When the science fiction author Kurt Vonnegut wrote “Cat’s Cradle,” he added a disclaimer stating the devastating “ice-nine” substance in his 1963 classic novel was not real.
However, it turns out his basic idea was just a little more than a half-century ahead of his time.
University of Pittsburgh researchers recently discovered a liquid polymer — a large molecule made up of smaller molecules — that solidifies when mixed with water at room temperature, much like Vonnegut’s ice-nine.
“Liquid mixtures don’t just naturally freeze up unless there are special conditions,” said John Keith, assistant professor of chemical and petroleum engineering and Richard King Mellon Faculty Fellow in Energy at Pitt’s Swanson School of Engineering.
In “Cat’s Cradle,” ice-nine was accidentally introduced to the world’s oceans, freezing all the water in them and then causing apocalyptic changes in weather patterns in a quick fashion. But the polymer discovered by Keith and researcher Sachin Velankar, associate professor of chemical and petroleum engineering, can only freeze small amounts of water at a time, and the freezing process can take many hours or even a whole day — no ice-nine-style destruction to worry about.
“Freezing large bodies of water would require enormous quantities of polymer as well as lots of mixing,” Velankar said.
The polymer, known as polyoxacyclobutane, when mixed with water, takes the mixture’s freezing point from the 32 degrees Fahrenheit needed for water alone to about 100 degrees Fahrenheit. The fictitious ice-nine raised the temperature at which water would freeze to 114.4 degrees.
Keith and Velankar have been researching this mixture that seems to defy the second law of thermodynamics, which states that within an isolated system, entropy always increases.
“When you mix two pure components together, the entropy (or the degree of disorder), always increases,” said Keith. “That disorder almost always causes mixtures to have a lower freezing point than either of the components individually, not higher.”
The mixture of salt and water, for example, freezes at lower temperatures than either salt or water individually. This quality makes salt well-suited for melting ice on roads and sidewalks in the winter.
“The weird thing about this case is the two compounds start out as liquids, but when you mix them at any temperature cooler than a hot summer day, the mixture will automatically freeze,” Velankar said. “At higher temperatures, the mixture melts and the polymer reacts to water like oil does. They separate and avoid each other.”
The researchers explained that this idea of two different compounds crystallizing when mixed is not unusual in metals. Sometimes, mixtures of metals can melt at a higher temperature than the individual metals do separately. However this is very rare for liquids.
The team has investigated using this polymer to remove salt from water samples as well as to create computer memory devices with Pitt chemistry professor Alexander Star. Pitt’s Center for Research Computing also contributed computing resources.
The Pitt team’s discovery was made during a collaborative effort between Delaware-based DuPont and Robert Enick, vice chair of research for Pitt’s chemical engineering department. A Japanese research team in the late ’60s characterized similar water freezing with a heavier form of the polymer that was solid and required pre-melting before freezing. The Pitt team found that freezing can also happen with lighter forms of the polymer that are liquid at room temperature.
“Now that we know an example of a polymer-water mixture with these qualities, we can search for other mixtures that might have other interesting properties,” said Keith. “I’m very optimistic that this is an exciting new class of crystalline materials that spontaneously form from mixtures of liquids.”