Phosphate-free dishwasher sparkle needs a chemical combo

We’ve all been there. However careful you are when you fill the dishwasher, that never guarantees every item will come out clean. It’s especially annoying when the wine glasses come out spotty, rather than sparkling. The problem is calcium carbonate - limescale - being left behind by the water, and it’s more of a problem in hard water areas.

The solution, until recently, was to add a phosphate-based ‘inhibitor’ to prevent the calcium build-up from forming, and inhibitors were added to all kinds of detergents to enhance their cleaning power. It later became apparent that, while phosphate is non-toxic, it can build up in the environment and have damaging effects. Living organisms need phosphorus to survive, but high levels in bodies of water leads to excessive plant growth and algal blooms, low oxygen levels and a decrease in biodiversity.

The EU banned phosphates in consumer laundry detergent in 2013, and a ban on the use of phosphates in dishwasher detergent followed last year. Other countries already have, or are proposing, similar bans, and scientists are searching for less damaging chemicals that can do the same job. However, as yet we don’t fully understand whether inhibitors prevent calcium deposits from forming completely, or simply inhibit their growth. In addition, six different forms of calcium carbonate, known as polymorphs, have been identified, and they don’t all react with inhibitors in the same way.

A team of researchers developed a lab-based model dishwasher system, which they used to rapidly screen 28 chemical candidates. Each chemical was included in an experimental batch of dishwasher detergent, and used to wash up both glass and plastic (PMMA, Poly-(methyl methacrylate)) test items. The researchers then used powder X-ray diffraction (PXRD) on Diamond’s Small Molecule Single-Crystal Diffraction beamline (I19) to examine the calcium deposits. Two PXRD patterns were collected for each sample, one in situ on the test item, and one on free material scraped from the surface. Author Prof Jonathan Steed from Durham University explains: “The PXRD pattern collected with substrate is often affected by the preferred orientation of the crystalline material, and the powder scraped from the substrate gives better information on the polymorph distribution ratio.”

The team found that, while no one chemical could match the performance of the phosphate-based inhibitor, two used in combination did the trick. While one inhibitor prevents the growth of the argonite polymorph on glass, the other inhibits the growth of calcite on plastic. Their combined cleaning power was confirmed in a commercial dishwasher - a truly sparkling result!
 

Related publication:

Hong, Y et al. Phosphate-Free Inhibition of Calcium Carbonate Dishwasher Deposits. Crystal Growth & Design 18 (3), 1526–1538 (2018). DOI:10.1021/acs.cgd.7b01508.