Features:

PLOTTING PORES WITH SIMULATIONS, CLUSTERS
by Trish Barker, NCSA Public Information Specialist

Gels can be found low -- nestling amid the packing material for consumer electronics and leather goods to absorb damaging moisture -- and high -- traveling through space to capture comet dust.

The pebbles of gel in your shoebox and comet-trailing aerogel, the lightest man-made material, share a common origin in the sol-gel process, a versatile technique for making ceramic and glass materials. In general, the method involves the transition of a system from a liquid filled with floating "sol" particles into a solid, but still highly porous, "gel" phase.

Scientists understand how to make a range of materials using this process -- from thin film coatings to the aerogel sent on a comet quest by NASA. What scientists lack is a detailed, microscopic portrait of the complex interior structure of a given porous material. That's one of the questions being tackled by Lev Gelb, an assistant professor of chemistry at Washington University in St. Louis (http://www.chemistry.wustl.edu/~gelb/). Gelb uses computational resources at NCSA to model the spongelike structure of sol-gel materials. Detailed models of the structure of these materials could be used to explore how gels interact and what tasks different gels can perform. A thorough understanding of gel formation might also make it possible to engineer materials for specific applications.

Creating complex structures

The sol-gel process begins with an aqueous solution of an organosilicon "precursor" compound. The precursor molecules react with each other, connecting to form small particles and then sticking together in larger clumps. This is the "sol" stage, in which solid particles are suspended in the liquid solvent.

What's forming in the solution is "a very complicated branched polymer with many loop structures," Gelb explains. As these polymers continue to interlock and adhere to one another, a liquid that once contained solid pieces becomes a gelatin-like solid riddled with tiny pores.

The structure of the resulting gel "depends pretty sensitively on exactly the solution you start with" and on the temperature, Gelb says, and each gel has a particular "average pore size." The pore size of a material influences the rate at which fluids flow in it, its ability to separate molecules based on their size, and its electrical, optical, and mechanical properties.

The sol-gel process can produce dense silica xerogels that are used in chromatography and studies of gas separations; highly porous aerogels that are used for thermal insulation; and thin films for use in sensors, electronics, and optics.

In all of these cases, "the final material is amorphous," Gelb says. "The material does not have a simple periodic crystal structure." In other words, the sizes, shapes, and distribution of the pores vary unpredictably.

Simulating the sol-gel process

To understand the complex structure of these materials, Gelb has turned to computational simulation. His strategy is to simulate each of the steps involved in the experimental preparation of the materials in order to model the resulting structure. Gelb is approaching the problem in two ways. First, his team is developing new models for atomic-level simulations of sol-gel reactions, applying these models, and then comparing the results to structures known from experiment. Secondly, the team is also developing and applying a coarse-grained sol-gel model that is less computationally expensive.

Employing the widely used quantum chemistry codes Gaussian and NWChem on NCSA's IBM p690 cluster, Gelb models the evolution of sol-gel systems using a parallel molecular dynamics code developed by his group.

Gelb has carried out large-scale simulations of the polymerization of silicic acid in aqueous solution, modeling systems with differing water-to-silicon ratios, silicic acid concentrations, and temperatures. Activation energies calculated from the simulations compared favorably with experimental results, encouraging Gelb to attempt to model more complex systems.

These simulations involve several tens of thousands of individual atoms and millions of femtosecond intervals and require thousands of hours of computation. It can take months to simulate mere nanoseconds.

"The problem is that atoms move very quickly and the forces they exert on each other are very strong," Gelb says. This makes simulating materials at the level of individual atoms very computationally expensive.

Gelb is also developing a coarse-grained model that will track sol particles rather than individual atoms and will represent the solvent with a density field rather than considering each solvent molecule individually. While this method relies on approximations that make it somewhat less detailed than the atom-by-atom approach, it has the benefit of being less computationally intensive.

Capillary condensation

Gelb is also looking at the pores within sol-gel materials from a slightly different perspective, one that considers the relationship between pore structure and adsorptive properties.

Adsorption is employed in many separation and refinement techniques, as well as in chromatography, so understanding how pore structure relates to the adsorptive qualities of a material provides an opportunity to improve applications that use sol-gel materials.

The first step is to understand the distribution of pore sizes in a given material. Scientists typically analyze pore size distribution by leveraging the phenomenon of capillary condensation.

Imagine a closed system containing a very thin tube and a vapor. If the pressure on the system is increased toward the vapor pressure of the liquid (at which point the vapor will condense into a liquid), the vapor will condense in the tube at a lower pressure than it will condense elsewhere. The pressure that is required to convert the vapor in the tube to a liquid is determined by the radius of the tube. This means that the radius of a tube (or pore) can be determined by observing at what pressure a vapor condenses within the tube.

The problem with this technique is that sol-gel materials are much more complex than tiny, isolated tubes. Using this method requires scientists to assume that the pores are regular in size, which they aren't, and that what occurs in one pore has no affect on its neighbors.

"So how descriptive is this measure?" Gelb asks. Instead of really describing the material in question, the use of capillary condensation gives scientists insight into the characteristics of a bunch of hypothetical cylinders.

Gelb's goal is to develop realistic models of the structures of various materials and then to use those structures in molecular dynamics simulations of gas adsorption in these materials. Through simulation, Gelb intends to produce a pore-by-pore view of adsorption; it would be difficult, perhaps impossible, to produce this view experimentally.

Furthermore, by working with computational models, Gelb has the flexibility to make adjustments in structure and observe how these changes affect adsorptive properties. This could lead to identifying and engineering sol-gel materials with properties uniquely suited to a wide range of uses, from earthly concerns such as chromatography and packaging to chasing comets.