🔬 materials science

Highly Stable Glasses of cis-Decalin and cis/trans-Decalin Mixtures

This study reports the first successful creation of highly stable vapor-deposited glasses from cis-decalin and cis/trans-decalin mixtures, demonstrating that these molecular mixtures, particularly the 50/50 composition, exhibit exceptional kinetic stability and low heat capacity comparable to or exceeding that of pure molecular ultrastable glasses.

Original authors: Katherine R. Whitaker, Daniel J. Scifo, M. D. Ediger, Mathias Ahrenberg, Christoph Schick

Published 2026-08-25

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CC BY 4.0

Original authors: Katherine R. Whitaker, Daniel J. Scifo, M. D. Ediger, Mathias Ahrenberg, Christoph Schick

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Glass is a material that defies simple categorization. It is a solid that lacks the orderly, repeating structure of a crystal, yet it does not flow like a liquid. Instead, it is a frozen snapshot of a disordered state, where molecules are locked in place but retain the chaotic arrangement of a fluid. This disorder is not a flaw; it is a feature that allows scientists to tune the properties of glass by changing what it is made of. However, the way glass is made matters just as much as its ingredients. A glass cooled quickly from a liquid is usually less stable and holds more energy than one cooled slowly. For decades, researchers have known that if you cool a liquid very slowly, or let it sit and age for a long time, the molecules have more time to find a snug, efficient packing arrangement, resulting in a glass that is denser and more stable.

Recently, scientists discovered a shortcut to this ideal state. By depositing molecules one by one from a vapor onto a cool surface, they can build a glass layer by layer. As each new molecule lands, it sits on a surface that is still mobile, allowing it to wiggle and settle into a perfect spot before being buried by the next layer. This process creates what are called "ultrastable glasses." These materials are so well-packed that they behave as if they had been cooled for thousands of years, yet they are formed in a matter of minutes. Until now, this phenomenon had only been observed in pure substances. The big question was whether this trick could work for mixtures of different molecules, and whether it could work for materials that are notoriously difficult to turn into glass.

A team of researchers at the University of Wisconsin–Madison and the University of Rostock set out to answer these questions using a substance called decalin. Decalin comes in two shapes, known as isomers: cis-decalin and trans-decalin. The researchers wanted to see if they could create ultrastable glasses from pure cis-decalin, from pure trans-decalin, and from various mixtures of the two. They used a specialized instrument called a nanocalorimeter, which is sensitive enough to measure the heat capacity of a tiny film just a few hundred nanometers thick. By depositing the vapor onto a surface held at specific temperatures and then carefully heating the film, they could watch how the glass transformed back into a liquid.

The results were striking. The team successfully created ultrastable glasses from cis-decalin and from mixtures containing both cis and trans forms. They found that by depositing the material at a temperature roughly 85 percent of the glass's transition point, they could produce a film that was significantly more stable than glass made by simply cooling the liquid. For the 50/50 mixture of the two shapes, the resulting glass was so stable that it took a transformation time equivalent to roughly 25,000 times the natural relaxation time of the liquid to turn back into a fluid. In practical terms, this means the glass was incredibly resistant to change. The heat capacity of these new glasses was also lower than that of ordinary glass, indicating that the molecules were packed more tightly and efficiently.

Perhaps the most surprising finding was that this method worked for a mixture of two different molecules. This is the the first time such stable glasses have been formed from a blend of substances. The researchers tested mixtures with different ratios of the two decalin shapes, from a quarter of one to three-quarters of the other, and found that stable glasses formed across the board. This suggests that the ability to create these high-performance materials is not limited to single, pure chemicals. It opens the door to engineering glasses with specific properties by simply adjusting the recipe. The study also revealed that even a very poor glass-former, like pure trans-decalin, could be incorporated into a stable glass if mixed with the other form, provided the mixture contained enough of the more cooperative partner.

Not every attempt was successful, however. The researchers tried to make a stable glass from pure trans-decalin, but the material crystallized before it could settle into a glassy state. This failure highlights that while the method is powerful, it is not universal; some materials simply refuse to stay disordered under these conditions. Furthermore, the study showed that the stability of the glass depends heavily on the temperature of the surface during deposition. If the surface is too hot, the molecules do not pack tightly enough. If it is too cold, they freeze in place before finding their best arrangement. The sweet spot was found to be a narrow range of temperatures where the molecules have just enough mobility to settle in perfectly.

These findings challenge the idea that ultrastable glasses are a rare curiosity limited to specific, simple molecules. The work demonstrates that the phenomenon is robust enough to handle mixtures and even very fragile materials that usually resist forming glasses. The researchers also ruled out the possibility that these stable properties were caused by tiny, hidden crystals forming within the glass. If that were the case, a mixture of two different molecules would likely not form such a uniform, stable structure. Instead, the evidence points to a truly amorphous state where the molecules are simply packed with extraordinary efficiency.

The implications of this work extend beyond the laboratory. Stable glasses have potential uses in organic electronics and pharmaceuticals, where the stability of a material is crucial for performance and shelf life. For instance, many drugs are sold as amorphous mixtures to improve how they dissolve in the body, but these mixtures can be unstable and prone to crystallizing over time. The ability to create stable glasses from mixtures suggests a new way to lock these materials into a durable state. By understanding how to control the packing of molecules during deposition, scientists may be able to design materials that are stronger, more stable, and better suited for advanced technologies. The study confirms that the rules governing these materials are more flexible than previously thought, offering a new toolkit for materials science.

Technical Summary: Highly Stable Glasses of cis-Decalin and cis/trans-Decalin Mixtures

Problem and Context
Vapor-deposited glasses have been shown to exhibit extraordinary properties, including high kinetic stability, high density, and low heat capacity ( $C_p$ ), often equivalent to liquid-cooled glasses aged for thousands to millions of years. These "ultrastable" glasses are typically formed by depositing molecules onto substrates at temperatures near $0.85 T_g$ (where $T_g$ is the glass transition temperature). While this phenomenon has been observed in over a dozen organic molecules, two specific avenues remained unexplored: the formation of stable glasses from molecular mixtures and the formation of stable glasses from highly fragile glassformers. Previous studies suggested that stable glass formation might be restricted to systems with intermediate fragility, as attempts with low-fragility systems like glycerol and water had failed. Furthermore, it was unknown whether the unique properties of stable glasses could be achieved in mixed molecular systems, which are technologically relevant in pharmaceuticals and organic electronics.

Methodology
The authors employed in situ AC nanocalorimetry to measure the reversing heat capacity of vapor-deposited films of cis-decalin and various cis/trans-decalin mixtures.

Sample Preparation: Thin films (~570 nm) were prepared by physical vapor deposition onto nanocalorimeter sensors held at controlled substrate temperatures ( $T_{sub}$ ) ranging from $0.72 T_g$ to $1.02 T_g$ .
Composition Control: Mixtures were created via co-deposition from separate crucibles. The 50/50 cis/trans mixture was prepared by adjusting leak valve pressures to achieve a gas-phase composition of 50% cis-decalin and 50% trans-decalin.
Characterization: Reversing heat capacity was measured during temperature ramps (2 K/min) to determine the onset temperature ( $T_{onset}$ ) and $C_p$ . For the 50/50 mixture, quasi-isothermal annealing experiments were conducted at 136.6 K to quantify kinetic stability by measuring the transformation time from the stable glass to the supercooled liquid.
Calibration: Temperature calibration was performed using phase transitions of cyclopentane and the dynamic glass transition of toluene, with additional verification via dielectric relaxation data for the decalin mixture.

Key Results

Formation of Stable Glasses from Mixtures: The study successfully demonstrated that cis/trans-decalin mixtures (specifically 25/75, 50/50, and 75/25 ratios) form highly stable glasses when vapor-deposited. These as-deposited glasses exhibited lower heat capacities and higher onset temperatures compared to their liquid-cooled counterparts. This represents the first report of highly stable glass formation in molecular mixtures.
High Fragility Systems: The 50/50 cis/trans-decalin mixture, characterized by a fragility index ( $m$ ) of 145–147 (the highest reported for molecular glassformers), successfully formed an ultrastable glass. This confirms that stable glass formation is not restricted to intermediate fragility systems but extends to highly fragile materials.
Thermodynamic and Kinetic Properties:
- Heat Capacity: The as-deposited cis-decalin glasses showed a maximum $C_p$ decrease of $4.5 \pm 1\%$ relative to the ordinary glass, while the 50/50 mixture showed a decrease of $2.5 \pm 0.5\%$ . The maximum reduction occurred at the lowest accessible deposition temperatures (~0.75 $T_g$ ) for cis-decalin and near 0.86 $T_g$ for the mixture.
- Kinetic Stability: The 50/50 mixture exhibited high kinetic stability, with an onset temperature increase of up to 7 K relative to the ordinary glass. Isothermal annealing of a ~500 nm film deposited at $0.86 T_g$ revealed a transformation time equivalent to $10^{4.4}$ times the structural relaxation time ( $\tau_\alpha$ ) of the supercooled liquid at the annealing temperature.
- Substrate Temperature Dependence: Both cis-decalin and the mixture showed a non-monotonic relationship between substrate temperature and stability. Maximum kinetic stability (highest $T_{onset}$ ) was observed in the range of $0.84–0.94 T_g$ . At lower temperatures, kinetic stability decreased as molecules lacked sufficient mobility to find efficient packing.
Failure Cases: Attempts to form stable glasses from pure trans-decalin were unsuccessful, likely due to crystallization during deposition. Similarly, pure cis-decalin is a poor glassformer, and this study provides the first reported $T_g$ for a completely amorphous cis-decalin sample.

Significance and Claims
The paper claims two primary contributions to the field of glass science:

Extension to Mixtures: It establishes that stable glasses can be formed from molecular mixtures, not just pure components. This suggests that the tunability of glass properties via composition—a hallmark of glass technology—extends to the regime of ultrastable glasses. The authors argue that the successful formation of stable glasses from mixtures (where mixed crystals are unlikely) provides strong evidence against the hypothesis that the enhanced stability of vapor-deposited glasses is due to the presence of nanocrystals.
Extension to High Fragility: The work demonstrates that extremely fragile glassformers (like the 50/50 decalin mixture) can form stable glasses. This broadens the known range of fragilities capable of supporting stable glass formation, suggesting that surface mobility is a robust mechanism across diverse molecular systems.

The authors conclude that these findings have potential relevance for organic electronics and amorphous pharmaceuticals, where mixtures are common. They also note that the ability to form stable glasses from poor glassformers (like cis-decalin) when mixed with a second component offers a pathway to stabilize materials that would otherwise crystallize.

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