Directory: Faculty

Valeria Molinero PHYSICAL & MATERIALS CHEMISTRY Assistant Professor Ph.D. University of Buenos Aires, 1999. Postdoctoral Researcher, California Institute of Technology, 2000-2003. Associate Researcher, Arizona State University, 2005-2006. Associate Scientist, California Institute of Technology, 2003-2006. Phone: (801) 585-9618 Office: 324 INSCC Email: valeria.molinero@utah.edu Publications

Activities & Awards

• Beckman Young Investigator Award, 2009
• Helmholtz Award, International Association for the Properties of Water and Steam, 2005

Research Interests

We use computer simulations and statistical mechanics methods to investigate the interplay between microscopic structure, dynamics and phase transformations in disordered materials. The systems of interest encompass from monatomic liquids to complex nanostructured polyelectrolyte membranes.

Molecular transport in membranes and glasses. Understanding the origins of diffusion within amorphous solids is central to development of several technologies: i) Biopreservation and drug storage depend on controlling the high mobility that water, ions and oxygen tend to have in the carbohydrate glasses used as preservation media, ii) Fuel cells depend on the ability of ions to migrate across a glassy or rubbery medium, iii) Matrix-decoupled diffusion is key to the performance of ionic glasses as electrolytes in electrochemical solid-state batteries. The commonality to these examples is the existence of small species (ions, molecules) whose diffusivity is several orders of magnitude higher than predicted from the viscosity of the material.

A goal of our research is to find the common factors that control decoupled diffusion of ions, water and other small molecules in amorphous solids with variable structural complexities. We address questions ranging from the role of the matrix in assisting the mobility, to the design of fuel cell membranes and the study of water, gas, and ion diffusion within them. We explore the relationship between the microscopic distribution of water, the relaxation times of the matrix, the type of interactions (and how much can they be simplified), and water transport in systems where water is fully miscible (e.g. carbohydrates, hydrogels) and segregated into a distinct nanophase (e.g. hydrophobic polyelectrolytes, nanostructured solids).

Figure, left. Water diffuses decoupled from matrix translation in a glucan at a temperature 40% above the glass transition. For clarity, water and carbohydrates are shown in different panels, both colored at t=0.

Glass formation vs crystallization in water-like liquids. Several liquids can be easily cooled below their melting point, forming a supercooled liquid and, on further cooling, a glass. Water is not one of those liquids: Bulk water is a bad glass former and easily crystallizes under normal cooling rates. Nevertheless water dispersed in nanoscopic dimensions and confined in the matrix of several materials can avoid the thermodynamic fate of ice formation and remain liquid-like well below the melting point.

We study the factors that determine the crystallization of water and other tetrahedral liquids, such as silicon and germanium. These tetrahedral liquids present polyamorphism -two different amorphous phases of different density- and have similar shape of the phase diagram.

The study of monatomic models of tetrahedral liquids derived from a silicon potential (Molinero et al. Phys. Rev. Lett. 97 (2006) 075701) suggests that the glassforming conditions can be predicted from the thermodynamic phase diagram, and points to the central role of the polyamorphic transformation in the pathway to crystallization.

The state of water in complex materials. Water is ubiquitous in biological and synthetic materials, where is often confined by a matrix. A goal of our research is to understand how the structure of the material and intermolecular interactions determine the distribution of water and the stability and metastability of water’s liquid state.

We focus on understanding disordered systems with soft confinement, where the matrix is a rubbery polymer or a viscous supercooled liquid. A central quest of our research is to find structural indicators of glassforming ability for water that can be used to characterize water metastability in a variety of environments without doing costly nucleation studies for each new material.

Hydrated Nafion in a multi-scale heterogenous material
Coarse Grained Modeling	Atomistic & Coarse Grained Modeling	Atomistic Simulations

Figure above. Zoom on the nanostructure of hydrated Nafion, the state of the art fuel cell membrane. The description of the structure and its relaxation requires multi-scale modeling techniques.

Multiscale modeling. A significant part of our research is directed to the development of new atomistic and coarse grained models of complex systems.

Parallel computing and the continuous increase in computer power enables the simulation of big periodic systems, composed by hundreds of thousand of atoms. But time can not be parallelized, and slow processes still demand extremely long simulations. To expand the simulation times, we use a multiscale approach in which we use atomistic simulations to develop coarser models of the same system that are ~10,000 faster than the atomistic ones. This method have proved very successful for the study of the microsecond scale relaxation in carbohydrate glasses (Molinero et al, Phys. Rev. Lett., 95 (2005) 045701) and we are now expanding it to model new molecular and polymeric materials.

 

Selected Publications

• "Water modeled as an intermediate element between carbon and silicon”, E. B. Moore and V. Molinero, J. Phys. Chem. B 113 (2009) 4008-4016
• "Vitrification of a monatomic metallic liquid”, M. H. Bhat, V. Molinero, E. Soignard, V. C. Solomon, S. Sastry, J. L Yarger, and C. A. Angell. Nature 448 (2007) 787-790
• "Tuning of tetrahedrality in a silicon potential yields a series of monatomic (metal-like) glassformers of very high fragility", V. Molinero, S. Sastry, and C. A. Angell. Phys. Rev. Lett., 97 (2006) 075701.
• "Multi-paradigm multi-scale simulations for fuel cell catalysts and membranes" W. Goddard III, B. Merinov, A. van Duin, T. Jacib, M. Blanco, V. Molinero, S.S. Jang And Y.H. Jang.  Molecular Simulations, 32 (2006) 251–268.
• "Molecular modeling of carbohydrates with no charges, no hydrogen bonds and no atoms" V. Molinero, and W.A. Goddard III.ACS Symposium Series No. 930, NMR Spectroscopy and Computer Modeling of Carbohydrates, J.F.G. Vliegenthart and R. Woods Ed., ACS, Washington DC, 2006.
• "Microscopic mechanism of water diffusion in glucose glasses" V. Molinero, and W.A. Goddard III.Phys. Rev. Lett., 95 (2005) 045701.
• "Fluorinated imidazoles as proton carriers for water-free fuel cell membranes" W. Deng, V. Molinero, and W.A. Goddard III. J. Am. Chem. Soc., 2004, 126, 15644-15645.
• "The mechanisms of nonexponential relaxations in supercooled glucose solutions: the role of water facilitation" V. Molinero, T. Cagin and W.A. Goddard III. J. Phys. Chem. A 2004, 108, 3699 – 3712.
• "Nanophase-segregation and transport in Nafion 117 from molecular dynamics simulations: effect of monomeric sequence" S. S. Jang, V. Molinero, T. Cagin, and W.A. Goddard III. J. Phys. Chem. B 2004, 108, 3149-3157.
• "M3B: a coarse grain force field for the simulation of oligosaccharides and their water mixtures" V. Molinero, and W.A. Goddard III. J. Phys. Chem. B 2004, 108, 1414-1427.
• "Sugar, water and free-volume networks in concentrated sucrose solutions".V. Molinero, T. Cagin, and W.A. Goddard III. Chem. Phys. Lett. 2003, 377, 469-474.
• "Dynamics of solvation-induced structural transitions in mesoscopic binary clusters". V. Molinero, D. Laria, and R. Kapral.  Phys. Rev. Lett. 2000, 84, 455-458.
• "Electrostatic interactions at self assembled molecular films of charged thiols on gold". V. Molinero, and E. J. Calvo. J. Electroanal. Chem. 1998, 17, 17-25.

Patents

U.S. patent US60578034, "Fluorinated imidazoles as proton carriers for water-free proton exchange membranes for fuel cells". Inventors: W.A. Goddard III, W. Deng, and V. Molinero.