Profile of Valeria P. Molinero

Growing up in Buenos Aires, Argentina, Valeria Molinero nursed an early love of learning and discovery, thanks to her parents. “When I encountered the natural sciences in school, I started liking them all,” she says. “And then I saw chemistry, and I liked it, and I decided to just study chemistry,” says Molinero.

Valeria Molinero. Image credit: Brian Maffly (University of Utah, Salt Lake City, UT).

Molinero went on to a long and fruitful career as a physical chemist, recognized for her work on modeling water and understanding the formation of ice and clathrate hydrates. She has combined theory and molecular simulations to address a variety of questions, including the properties of water and substances such as silica that share tetrahedral topology. In her Inaugural Article, Molinero presents a machine learning–based model of water that describes both a liquid–liquid phase transition of supercooled water and ice crystallization (1).

Molinero is a professor of theoretical chemistry at the University of Utah. In 2019, her article exploring the formation of the smallest droplets of ice received the PNAS Cozzarelli Prize (2). “I’m proud of the work we did, that was really a highlight,” says Molinero. She was elected to the National Academy of Sciences in 2022.

From Electrochemistry to Water

Molinero worked on electrochemistry as an undergraduate student at the University of Buenos Aires. When a friend told her about a professor who used a combination of theory and molecular simulations in his research, she was intrigued and went to talk to Daniel Laria. “He told me about the idea of doing experiments in a computer and answering very basic questions with a lot of control, and I really enjoyed that aspect and the possibilities of formulating new realities in the computer and seeing how they evolve. So that’s how I got into theory and modeling,” says Molinero. She completed a PhD in chemistry from the University of Buenos Aires in 1999.

After her PhD, she joined the lab of William Goddard at Caltech for postdoctoral research. A few years later, she met Austen Angell from Arizona State University and simultaneously pursued postdoctoral research with Goddard and Angell. “Austen Angell has been a very influential person in my career, and my work with him brought me more into the areas that I would say have defined my career since,” Molinero says. Those areas include the chemistry of water and supercooled liquids, which she focused on when starting her own lab at the University of Utah in 2006.

At the University of Utah, Molinero developed an approach sparked by her previous work. As she conducted experiments and simulations for silicon and germanium, she had started to notice relationships between the physics and phase behavior of group IV elements—such as silicon, germanium, and carbon—and the behavior of water. “The physics looked deceptively similar,” she says. The elements were all tetracoordinated and formed diamond-like crystals, such as when water formed ice.

Based on the similarities, Molinero developed a new approach to modeling water and understanding water as if it were an element of the periodic table with properties intermediate between carbon and silicon (3). “I developed a model that represents water as a single particle, and this single particle has the ability to form tetrahedral bonds like silicon, but the [bonds] are stronger, more tetrahedral,” she says. “On the one hand, it’s a simple model, so it has been very accessible and a lot of people use it, but, at the same time, it’s actually a link between completely different substances that are not chemically related, so it gives you a new framework for thinking about water,” says Molinero.

The model enabled Molinero to find quasicrystals of water, as well as anomalous phases in confined water that had not been previously reported. “We ended up addressing a lot of the questions of phase transitions in water, including important questions about supercooled water that were previously difficult to do simulations of,” she says.

Venturing into New Areas

At the University of Utah, Molinero also started studying the crystallization of water, particularly the interplay between crystallization and structural changes in supercooled water. “At this point, I’m well-known for phase transitions and crystallization, but I had not done any crystallization before I got to Utah, and even said at one point, ‘I will never do crystallization,’" she says. Among other findings, she was able to decipher some of the factors controlling the crystallization of water into ice (4). “We showed through molecular simulations with the model I have developed that when the structure of water changes the most, the crystallization rate of water is the maximum,” says Molinero. “We predicted a maximum crystallization rate in real water at around 225 Kelvin, and indeed, it was found in experiments later to be at 228 Kelvin or so,” she says.

These studies have had a broad impact. “The most known and impactful research I have done has to do with understanding what determines the formation of ice,” she says. “I used to laugh that I’m famous for showing that cold water forms ice,” says Molinero. Yet the commonplace nature of this transformation belies its complexity and significance. “It’s one that has a lot of implications in nature, and, at the same time, it has been elusive,” says Molinero. “Ice formation induced by particles or organisms or surfaces dominates the formation of ice in clouds, and ice formation in clouds is what controls precipitation,” she says. Predicting which particles can form ice and uncovering why and how the particles promote ice formation are questions into which Molinero has made serious inroads.

Her work also led her to study how microorganisms, insects, and fish control ice formation to survive subzero temperatures. For example, some bacteria contain ice-nucleating proteins in their outer membranes and use the proteins to form ice on plants. “When you see in the winter that a night is cold and the plants blacken, it’s not because ice is spontaneously forming, it’s because these bacteria are promoting the formation of ice tissue, and it’s superimportant for crops because the crops can freeze and become damaged,” she says. The same bacteria can also become aerosolized and participate in ice formation in low-lying clouds, contributing to precipitation.

Molinero has used models and molecular simulations to study how bacteria and insects promote ice nucleation using proteins. She recently published a study on ice nucleation in fungi, finding commonalities between the organisms (5). “Many organisms that control ice formation produce proteins that are typically not very large, and in all of these organisms, the commonality is that these proteins come together to produce larger aggregates that enable ice formation,” she says. Molinero continues to study this process, including what causes different proteins to aggregate and why some organisms that have no apparent need for such proteins—such as mammals—possess them.

One of Molinero’s students suggested that she study clathrate hydrates—substances made up of water and hydrophobic molecules. “In principle, these do not mix, but if these hydrophobic molecules are small enough and you apply pressure and you cool, you form crystals that have a huge ratio of gas to molecules, typically with propane gases or methane gases,” she says. “When my student told me about them, I didn’t know what they were, and as I read about them, I found them intriguing,” she says.

Clathrate hydrates have attracted broad interest because of their potential applications. “They’re actually the most abundant fossil fuel reserves on Earth,” explains Molinero. Her research addressed some unanswered questions about clathrate hydrates. “We proposed what is now considered the mechanism of formation of clathrate hydrates,” she says (6).

Molinero says she has always enjoyed finding new topics to study. “I don’t know whether we are brave or foolish, but I’m not afraid to study things that don’t look like they can be done,” she says. “We go into these areas and get to a question that looks impossible, and then we develop some models, some methods, and we move forward,” says Molinero.

Back to Water

Over the course of her career, Molinero has often branched out beyond water, including work on zeolites and on improving current production in fuel cells. However, for her Inaugural Article, Molinero returned to her original interest (1). Researchers had proposed that as water cools into a supercooled state, at some point, liquid water of one density could transform into liquid water of a different density. However, Molinero’s original model did not enable her to study this putative liquid–liquid transition. “The Inaugural Article, in some sense, is like going back to this question, and now we can study this liquid–liquid transition,” she says. “This model enables us… to look at what happens with crystallization as we approach this transformation between two liquid phases. What we find is that the point at which the liquid transforms from one phase to the next—the liquid–liquid transition line—is the same point at which the crystallization rate is maximum, so that the system is more susceptible toward crystallization when you have this liquid–liquid transition,” explains Molinero.

Molinero plans to use this model to address other questions. “Many of these questions have been debated for a long time, and now this model enables you to do the experiment in the computer, and we’re hoping that with this we are opening the possibility of unraveling many more of the questions about supercooled water,” she says. As Molinero’s models have improved, so have computers. “As a PhD student, it was really hard to do any computation,” she says. “Now, I never feel that what I want to do is limited by computing,” says Molinero.

Throughout her career, Molinero has found ways to apply lessons from one project to another. “What I like is to learn broadly, and whether it’s water or silicon or silica, the questions are different, but there’s always a unifying theme,” she says. “I’m excited by newness, and I look forward to drifting into new areas and all the new problems to come,” says Molinero.

Despite her penchant for novelty, Molinero says one thing has remained constant in her career: “I love what I do; it often makes me feel how I felt when I was playing as a child, and you always want to feel this kind of joy in what you do.”