is particularly interested in accurately depicting the behavior of organic molecules whether they’re in their fluid or vapor states of matter. And in the process of pinning these slippery states down, she’s come up with new mathematical theories for other engineers and chemists to use in designing chemical processes that use hydrocarbons.

(As in “oil” and “polymers.” Pretty significant players in the industrial world, not to mention being the building blocks for other organic compounds.) 

McCabe is a key player herself, leading a team of researchers at Vanderbilt involved in refining computational molecular modeling techniques. She and others are working to better understand molecular behavior and to more accurately predict how large numbers of molecules will react to each other. 

You might think that these sorts of chemical interactions had been figured out when quantum mechanics was invented, and now it’s just a question of plugging in the numbers.  

Not so. True, physics can tell us what a handful of atoms might do together, but when you throw in the hundreds of molecules interacting at the nanoscale, quantum mechanics’ first principles computation becomes very difficult. (Nanoscale refers to quantities of matter the size of a billionth of a meter or smaller; nanotechnology refers to fabrication of devices no larger than the size of 100 nanometers.) 

Enter computer simulation, which uses accurate approximations to make the interactions between the molecules simple enough to work with. Computer simulation of molecular behavior is one of the major driving forces behind the nanotechnology explosion. Without these techniques, the new materials and devices may not be economical or feasible to produce, because in some cases the testing costs alone would be prohibitive. 

“Experimental measurements can be very costly and time consuming,” McCabe said. “Computer modeling and simulation are proving to be attractive and valuable means with which to fill in the gaps in experimental literature and obtain important information.” 

Computational molecular modeling has additional intuitive appeal because the visualization of the results from a molecular simulation can show how molecules interact. Understanding the microscopic basis for various properties is very valuable to engineers, because they can use this understanding to streamline the design of chemical processes and make these processes more manageable.  

After working with other researchers on refining the Statistical Associating Fluid Theory, a molecular-based equation of state, that describes the relationship between pressure, density, and temperature in fluid and vapor phases, she began to look at how to correct the theory to describe the molecular behavior evidenced by more complex systems. In particular, she is using the theory to understand new environmentally friendly chemical processes.  

One of her recent advances is called crossover theory. It describes the changes that occur in the properties of a fluid when it is close to its critical point (the specific temperature, pressure and density at which there is no difference between liquid and gas).  Until the crossover behavior was taken into account, the equations engineers used to describe fluids failed to accurately describe molecular properties near the critical point, which is particularly important in several environmental applications. 

The National Science Foundation (NSF) has helped fund this research, and the techniques she has developed will help engineers working with a variety of lubricants and solvents to have more definitive understandings of how the fluids will behave under various conditions. This information is critically important to have at the front-end of the design process, especially for the new, highly miniaturized devices and systems being produced in the nanotechnology realm. 

Her work promises to make important contributions to the effort to understand fluid dynamics, which are involved in everything from blood circulation and microfluidic channels in biosensors to computer chip production. “The ability to accurately predict the thermodynamic properties of fluids is central to process design and has long been a goal of chemical engineers,” McCabe says. 

Plugged in to nanotechnology research at Vanderbilt as well as with researchers around the world, McCabe is helping others leverage her techniques to apply them to a variety of chemical engineering challenges. 

 “We first applied these techniques to simple industrially relevant fluids, and then moved on to carbon dioxide, water and mixtures, as well as complex systems with nanoscale structure” McCabe said. “Our goal is to develop methods that produce results that are significantly closer to reality.”