A change of (crystal) face slows scale.

Biological molecules function by having folds, kinks and charged regions that "recognize" specific substances and act as docking sites. When a substance docks, this alters biologic activity. To deactivate biological molecules, pharmacologists trick them with synthetic chemicals designed to fit into these sites and block the normal attachment by a natural substance.

That principle, molecular recognition, also applies to inorganic materials, chemist Roger J. Davey and his colleagues at ICI Chemicals and Polymers Limited, Runcorn, England, demonstrate in the Oct. 10 NATURE. Learning about molecular recognition can aid the design of antiscalants to prevent the buildup of barium sulfate, whose crystals cause problems inside water pipes, they note.

Crystals grow into characteristic shapes by adding material at particular growth sites. Barium sulfate crystallizes in the form of rhombic plates. But after adding different chemicals to solutions of barium sulfate, the ICI team discovered that a compound with two phosphonate groups linked by a chain of three atoms caused the rhombic plates to develop pinched corners. When the scientists increased the phosphonate's concentrations, disks, not rhombic plates, developed.

They concluded that the two phosphonate groups "looked like" the bottom of a sulfate to the crystal surface. But when the ions due to form the next layer reach the growth site with the phosphorous compound attached, "they feel the presence of the phosphonate," says Davey. As a result, the crystal stops growing and deposition ceases.

"What is new is the understanding we've put into the design of these phosphonates," notes Davey. Now chemists know very specifically what arrangement of atoms works best to prevent barium-sulfate buildup, he adds.