Salt stretching can have effect for desalination

Salt can be stretched. That is the unexpected discovery made by researchers at the US government's Sandia Laboratory reported in the lab's May Nano Letters (NW Moore et al, Nano Letters, 2009. DOI:10.1021/nl9004805).

That a block of salt can stretch rather than remain inert might affect world desalination efforts, which involve choosing particular sizes of nanometer-diameter pores to strain salts from brackish water. Understanding unexpected salt deformations also may lead to better understanding of sea salt aerosols, implicated in problems as broad as cloud nucleation, smog formation, ozone destruction and asthma triggers, the researchers write.

The discovery came about as researchers were examining the mechanical properties of salt in the absence of water. They found unexpectedly that the brittle substance appeared malleable enough to distort over surprisingly long distances by clinging to a special microscope's nanometer-sized tip as it left the surface of the salt.

"It's not supposed to do that," said Sandia principal investigator Jack Houston. "Unlike, say, gold, which is ductile and deforms under pressure, salt is brittle. Hit it with a hammer, it shatters like glass."

More intense examination showed that surface salt molecules formed a ductile meniscus with the exploratory tip as it withdrew from penetrating the cube. In this, it resembled the behavior of the surface of water when an object is withdrawn from it. But unlike water, the salt meniscus did not break from its own weight as the tip was withdrawn. Instead it followed the tip along, sliding away as it thinned and elongated from 580 nanometers (nm) to 2,191 nm in shapes that resembled nanowires.

A possible explanation for salt molecules peeling off the salt block, said Houston, is that "surface molecules don't have buddies." That is, because there is no atomic lattice above them, they are more mobile than the internal body of salt molecules forming the salt block.

Salt showing signs of surface mobility at room temperatures was "totally surprising," said Houston, who had initially intended to study more conventionally interesting characteristics of the quarter-inch square, eighth-inch long salt block.

Other researchers on this work include Sandia's Nathan Moore, with Hunhang Luo and Scott Mao from the University of Pittsburgh.

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