrections due to complex dissociation in the carrier droplet and finding of agents that may stabilize the protein interfaces.

[5] Myong In Oh and Styliani Consta. Stability of a Transient Protein Complex in a Charged Aqueous Droplet with Variable pH. JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 8(1):80--85, JAN 5 2017.

Electrospray ionization mass spectrometry (ESI-MS) has the potential to become a high-throughput robust experimental method for the detection of protein-protein equilibrium constants. Poorly understood processes that affect the stability of weak noncovalent protein complexes in the intervening droplet environment are a significant factor that precludes the advancement of the method. We use molecular dynamics to study the stability of a ubiquitin and ubiquitin-associated domain complex (RCSB PDB code 2MRO) in an aqueous droplet with changing size and charge concentration. We present evidence that a weak protein complex changes conformation and may dissociate in shrinking droplets. Then, the droplets containing these dissociated proteins divide. Our findings suggest that in some cases ESI-MS does not measure the correct association constants. The study intends to stimulate research for systematic development of experimental protocols that stabilize weakly bound protein interfaces in droplets.

[6] Mahmoud Sharawy and Styliani Consta. Characterization of “Star” Droplet Morphologies Induced by Charged Macromolecules. JOURNAL OF PHYSICAL CHEMISTRY A, 120(44):8871--8880, NOV 10 2016.

“Star” morphologies of charged liquid droplets are distinct droplet conformations that, for a certain charge squared to volume ratio, have lower energy than their spherically shaped analogues. For these shapes to appear, the charge should be carried by a single ionic species. A typical example of a charge carrier that we employ in this study is a fully charged double-stranded oligodeoxynucleotide (dsDNA) in an aqueous and an acetonitrile droplet. We-characterize the structure and dynamics of the star-shaped droplets. We find that by increasing the charge squared to volume ratio, the droplet evolves from spherical to “spiky” shapes, by first passing from droplet sizes that undergo enhanced shape fluctuations relative to those of the larger spherical droplets. These fluctuations mark the onset of the instability. We also find that in-the spiky droplet, the orientation of the solvent molecules in the first shell about the dsDNA is very close to that in the bulk solution. However, this orientation is substantially different farther away from the dsDNA. With regards to dynamics, the motion of the spikes is reflected in the autocorrelation functions of rotationally invariant order, parameters that show a damped oscillator form of decay, indicative of the elastic motion of the spikes. We compare the formation of spikes with that of the ferrofluids and the dielectric materials in an electric field, and we conclude that they represent a different entity that deserves its own characterization. The study provides insight into the manner in which the charge distribution may give rise to well-controlled droplet morphologies and calls for experiments in this direction.

[7] Styliani Consta, Myong In Oh, and Anatoly Malevanets. New mechanisms of macroion-induced disintegration of charged droplets. CHEMICAL PHYSICS LETTERS, 663:1--12, OCT 16 2016.

Molecular modeling has revealed that the presence of charged macromolecules (macroions) in liquid droplets dramatically changes the pathways of droplet fission. These mechanisms are not captured by the traditional theories such as ion-evaporation and charge-residue models. We review the general mechanisms by which macroions emerge from droplets and the factors that determine the droplet fission. These mechanisms include counter-intuitive “star” droplet formations and extrusion of linear macroions from droplets. These findings may play a direct role in determining macromolecule charge states in electrospray mass spectrometry experiments. (C) 2016 Elsevier B.V. All rights reserved.

[8] Sepideh Soltani, Myong In Oh, and Styliani Consta. Effect of solvent on the charging mechanisms of poly(ethylene glycol) in droplets. JOURNAL OF CHEMICAL PHYSICS, 142(11), MAR 21 2015.

We examine the effect of solvent on the charging mechanisms of a macromolecule in a droplet by using molecular dynamics simulations. The droplet contains excess charge that is carried by sodium ions. To investigate the principles of the charging mechanisms of a macromolecule in a droplet, we simulate aqueous and methanol droplets that contain a poly(ethylene glycol) (PEG) molecule. We find that the solvent plays a critical role in the charging mechanism and in the manner that the sodiated PEG emerges from a droplet. In the aqueous droplets, the sodiated PEG is released from the droplet while it is being charged at a droplet charge state below the Rayleigh limit. The charging of PEG occurs on the surface of the droplet. In contrast to the aqueous droplets, in the methanol droplet, the sodiated PEG resides in the interior of the droplet and it may become charged at any location in the droplet, interior or surface. The sodiated PEG emerges from the droplet by drying-out of the solvent. Even though these two mechanisms appear to be phenomenologically similar to the widely accepted ion-evaporation and charge-residue mechanisms, they have fundamental differences from those. An integral part of the mechanism that the macromolecular ions emerge from droplets is the droplet morphology. Droplet morphologies give rise to different solvation interactions between the solvent and the macromolecule. In the water-sodiated PEG system, we find the extrusion of the PEG morphology, while in methanol-sodiated droplet, we find the “pearl-on-the-necklace” morphology and the extrusion of the sodiated PEG in the last stage of the desolvation process. These findings provide insight into the mechanisms that macromolecules acquire their charge in droplets produced in electrospray ionization experiments. (C) 2015 AIP Publishing LLC.

[9] Styliani Consta and Anatoly Malevanets. Disintegration mechanisms of charged nanodroplets: novel systems for applying methods of activated processes. MOLECULAR SIMULATION, 41(1-3, SI):73--85, FEB 11 2015.

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 Rayleigh's 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.

[10] Styliani Consta, Myong In Oh, and Sepideh Soltani. Advances in the theoretical and molecular simulation studies of the ion chemistry in droplets. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY, 377(SI):557--567, FEB 1 2015.

We review recent theoretical and computational advances in the understanding of ejection mechanisms of solvated ions and macromolecular ions (macroions) from highly charged nanodrops. The excess charge is carried by simple ions or macroions. On the one hand, the principal factors that lead to the instability of a droplet due to its high charge are captured by fundamental theories. These theories consider the competition between the electrostatic interactions between ions of the same sign that tend to fragment the droplet and the surface tension that tends to keep the droplet connected. On the other hand, a detailed description of the molecular mechanism of the fragmentation in these complex systems has still not been developed. Development of a comprehensive model for the droplet fragmentation is further complicated by chemical modifications of the macromolecule by binding to alkali ions, divalent ions or protons in a constantly changing droplet environment. We describe our major findings in understanding the disintegration mechanisms of charged droplets. For droplets containing simple ions, we utilise computational methodology for activated processes to describe the ejection of solvated ions. We found that the transition state involves bottle-neck configurations where the ions leave a droplet by shape fluctuations that are similar to “Taylor” cones or string formations of the solvent. In the presence of macromolecular ions, due to the connectivity of the charge along the macromolecular backbone, droplets take different shapes from those that contain simple ions. By using molecular dynamics simulations we classified these droplet states as contiguous extrusion of the macroion from a droplet, drying-out, star-like formation of solvent surrounding a macroion and pearl formation along the macromolecular chain. Quantitative analysis of each of the droplet states calls for its own theoretical description to generalise the simulation findings. Using a combination of analytical theory and molecular simulations we proposed a theoretical model for the contiguous extrusion of a macromolecule from a droplet. (C) 2014 Elsevier B.V. All rights reserved.

[11] Falana Aziza Sheriff and Styliani Consta. Charge-induced instabilities of droplets containing macromolecular complexes. CANADIAN JOURNAL OF CHEMISTRY, 93(2):173--180, FEB 2015.

Solvated macromolecular complexes are ubiquitous in nature, notably in biological systems containing proteins and nucleic acids. Studies of the interactions within a macromolecular complex and between the complex and the solvent in droplet environments are critical for understanding the stability of macromolecular complexes in electrospray ionization (ESI) and nanofluidic experiments. In this study, two distinct cases of macromolecular complexes in aqueous nanodrops are examined by using molecular dynamics simulations: (i) a pair of sodiated poly(ethylene) glycol (PEG) macroions and (ii) a double-stranded DNA (dsDNA). PEG represents a case in which the surface energy of the aqueous droplet is larger than the solvent-macromolecule energy. Conversely, in a droplet solvating dsDNA, the solvent-macromolecule interaction energy overcomes the solvent interaction energy. We report that charge-induced instabilities previously identified for single macroions also appear in the case of complexes, but with a higher level of complexity. In the case of a pair of PEG macroions, we found that their conformations on the surface of a droplet “sense” each other. The charged PEGs are each released from a droplet at different times through contiguous extrusion or drying-out mechanisms. In the case of the DNA, the charge-induced instability manifests as a spine droplet morphology. Narrow regions of the spines promote break down of the hydrogen bonds that hold the dsDNA together. The dsDNA separates into two single strands as it is increasingly exposed to vacuum. These findings elucidate charge-induced instabilities of macromolecular complexes in droplets, which are critical intermediates in ESI and nanofluidic experiments.

[12] Mahmoud Sharawy and Styliani Consta. How do non-covalent complexes dissociate in droplets? A case study of the desolvation of dsDNA from a charged aqueous nanodrop. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 17(38):25550--25562, 2015.

We present the desolvation mechanism of a double-stranded oligodeoxynucleotide (dsDNA) from an aqueous nanodrop studied by using atomistic molecular dynamics methods. The central theme of this study is the stability of a non-covalently bound complex, in general, and that of a dsDNA in particular, in a droplet environment. Among the factors that may affect the stability of a complex in an evaporating droplet we examine the increase in ion concentration and the distinct droplet morphologies arising from the charge-induced instability. We explore in detail a large set of aqueous nanodrops with excess negative charge, which comprise a dsDNA and Na+, Cl- ions at various concentrations. We find that for a square of the charge to volume ratio above that of the Rayleigh limit the droplet attains distinct “spiky” morphologies that disperse the charge in larger volume relative to that of the spherical drop. Moreover, it is found that it is possible for a non-covalent complex to remain associated in an unstable droplet as long as there is enough solvent to accommodate the instability. In the presence of Na+ and Cl- ions, the Na+ ions form adducts with the double helical DNA in the minor groove, which help stabilise the duplex state in the gas phase. The negative ions may be released from the droplet. In a DNA-containing droplet with a net charge that is less negative than 50% of the dsDNA charge, the DNA maintains a double-stranded state in the gas phase. Several of our findings are in good agreement with experiments, while the spiky droplet morphology due to the charge-induced instability calls for new experiments. The results shed light on the association properties of complexes of macromolecules in droplet environments, which are critical intermediates in electrospray ionisation experiments.

[13] Mahmoud Sharawy and Styliani Consta. Effect of counterions on the charging mechanisms of a macromolecule in aqueous nanodrops. JOURNAL OF CHEMICAL PHYSICS, 141(10), SEP 14 2014.

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. (c) 2014 AIP Publishing LLC.

[14] Styliani Consta. Molecular modeling of the ejection mechanisms of macromolecules from charged droplets. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 248, AUG 10 2014. 248th National Meeting of the American-Chemical-Society (ACS), San Francisco, CA, AUG 10-14, 2014.
[15] M. Paliy, S. Consta, and J. Yang. Interactions between Carbon Nanoparticles in a Droplet of Organic Solvent. JOURNAL OF PHYSICAL CHEMISTRY C, 118(29):16074--16086, JUL 24 2014.

We report the first detailed molecular dynamics study of two carbon nanoparticles (nanotubes or fullerenes) embedded in a droplet of chloroform. The carbon nanoparticles are negatively charged and are accompanied by the positive counterions of sodium. On the one hand, this setup is inspired by the carbon nanoparticle salts-”nanotubides” and “fullerides”-proposed recently as a convenient way to address the difficult problem of the dispersion of carbon nanoparticulates in the organic solvents. On the other hand, the electrospray of carbon nanoparticle suspensions has been shown to be a promising technique for the production of thin nanoparticle coatings. We explore the combination of these two approaches by using molecular simulations. We find that depending on the overall electrostatic charge balance the two nanoparticles can exist in the droplet either in the “salted-out” bound state or in a “solvent-separated” state. The latter opens up the possibility for the efficient dispersion of the nanoparticles in the charged droplet environment. We also find that depending on the charge of the nanoparticles the mother droplet may break evenly or unevenly with respect to the mass but evenly with respect to charge. The understanding of the fragmentation paths and the intermolecular interactions of the nanoparticles in droplets provides insight into the manner that the droplet chemistry can be manipulated so that effective dispersion of nanoparticulates can be achieved.

[16] Andrei Buin, Haiyan Wang, Styliani Consta, and Yining Huang. A study of conformational equilibrium of 1,1,2-trichloroethane in FAU-type zeolites. MICROPOROUS AND MESOPOROUS MATERIALS, 183:207--217, JAN 1 2014.

We study the effects of loading and nature of the zeolitic framework in the trans reversible arrow gauche equilibrium of 1,1,2-trichloroethane (TCE) in zeolites with Faujasite (FAU) structure including sodium Y (Na-Y) and siliceous Y (Si-Y). TCE is used as a prototype molecule for the study of zeolite-chlorinated hydrocarbon interactions that determine conformational equilibrium. Conformational changes of TCE were sampled using empirical force fields and quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) in combination with replica exchange molecular dynamics (REMD). The population ratio of the conformers at high loading was compared with that found by Fourier transform (FT)-Raman spectroscopy and the results are in very good agreement. Comparison between the infinitely dilute limit of loading, and higher loading shows that the population ratio of the conformers at equilibrium depends strongly on the Si/Al ratio and consequently, the presence of charge compensating cations as well as the loading. At very low loading, in both Na-Y and Si-Y the population of the trans conformer is the largest with the exception of the empirical modeling that predicts the gauche conformer to be dominant in Na-Y. At the higher loading, in Na-Y the population the gauche conformer constitutes the great majority. In Si-Y the population of the trans conformer continues to dominate but the percentage of the gauche conformer increases substantially relative to the loading at the infinitely dilute limit. Such different conformational behaviors provide insight into the factors responsible for diffusion mechanisms and adsorption energies in the FAU-type zeolitic frameworks. (C) 2013 Elsevier Inc. All rights reserved.

[17] Anatoly Malevanets and Styliani Consta. Variation of droplet acidity during evaporation. JOURNAL OF CHEMICAL PHYSICS, 138(18), MAY 14 2013.

Variation of acidity and associated chemical changes of macromolecules in evaporating droplets is of central importance in electrosprayed aerosols. We study changes in acidity during evolution of a droplet that is composed of solvent and a charge binding macromolecule. We analyze the acidity of the droplet using analytical theory and stochastic modeling. We derive a universal relation for the minimum pH of a droplet in the presence of a protein and the results are confirmed by the stochastic modeling of ubiquitin and lysozyme at varying values of pH. We establish that in acidic droplets, once the number of solvated charges reaches the macroion charge, the further droplet evaporation, counter-intuitively, reduces the number of charges on the macromolecule and increases the droplet pH. (C) 2013 AIP Publishing LLC.

[18] Styliani Consta and Anatoly Malevanets. Classification of the ejection mechanisms of charged macromolecules from liquid droplets. JOURNAL OF CHEMICAL PHYSICS, 138(4), JAN 28 2013.

The relation between the charge state of a macromolecule and its ejection mechanism from droplets is one of the important questions in electrospray ionization methods. In this article, effects of solvent-solute interaction on the manifestation of the charge induced instability in a droplet are examined. We studied the instabilities in a prototype system of a droplet comprised of charged poly(ethylene glycol) and methanol, acetonitrile, and water solvents. We observed instances of three, previously only conjectured, [S. Consta, J. Phys. Chem. B 114, 5263 (2010)] mechanisms of macroion ejection. The mechanism of ejection of charged macroion in methanol is reminiscent of “pearl” model in polymer physics. In acetonitrile droplets, the instability manifests through formation of solvent spines around the solvated macroion. In water, we find that the macroion is ejected from the droplet through contiguous extrusion of a part of the chain. The difference in the morphology of the instabilities is attributed to the interplay between forces arising from the macroion solvation energy and the surface energy of the droplet interface. For the contiguous extrusion of a charged macromolecule from a droplet, we demonstrate that the proposed mechanism leads to ejection of the macromolecule from droplets with sizes well below the Rayleigh limit. The ejected macromolecule may hold charge significantly higher than that suggested by prevailing theories. The simulations reveal new mechanisms of macroion evaporation that differ from conventional charge residue model and ion evaporation mechanisms. (C) 2013 American Institute of Physics. []

[19] Styliani Consta and Anatoly Malevanets. Manifestations of Charge Induced Instability in Droplets Effected by Charged Macromolecules. PHYSICAL REVIEW LETTERS, 109(14), OCT 4 2012.

Ion-release processes from droplets that contain excess charge are of central importance in determining the charge-state distributions of macromolecules in electrospray ionization methods. We develop an analytical theory to describe the mechanism of contiguous extrusion of a charged macromolecule from a droplet. We find that the universal parameter determining the system behavior is the ratio of solvation energy per unit length to the square of the ion charge density per unit length. Systems with the same value of the ratio will follow the same path in the course of droplet evaporation. The analytical model is compared with molecular simulations of charged polyethylene glycol macroion in aqueous droplets, and the results are in excellent agreement.

[20] Jun Kyung Chung and Styliani Consta. Release Mechanisms of Poly(ethylene glycol) Macroions from Aqueous Charged Nanodroplets. JOURNAL OF PHYSICAL CHEMISTRY B, 116(19):5777--5785, MAY 17 2012.

Ion-release processes in nanodroplets that contain excess charge are of central importance in atmospheric aerosols as well as in determining the charge state distributions of macroions that are detected in electrospray mass spectrometry (ESMS) experiments. We performed molecular simulations of systems of a poly(ethylene glycol) (PEG) associated with various ions (Na+, Li+,Ca2+) in aqueous charged nanodroplets in order to investigate the manner that the macroion emerges from an aqueous nanodroplet as well as its final charge state. In the study we focused on a specific region of the parameter space with respect to charge and size of droplets that is close to the Rayleigh limit. We found that for sizes of droplets with linear dimensions of several nanometers and length of PEG up to 100 monomers, the PEG macroion emerges from the droplet following a three-step process: (i) phase separation, (ii) gradual extension of the macroion out of the droplet, and (iii) drying-out of the solvent or spontaneous detachment of the macroion from the droplet. The third step is determined by the ratio of charge on the macroion to the ions in the water portion of the droplet. The chemical transformation that is caused in PEG by the transfer of ions from the solvent into PEG determines its release mechanism. When the charge is carried by macroions, the charge-induced instability manifests by following one of the expected scenarios of Rayleigh instability; however, the assumptions of the Rayleigh model break down. We also examined the release of the macroion below the Rayleigh limit, and we found that the macroion emerges from the droplet by drying-out of the solvent. On balance of phenomenological evidence, we concluded that the ion-evaporation mechanism (IEM) in its most common meaning is not the followed mechanism for the parameter space of the systems that we studied. The final charge state of the macroion is in excellent agreement with the experimental data of Fenn and co-workers.

[21] M. Paliy, O. M. Braun, and S. Consta. Friction in a Thin Water Layer: Dissociative versus Nondissociative Friction. JOURNAL OF PHYSICAL CHEMISTRY C, 116(16):8932--8942, APR 26 2012.

An ultrathin water film confined between two substrates in moving contact is studied using Langevin molecular dynamics with a coordinate- and velocity-dependent damping coefficient. The water molecules are modeled with the Central-Force model that allows for the dissociation of water molecules into H+ and OH-. Two different friction scenarios are found depending on the applied pressure and the strength of the interaction of water with substrates. Under low loads, the water molecules stay intact during the frictional sliding. However, when the applied pressure increases past a value of similar to 20 GPa, the water molecules begin to dissociate and recombine immediately in the course of sliding, which results in a large increase of friction. The rate of such dissociation is found to be roughly proportional to the speed of driving. The relation of the observed phenomena to the “superionic” and “ionic fluid” states of water and its relevance in practical friction situations is discussed.

[22] Andrei Buin, Jianrong Ma, Yining Huang, Styliani Consta, and Zhang Hui. Conformational Changes of trans-1,2-Dichlorocyclohexane Adsorbed in Zeolites Studied by FT-Raman Spectroscopy and Molecular QM/MM Simulations. JOURNAL OF PHYSICAL CHEMISTRY C, 116(15):8608--8618, APR 19 2012.

The conformational behavior of trans-1,2-dichlorocyclohexane (T12D) adsorbed inside zeolites with Faujasite structure (FAU) including sodium Y (Na-Y) and siliceous Y (Si-Y) was investigated by FT-Raman spectroscopy and molecular simulations. The results have clearly shown that the conformational and dynamic properties of T12D strongly depend on the presence of charge-balancing cations as well as Si/Al ratio. Upon loading into Na-Y, the population of the diequatorial (ee) conformer increases compared with pure T12D liquid due to the strong interaction with the extra-framework Na+ ions. Molecular simulations of T12D in Na-Y and Si-Y have also been carried out. The T12D molecule was modeled by quantum and semiempirical quantum chemistry methods and embedded in a zeolite framework that was described by empirical force field. Conformational changes were sampled using quantum mechanics/molecular mechanics molecular dynamics and replica exchange molecular dynamics. Molecular simulations revealed that the ee conformer is preferable versus the diaxial (aa) conformer in both Na-Y and Si-Y frameworks. However, in the Na-Y supercage environment, the Na+ ions polarize the ee conformer stronger than in Si-Y. This leads to a larger shift of the conformational equilibrium in favor of the ee component in Na-Y relative to Si Y. Computations of the equilibrium population of aa and ee conformers using the dihedral CI-C-C-Cl angle distributions of aa and ee showed good quantitative agreement with the experimental findings with respect to the dominant ee conformer in both Na-Y and Si-Y.

[23] Doaa M. Ragab, Sohrab Rohani, and Styliani Consta. Controlled release of 5-fluorouracil and progesterone from magnetic nanoaggregates. INTERNATIONAL JOURNAL OF NANOMEDICINE, 7:3167--3189, 2012.

Background: The potential use of magnetic nanoparticles in biomedical applications has witnessed an exponential growth in recent years. Methods: In this study, we used nanoaggregates of magnetic nanoparticles as carriers for controlled drug delivery. The nanoaggregates are formed due to the presence of the block copolymer of polyethylene oxide-polypropylene oxide (Pluronic F-68) and beta-cyclodextrin that surround the magnetic core of the nanoparticles. The administration of the drug carriers occurs by inhalation, and the drug is delivered systemically via the pulmonary route. We tested the delivery of 5-fluorouracil and progesterone, which are used as models of hydrophilic and hydrophobic drugs, respectively. Results: The estimated nanoaggregates' diameters are between 293 nm +/- 14.65 nm and 90.2 nm +/- 4.51 nm, respectively. In-situ and post-synthesis techniques are two approaches for drug loading. The polymer composition of nanoaggregates and initial drug concentration showed a significant effect on both the drug entrapment efficiency and release kinetics. Average drug entrapment efficiencies ranged between 16.11% and 83.25%. In-situ loaded samples showed significantly slower release rates. The drug release mechanism is investigated by mathematical curve fitting to different drug release kinetics models. In most cases, the Peppas model has shown good correlations (coefficients of correlation, R-2, between 0.85 and 0.99) with the examined release profiles. The estimated release indices are below 0.5, which indicates the Fickian diffusion mechanism. For samples with an initial burst effect, the modified Peppas model can provide a better understanding of the drug release mechanism, both in the samples loaded with progesterone, or those high polymer concentrations. Conclusion: Our work showed prolonged delivery of drugs (5-fluorouracil and progesterone) by diffusion from nanoaggregates, with the potential to reduce dose-related adverse effects.

[24] Andrei Buin, Styliani Consta, and Tsun-Kong Sham. Mechanisms of Phase Transformations of TiO2 Nanotubes and Nanorods. JOURNAL OF PHYSICAL CHEMISTRY C, 115(45):22257--22264, NOV 17 2011.

Phase transformations of titanium dioxide (TiO2) nanotubes and nanorods at elevated temperatures are studied using molecular dynamics (MD) and replica exchange molecular dynamics (REMD) utilized here in the same way as simulated annealing. In the study, TiO2 nanotubes with amorphous (amT) and anatase structures (anT) as well as TiO2 amorphous nanorods (amR) that are amenable to experimental investigation are considered at various temperatures. It is found that amT and amR transform into a rutile rod, while anT transforms into a brookite nanotube. It is demonstrated that transformation of anT starts from TiO4 and TiO5 complexes found in the surfaces of the system, in contrast to amT and amR, where initial grains of the new phase may develop throughout the entire system starting from TiO5 and TiO6 complexes. The evolution of the number of TiOx (x = 4, 5, 6, 7) complexes indicates that the transformation of amT and amR occurs almost suddenly relative to the transformation of anT. The initial grains of transformation of amT have a structure close to rutile, while those of anT have brookite features. To our knowledge, we report the first simulations of phase transformations of TiO2 nanotubes and nanorods where simulations are performed beyond mu s.

[25] Styliani Consta and Jun Kyung Chung. Charge-Induced Conformational Changes of PEG-(Na-n(+)) in a Vacuum and Aqueous Nanodroplets. JOURNAL OF PHYSICAL CHEMISTRY B, 115(35):10447--10455, SEP 8 2011.

Charged-induced conformational changes of sodiated poly(ethylene glycol) (PEG-(Na+)(n)) in a vacuum and water droplets were studied using molecular simulations. In a vacuum, compact and partially unwound conformations were identified by analyzing occupation probabilities on reaction surfaces defined by the accessible surface area of the macroion, the distance between the centers of mass of the PEG molecule and of the cations and radius of gyration of the macromolecule. The critical charge of the macromolecule for which there is coexistence of various conformations was estimated using the Rayleigh criterion for the breakdown of highly charged droplets and compared with that observed in the simulations. The simulation findings agreed well with the Rayleigh prediction. The properties of sodiated PEG macromolecules in the presence of solvent and ions were also investigated. It was found that the macroion becomes saturated with charge. The highly charged state leads to an extended conformation that is partially expelled by the droplet. However, a portion of the chain is still in contact with the solvent. Drying-out of the solvent leaves the macroion in a stretched necklace-like conformation. The mechanism of release of sodiated PEG from an aqueous droplet was shown to be distinctly different from that of a protonated polyhistidine in a water nanodroplet, demonstrating sensitivity of the disintegration mechanism of the nanodroplet to the degree of hydrophilicity of the macroion.

[26] Yi Liu, Styliani Consta, and William A. Goddard, III. Nanoimmiscibility: Selective Absorption of Liquid Methanol-Water Mixtures in Carbon Nanotubes. JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 10(6, SI):3834--3843, JUN 2010.

Despite the continuing research interests in CNT-liquid systems, the microscopic structure and transport behavior of liquid mixtures in carbon nanotubes (CNTs) remain poorly understood. Methanol and water liquids are completely miscible across the entire range of concentration; however, recent research reveals that they are immiscible at a molecular level. In this work, we carried out classical molecular dynamics to study the molecular distribution, structure ordering, clustering and transport behavior of liquid methanol-water mixtures within CNT confinement. We found that CNTs preferentially absorbed methanol over water molecule even though the latter has a smaller molecular size, indicating that chemical effect such as molecular hydrophilicity plays a crucial role in the molecular absorption of CNTs. Due to the selective absorption of CNTs, methanol aqueous solution changes from microscopically immiscible to macroscopically immiscible at nanoscale. This nanoscale immiscibility may be utilized in various applications of CNTs including direct methanol fuel cells, nanosensors, molecular sieves, nanofluidic chips, and capsules for drug delivery.

[27] Styliani Consta. Manifestation of Rayleigh Instability in Droplets Containing Multiply Charged Macroions. JOURNAL OF PHYSICAL CHEMISTRY B, 114(16):5263--5268, APR 29 2010.

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.

[28] Yi Liu, Styliani Consta, Yujun Shi, R. H. Lipson, and William A. Goddard, III. Prediction of the Size Distributions of Methanol-Ethanol Clusters Detected in VUV Laser/Time-of-Flight Mass Spectrometry. JOURNAL OF PHYSICAL CHEMISTRY A, 113(25):6865--6875, JUN 25 2009.

The size distributions and geometries of vapor clusters equilibrated with methanol-ethanol (Me-Et) liquid mixtures were recently studied by vacuum ultraviolet (VUV) laser time-of-flight (TOF) mass spectrometry and density functional theory (DFT) calculations (Liu, Y.; Consta, S.; Ogeer, F.; Shi, Y. J.; Lipson, R. H. Can. J. Client. 2007, 85, 843-852). On the basis of the mass spectra recorded, it was concluded that the formation of neutral tetramers is particularly prominent. Here we develop grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) frameworks to compute cluster size distributions in vapor mixtures that allow a direct comparison with experimental mass spectra. Using the all-atom optimized potential for liquid simulations (OPLS-AA) force field, we systematically examined the neutral cluster size distributions as functions of pressure and temperature. These neutral cluster distributions were then used to derive ionized cluster distributions to compare directly with the experiments. The simulations suggest that supersaturation at 12 to 16 times the equilibrium vapor pressure at 298 K or supercooling at temperature 240 to 260 K at the equilibrium vapor pressure can lead to the relatively abundant tetramer population observed in the experiments. Our simulations capture the most distinct features observed in the experimental TOF mass spectra: Et3H+ at m/z = 139 in the vapor corresponding to 10:90% Me-Et liquid mixture and Me3H+ at m/z = 97 in the vapors corresponding to 50:50% and 90: 10% Me-Et liquid mixtures. The hybrid GCMC scheme developed in this work extends the capability of studying the size distributions of neat clusters to mixed species and provides a useful tool for studying environmentally important systems such as atmospheric aerosols.

[29] Y. Liu, S. Consta, F. Ogeer, Y. J. Shi, and R. H. Lipson. Geometries and energetics of methanol-ethanol clusters: a VUV laser/time-of-flight mass spectrometry and density functional theory study. CANADIAN JOURNAL OF CHEMISTRY-REVUE CANADIENNE DE CHIMIE, 85(10):843--852, OCT 2007.

Hydrogen-bonded clusters, formed above liquid methanol (Me) and ethanol (Et) mixtures of various compositions, were entrained in a supersonic jet and probed using 118 nm vacuum ultraviolet (VUV) laser single-photon ionization/time-of-flight mass spectrometry. The spectra are dominated by protonated cluster ions, formed by ionizing hydrogen-bonded MemEtn neutrals, m = 0-4, n = 0-3, and m + n = 2-5. The structures and energetics of the neutral and ionic species were investigated using both the all-atom optimized potential for liquid state, OPLS-AA, and the density functional (DFT) calculations. The energetic factors affecting the observed cluster distributions were examined. Calculations indicate that the large change in binding energy going from trimer to tetramer can be attributed more to pair-wise interactions than to cooperativity effects.

[30] Kengo Ichiki and Styliani Consta. Disintegration mechanisms of charged aqueous nanodroplets studied by simulations and analytical models. JOURNAL OF PHYSICAL CHEMISTRY B, 110(39):19168--19175, OCT 5 2006.

The mechanism of fragmentation processes in aqueous nanodroplets charged with ions is studied by molecular dynamics (MD) simulations. By using constant-temperature MD, the evaporation of the water is naturally taken into account and sequences of ion fragmentation events are observed. The size of the critical radius of the charged droplet just before the fragmentation and the distribution of the sizes of the fragments are estimated. Comparison of the Rayleigh critical radius for fragmentation and simulation data is within 0.23 nm. This seemingly small difference arises from a large difference in the number of water molecules that makes fragmentation an activated process as in the ion evaporation mechanism (IEM). This finding is in agreement with the predictions of Labowsky et al. [Anal. Chim. Acta 2000, 406, 105-118] for charged aqueous drops. The size of the daughter droplets is larger than the prediction of Born's theory by 0.1 to 0.15 nm. The nature and the dynamics of the intermediate states of the fragmentation process characterized by a bridge formed between the mother droplet and the evaporating ion or thorned structures where the ion sits on the tip are important for the outcome of the size-distribution of the fragments, while they are is missing in Born's theory.

[31] Styliani Consta. Detecting reaction pathways and computing reaction rates in condensed phase. THEORETICAL CHEMISTRY ACCOUNTS, 116(1-3):373--382, AUG 2006.

Methods for the computation of rate constants that characterize classical reactions occurring in the condensed phase are discussed. While microscopic expressions for these transport properties are well known, their computation presents challenges for simulation since reactive events often occur rarely, and the long time scales that are typical for reactive processes are not accessible using simple molecular dynamics methods. Furthermore, the underlying free energy surface is very complex with many saddle points that prevent sampling of possible reaction pathways. As a result, the reaction coordinate may be a complex many-body function of the system's degrees of freedom. Since there is not an a priori way to define a “good” reaction coordinate, methods are being developed to assist in a systematic construction of a reaction coordinate. These methods are reviewed and examples of non-trivial reaction coordinates are presented.

[32] M. Paliy, O. M. Braun, and S. Consta. The friction properties of an ultrathin confined water film. TRIBOLOGY LETTERS, 23(1):7--14, JUL 2006.

An ultrathin water film confined between two substrates in moving contact is studied using Langevin molecular dynamics with coordinate- and velocity-dependent damping coefficient. The correlations between the structure of the water lubricant film and its frictional properties are found in a wide range of applied loads and for various strengths of interactions between water and surfaces. A self-organization of the film into a low-friction state under driving is observed to occur. Surprisingly, it is found that the “hydrophilic” surfaces exhibit a lower friction than the “hydrophobic” ones. The viscosity of the confined water estimated from the simulations is in a good agreement with the experimental value.

[33] S. Consta. Detecting reaction pathways and computing reaction rates in condensed phase. In Simos, T and Maroulis, G, editor, ADVANCES IN COMPUTATIONAL METHODS IN SCIENCES AND ENGINEERING 2005, VOLS 4 A & 4 B, volume 4A-4B of LECTURE SERIES ON COMPUTER AND COMPUTATIONAL SCIENCES, pages 709--712. Amer Chem Soc; Amer Phys Soc, 2005. International Conference on Computational Methods in Sciences and Engineering (ICCMSE 2005), Corinth, GREECE, OCT 21-26, 2005.

In this contribution, methods for the computation of rate constants that characterize classical reactions occurring in the condensed phase axe discussed. Methods of transition path sampling and minimal action are reviewed and their adaption in the study on physical fragmentation of charged aqueous clusters is discussed.

[34] OM Braun, M Paliy, and S Consta. Ordering of a thin lubricant film due to sliding. PHYSICAL REVIEW LETTERS, 92(25), JUN 25 2004.

A thin lubricant film confined between two substrates in moving contact is studied using Langevin molecular dynamics with the coordinate- and velocity-dependent damping coefficient. It is shown that an optimal choice of the interaction within the lubricant can lead to minimal kinetic friction as well as to low critical velocity of the stick-slip to smooth-sliding transition. The strength of this interaction should be high enough (relative to the strength of the interaction of lubricant atoms with the substrates) so that the lubricant remains in a solid state during sliding. At the same time, the strength of the interaction should not be too high, in order to allow annealing of defects in the lubricant at slips.

[35] S Consta, KR Mainer, and W Novak. Fragmentation mechanisms of aqueous clusters charged with ions. JOURNAL OF CHEMICAL PHYSICS, 119(19):10125--10132, NOV 15 2003.

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. (C) 2003 American Institute of Physics.

[36] GS Fanourgakis, YJ Shi, S Consta, and RH Lipson. A spectroscopic and computer simulation study of butanol vapors. JOURNAL OF CHEMICAL PHYSICS, 119(13):6597--6608, OCT 1 2003.

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)(+), nless than or equal to3; 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 assess the relative stability of the isomeric tetramers. Together, these experimental and theoretical results suggest that butanol tetramers are “magic-number” structures, and that the protonated ion signals of size n could be correlated with the neutral cluster of size n+1, provided the vapor pressures sampled in the supersonic jet exceeded equilibrium values. (C) 2003 American Institute of Physics.

[37] S Consta. Fragmentation reactions of charged aqueous clusters. JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, 591:131--140, AUG 30 2002.

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. (C) 2002 Elsevier Science B.V. All rights reserved.

[38] YJ Shi, S Consta, AK Das, B Mallik, D Lacey, and RH Lipson. A 118 nm vacuum ultraviolet laser/time-of-flight mass spectroscopic study of methanol and ethanol clusters in the vapor phase. JOURNAL OF CHEMICAL PHYSICS, 116(16):6990--6999, APR 22 2002.

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 pressuresgreater than or equal tothe equilibrium vapor pressure were computed using the grand canonical Monte Carlo and molecular dynamics techniques. Lastly, differences between these experiments and the results of bimolecular reaction studies are discussed. (C) 2002 American Institute of Physics.

[39] S Consta, TJH Vlugt, JW Hoeth, B Smit, and D Frenkel. Recoil growth algorithm for chain molecules with continuous interactions. MOLECULAR PHYSICS, 97(12):1243--1254, DEC 20 1999.

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.

[40] S Consta and R Kapral. Ionization reactions of ion complexes in mesoscopic water clusters. JOURNAL OF CHEMICAL PHYSICS, 111(22):10183--10191, DEC 8 1999.

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. (C) 1999 American Institute of Physics. [S0021-9606(99)51941-7].

[41] S Consta, NB Wilding, D Frenkel, and Z Alexandrowicz. Recoil growth: An efficient simulation method for multi-polymer systems. JOURNAL OF CHEMICAL PHYSICS, 110(6):3220--3228, FEB 8 1999.

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. (C) 1999 American Institute of Physics. [S0021-9606(99)51406-2].

[42] R Kapral, S Consta, and D Laria. 1996 Polanyi Award Lecture Proton reactions in clusters. CANADIAN JOURNAL OF CHEMISTRY-REVUE CANADIENNE DE CHIMIE, 75(1):1--8, JAN 1997. 79th Annual Conference of the Canadian-Society-for-Chemistry, ST JOHNS, CANADA, JUN 23-26, 1996.

Reactions in mesoscopic, molecular clusters may proceed by mechanisms and with rates that differ from those in bulk solvents. Two examples of reactions in large, liquid-state, molecular clusters are described to illustrate the distinctive features of these reactions: acid dissociation and proton transfer in aprotic, polar solvents. Both of these reactions involve proton dynamics so methods for dealing with mixed quantum-classical systems must be utilized to investigate the reaction dynamics. Surface versus bulk solvation effects play an important role in determining the reaction mechanisms as do the strong cluster fluctuations. Mechanisms for proton transfer within clusters that have no bulk analogs will be described.

[43] S Consta and R Kapral. Dynamics of proton transfer in mesoscopic clusters. JOURNAL OF CHEMICAL PHYSICS, 104(12):4581--4590, MAR 22 1996.

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 nonadiabatic 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. (C) 1996 American Institute of Physics.


Configuration interaction calculations have been carried out on the ground and ten excited states of XeH. The results are in agreement with the existing experimental data on the Rydberg spectra of this system. In addition, secondary minima are found in the first two excited states (A 2-SIGMA+ and B'2 PI) at internuclear distance of 5.0 bohr, where the two states have charge-transfer character, in agreement with recent experimental findings. The present calculations are not of sufficient accuracy to reproduce the Van der Waals minimum of the ground state.