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Valeria Molinero

Bio: Valeria Molinero is an academic researcher from University of Utah. The author has contributed to research in topics: Nucleation & Ice nucleus. The author has an hindex of 54, co-authored 135 publications receiving 9727 citations. Previous affiliations of Valeria Molinero include Facultad de Ciencias Exactas y Naturales & University of Buenos Aires.


Papers
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Journal ArticleDOI
TL;DR: mW mimics the hydrogen-bonded structure of water through the introduction of a nonbond angular dependent term that encourages tetrahedral configurations, and concludes that it is not the nature of the interactions but the connectivity of the molecules that determines the structural and thermodynamic behavior of water.
Abstract: Water and silicon are chemically dissimilar substances with common physical properties. Their liquids display a temperature of maximum density, increased diffusivity on compression, and they form tetrahedral crystals and tetrahedral amorphous phases. The common feature to water, silicon, and carbon is the formation of tetrahedrally coordinated units. We exploit these similarities to develop a coarse-grained model of water (mW) that is essentially an atom with tetrahedrality intermediate between carbon and silicon. mW mimics the hydrogen-bonded structure of water through the introduction of a nonbond angular dependent term that encourages tetrahedral configurations. The model departs from the prevailing paradigm in water modeling: the use of long-ranged forces (electrostatics) to produce short-ranged (hydrogen-bonded) structure. mW has only short-range interactions yet it reproduces the energetics, density and structure of liquid water, and its anomalies and phase transitions with comparable or better accu...

816 citations

Journal ArticleDOI
24 Nov 2011-Nature
TL;DR: Moore and Molinero as discussed by the authors showed that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water explains its anomalous thermodynamics and also controls the rate and mechanism of ice formation.
Abstract: The various anomalous properties of water have puzzled scientists for decades, and many hypotheses have been put forward to explain their origin. One mystery is the question of what determines the lowest temperature to which water can be cooled before it freezes to ice. Rapid crystallization at low temperatures hampers experimental studies, and simulations are usually prohibitively costly in terms of computer time. Using a simple water model that allows demanding calculations, Emily Moore and Valeria Molinero now show that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water controls the rate and mechanism of ice formation. The structural change also results in a peak in the rate of crystallization at 225 K; below this temperature, ice nuclei form faster than liquid water can equilibrate. This finding explains the observed thermodynamic anomalies, and why homogeneous ice nucleation rates depend on the thermodynamics of water. One of water’s unsolved puzzles is the question of what determines the lowest temperature to which it can be cooled before freezing to ice. The supercooled liquid has been probed experimentally to near the homogeneous nucleation temperature, TH ≈ 232 K, yet the mechanism of ice crystallization—including the size and structure of critical nuclei—has not yet been resolved. The heat capacity and compressibility of liquid water anomalously increase on moving into the supercooled region, according to power laws that would diverge (that is, approach infinity) at ∼225 K (refs 1, 2), so there may be a link between water’s thermodynamic anomalies and the crystallization rate of ice. But probing this link is challenging because fast crystallization prevents experimental studies of the liquid below TH. And although atomistic studies have captured water crystallization3, high computational costs have so far prevented an assessment of the rates and mechanism involved. Here we report coarse-grained molecular simulations with the mW water model4 in the supercooled regime around TH which reveal that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water explains its anomalous thermodynamics and also controls the rate and mechanisms of ice formation. The results of the simulations and classical nucleation theory using experimental data suggest that the crystallization rate of water reaches a maximum around 225 K, below which ice nuclei form faster than liquid water can equilibrate. This implies a lower limit of metastability of liquid water just below TH and well above its glass transition temperature, 136 K. By establishing a relationship between the structural transformation in liquid water and its anomalous thermodynamics and crystallization rate, our findings also provide mechanistic insight into the observed5 dependence of homogeneous ice nucleation rates on the thermodynamics of water.

608 citations

Posted Content
TL;DR: In this article, a coarse-grained model of water (mW) was developed, which is essentially an atom with tetrahedrality intermediate between carbon and silicon, and mimics the hydrogen-bonded structure of water through the introduction of a nonbond angular dependent term.
Abstract: Water and silicon are chemically dissimilar substances with common physical properties. Their liquids display a temperature of maximum density, increased diffusivity on compression, they form tetrahedral crystals and tetrahedral amorphous phases. The common feature to water, silicon and carbon is the formation of tetrahedrally coordinated units. We exploit these similarities to develop a coarse-grained model of water (mW) that is essentially an atom with tetrahedrality intermediate between carbon and silicon. mW mimics the hydrogen-bonded structure of water through the introduction of a nonbond angular dependent term that encourages tetrahedral configurations. The model departs from the prevailing paradigm in water modeling: the use of long-ranged forces (electrostatics) to produce short-ranged (hydrogen-bonded) structure. mW has only short-range interactions yet it reproduces the energetics, density and structure of liquid water, its anomalies and phase transitions with comparable or better accuracy than the most popular atomistic models of water, at less than 1% of the computational cost. We conclude that it is not the nature of the interactions but the connectivity of the molecules that determines the structural and thermodynamic behavior of water. The speedup in computing time provided by mW makes it particularly useful for the study of slow processes in deeply supercooled water, the mechanism of ice nucleation, wetting-drying transitions, and as a realistic water model for coarse-grained simulations of biomolecules and complex materials.

562 citations

Journal ArticleDOI
TL;DR: The properties of hydrated Nafion polyelectrolyte are attributed to its nanophase-segregre... as mentioned in this paper, which is widely used in polymer electrolyte membrane fuel cells due to its high proton conductivity.
Abstract: Nafion polyelectrolyte is widely used in polymer electrolyte membrane fuel cells (PEMFC) due to its high proton conductivity. The properties of hydrated Nafion are attributed to its nanophase-segre...

417 citations

Journal ArticleDOI
TL;DR: The "blob mechanism" unveiled in this work synthesizes elements of the labile cluster and local structuring hypotheses of clathrate nucleation and bears strong analogies to the two-step mechanisms of crystallization of proteins and colloids.
Abstract: The nucleation and growth of clathrate hydrates of a hydrophobic guest comparable to methane or carbon dioxide are studied by molecular dynamics simulations of two-phase systems. The crystallization proceeds in two steps: First, the guest molecules concentrate in "blobs", amorphous clusters involving multiple guest molecules in water-mediated configurations. These blobs are in dynamic equilibrium with the dilute solution and give birth to the clathrate cages that eventually transform it into an amorphous clathrate nucleus. In a second step, the amorphous clathrate transforms into crystalline clathrate. At low temperatures, the system can be arrested in the metastable amorphous clathrate phase for times sufficiently long for it to appear as an intermediate in the crystallization of clathrates. The "blob mechanism" unveiled in this work synthesizes elements of the labile cluster and local structuring hypotheses of clathrate nucleation and bears strong analogies to the two-step mechanisms of crystallization of proteins and colloids.

377 citations


Cited by
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01 May 1993
TL;DR: Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems.
Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

29,323 citations

Journal ArticleDOI
TL;DR: Light scattering experiments revealed that the radius of gyration had a linear dependence on the molar mass of the aggregates, which suggests that the particles are in the form of rods or ribbons, or at least some elongated structure.
Abstract: Equivalent weight (EW) is the number of grams of dry Nafion per mole of sulfonic acid groups when the material is in the acid form. This is an average EW in the sense that the comonomer sequence distribution (that is usually unknown to the investigator and largely unreported) gives a distribution in m in this formula. EW can be ascertained by acid-base titration, by analysis of atomic sulfur, and by FT-IR spectroscopy. The relationship between EW and m is EW ) 100m + 446 so that, for example, the side chains are separated by around 14 CF2 units in a membrane of 1100 EW. Common at the time of this writing are Nafion 117 films. The designation “117” refers to a film having 1100 EW and a nominal thickness of 0.007 in., although 115 and 112 films have also been available. Early-reported studies involved 1200 EW samples as well as special experimental varieties, some being rather thin. The equivalent weight is related to the property more often seen in the field of conventional ion exchange resins, namely the ion exchange capacity (IEC), by the equation IEC ) 1000/EW. The mention of the molecular weight of high equivalent weight (EW > 1000 g‚mol-1) Nafion is almost absent in the literature, although the range 105-106 Da has been mentioned. As this polymer does not form true solutions, the common methods of light scattering and gel permeation chromatography cannot be used to determine molecular weight as well as the size and shape of isolated, truly dissolved molecules. Studies of the structure of this polymer in solvent (albeit not a true solution) will be mentioned in the scattering section of this review. It should be noted that Curtin et al. performed size exclusion chromatography determinations of the molecular weight distribution in Nafion aqueous dispersions after they were heated to high temperatures (230, 250, and 270 °C).1 Before heating, there was a high molecular weight shoulder on a bimodal distribution, due to molecular aggregates, but this shoulder disappeared upon heating, which indicated that the aggregates were disrupted. The peaks for the monomodal distribution for the heated samples were all located at molecular weights slightly higher than 105 g‚mol-1. Also, light scattering experiments revealed that the radius of gyration had a linear dependence on the molar mass of the aggregates, which suggests that the particles are in the form of rods or ribbons, or at least some elongated structure. Nafion ionomers are usually derived from the thermoplastic -SO2F precursor form that can be extruded into sheets of required thickness. Strong interactions between the ionic groups are an obstacle to melt processing. This precursor does not possess the clustered morphology that will be of great concern in this article but does possess Teflon-like crystallinity which persists when the sulfonyl fluoride form is converted to, for example, the K+ form by reacting it with KOH in water and DMSO. Thereafter, the -SO3H form is achieved by soaking the film in a sufficiently concentrated aqueous acid solution. Extrusion of the sulfonyl fluoride precursor can cause microstructural orientation in the machine direction, * Address correspondence to either author. Phone: 601-266-5595/ 4480. Fax: 601-266-5635. E-mail: Kenneth.Mauritz@usm.edu; RBMoore@usm.edu. 4535 Chem. Rev. 2004, 104, 4535−4585

4,130 citations

Journal ArticleDOI
TL;DR: Atomistic modeling with ReaxFF provides a useful method for determining the initial events of oxidation of hydrocarbons under extreme conditions and can enhance existing combustion models.
Abstract: To investigate the initial chemical events associated with high-temperature gas-phase oxidation of hydrocarbons, we have expanded the ReaxFF reactive force field training set to include additional transition states and chemical reactivity of systems relevant to these reactions and optimized the force field parameters against a quantum mechanics (QM)-based training set. To validate the ReaxFF potential obtained after parameter optimization, we performed a range of NVT−MD simulations on various hydrocarbon/O2 systems. From simulations on methane/O2, o-xylene/O2, propene/O2, and benzene/O2 mixtures, we found that ReaxFF obtains the correct reactivity trend (propene > o-xylene > methane > benzene), following the trend in the C−H bond strength in these hydrocarbons. We also tracked in detail the reactions during a complete oxidation of isolated methane, propene, and o-xylene to a CO/CO2/H2O mixture and found that the pathways predicted by ReaxFF are in agreement with chemical intuition and our QM results. We o...

1,815 citations

Journal ArticleDOI
TL;DR: Simulations for various other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data, and a recently introduced algorithm can explain important features of Nafeon, including fast diffusion of water and protons through Nafions and its persistence at low temperatures.
Abstract: The structure of the Nafion ionomer used in proton-exchange membranes of H(2)/O(2) fuel cells has long been contentious. Using a recently introduced algorithm, we have quantitatively simulated previously published small-angle scattering data of hydrated Nafion. The characteristic 'ionomer peak' arises from long parallel but otherwise randomly packed water channels surrounded by partially hydrophilic side branches, forming inverted-micelle cylinders. At 20 vol% water, the water channels have diameters of between 1.8 and 3.5 nm, with an average of 2.4 nm. Nafion crystallites (approximately 10 vol%), which form physical crosslinks that are crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross-sections of approximately (5 nm)(2). Simulations for various other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data. The new model can explain important features of Nafion, including fast diffusion of water and protons through Nafion and its persistence at low temperatures.

1,239 citations

Journal ArticleDOI
TL;DR: The structural origin of chirality in different supramolecular structures through combinations of structural analysis methods has been investigated in this article, where the most ideal building blocks would need to display shape persistence in solution and in the solid state, since only this feature provides access to the use of complementary methods of structural analyses.
Abstract: Dendron-mediated self-assembly, disassembly, and self-organization of complex systems have been investigated. The most ideal building blocks would need to display shape persistence in solution and in the solid state, since only this feature provides access to the use of complementary methods of structural analysis. Most supramolecular dendrimers are chiral even when they are constructed from nonchiral building blocks and are equipped with mechanisms that amplify chirality. This poses additional challenges associated with the understanding of the structural origin of chirality in different supramolecular structures through combinations of structural analysis methods. While many supramolecular structures assembled from dendrimers and dendrons resemble some of the related morphologies generated from block-copolymers, they are much more complex and are not determined by the volume ratio between the dissimilar parts of the molecule.

1,061 citations