Can Polyoxometalates Be Considered as Superchaotropic Ions?

In this work, we have studied the influence of different anionic species, including typical Hofmeister ions (citrate(3-) and SCN-), hydrophobic ions (tetraphenyl borate, Ph4B-), and polyoxometallic clusters (Keggin-type POM, PW3-), on the electrophoretic mobility of colloidal particles of a different nature, charge, and wettability. We focused our attention on the adsorption properties of PW3- in comparison with Ph4B- because both species have previously been termed as superchaotropes. However, their different structural properties make it difficult to reconcile the same classification for both anions. We found that the influence of Ph4B- is similar to and surpasses the characteristic effect of chaotropic anions (SCN-), which particularly interacts more strongly with hydrophobic interfaces. On the contrary, we observed clear POM adsorption on the different particles investigated, regardless of their charge or wettability. These observations are complemented by a direct adsorption measurement that combines a quartz crystal microbalance and atomic force microscopy, confirming the strong and irreversible self-assembly of POM on different substrates. Our results indicate that polyoxometallic anionic clusters should better be described as small colloids instead of as large anions. The exceptional adsorption properties of POM clusters on any kind of interface suggest that their behavior goes beyond the classical Hofmeister effects, and therefore, these species should not be included in the Hofmeister series as superchaotropic anions.