MATERIALS
Rewritable paper goes technicolor ty of California, Riverside, who also works on rewritable paper and was not involved in this research. Using zinc-based inks and a different A team at Nanjing University of Posts & terpyridine, the team at Nanjing University printed a bar code pattern that was Telecommunications overcame these hurvisible only under UV light, and it lasted dles, using water-based inks and inexpenfor more than one sive materials to form year. The resolution colored metal-ligand of these patterns was complexes that remain comparable to that of stable for at least six conventional ink-jet months. printing, and the team The ligands are tersuggests the method pyridine molecules, could be useful for secontaining three nicurity tags. trogen atoms that coThe rewritable ordinate to metal ions paper can be erased such as iron, zinc, and and reused five times cobalt. The terpyridines with no significant also carry fluorene or loss in color intensity. triphenylamine groups However, the erasure that tune their optical process involves washproperties. ing with a solution of In one demonstrafluoride ions to disrupt tion, the researchAqueous metal-ion inks were the metal-ligand comers coated paper used to draw a sunflower on plexes, a significant with a thin film of a rewritable paper. drawback for compoly(ethylene glycol) mercial applications. Qiang Zhao, part of copolymer mixed with a terpyridine the research team, says they are already molecule. Adding aqueous solutions of working on more practical ways to erase different metal ions to the paper—eithe paper. ther by freehand drawing or by ink-jet The researchers estimate that printing printing—created long-lasting colored a page of text on their rewritable paper complexes that included brown, blue, yelcould cost less than one-fifth of a convenlow, and green hues. “It is exciting to see tional ink-jet print on fresh paper. “We are that by simply varying the metal ions, a number of distinct colors can be obtained, optimistic about the commercialization of rewritable paper,” Zhao says.—MARK and the color intensity remains stable for months,” says Yadong Yin at the Universi- PEPLOW, special to C&EN
Metal-ligand complexes produce long-lasting colors that can be erased on demand, allowing paper to be reused The paper industry has a significant environmental impact, from cutting down trees for raw material to consuming large amounts of energy and water to process that material. To curb that impact, chemists have been working on rewritable paper technologies that would allow people to print on a sheet of paper with special inks and then erase them to reuse the paper. A team now reports a form of rewritable paper that can display multicolored
N N N Terpyridine ligand
images and text for months, before being erased and reused time after time (Nat. Commun. 2018, DOI: 10.1038/ s41467-017-02452-w). Previous technologies have involved impregnating paper with colorless dyes that are activated by water sprayed from ink-jet printers; others use dyes that show up when exposed to ultraviolet light. But these systems have struggled to create multicolored images, and the printed text rarely lasts for more than a few days.
THEORETICAL CHEMISTRY
C R E D I T: NAT. CO M MU N.
Water’s hypercooling point revised A new calculation of the temperature at which water undergoes a unique phase change revises the point upward by about 100 °C, enabling researchers to test its existence for the first time (J. Chem. Phys. Lett. 2018, DOI: 10.1021/acs.jpclett.7b03068). When pure liquid water is supercooled below –5 °C, it freezes in two distinct steps. At about –20 °C, a thin network of ice crystals rapidly forms. The energy released by that phase change prevents the remaining liquid from freezing until roughly –30 °C.
Those steps can happen as one in a process called hypercooling, when the liquid is cold enough that the excess energy from the first step is absorbed by the frozen solid. Sigurd Bauerecker of Braunschweig University of Technology says there has been almost no research on hypercooled water because a 1997 book calculated the hypercooling point of water to be –160 °C. That’s well below the point at which water will spontaneously nucleate and freeze, making it impossible to test the calculation.
Bauerecker and colleagues recalculated the hypercooling point using more accurate data about the temperature dependencies of water’s heat capacity and the enthalpy of freezing and found it to be –64 °C. Several researchers have already shown they can supercool water below that temperature, putting the hypercooling point within reach of experimentalists and possibly nature. “Up to now, hypercooling did not matter for water,” Bauerecker says. “Now it comes into play.”—SAM LEMONICK JANUARY 15, 2018 | CEN.ACS.ORG | C&EN
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