Nanometer-Size Polyoxometalate Anions Adsorb Strongly on Neutral Soft Surfaces

Abstract

Polyoxometalates (POMs) are nanometric metal-oxide anions with unique chemical and physical properties. During last decade, significant efforts have been made to give POMs surface activity and self-assembly properties that are essential for catalysis applications and for producing organic-inorganic hybrid materials. In this work, POMs based surfactants are produced spontaneously through non-covalent interactions in water by mixing non-ionic surfactants with POM. The most common POMs of tungstosilicate and tungstophosphate, have indeed an unexpected strong tendency to adsorb on polar and neutral interfaces. Micelles in water and water/air interfaces were investigated by SAXS and ion flotation showing the POM anions adsorbed at the micelle surface and on monolayers of non-ionic surfactants. This general property of POM provides a unique opportunity for deeper understanding of many medicinal effects of POMs, i.e. their antiviral and antitumor activities that involves their specific adsorption on biological surfaces.

Polyoxometalate/Polyethylene Glycol Interactions in Water: From Nanoassemblies in Water to Crystal Formation by Electrostatic Screening

“…In a previous study it was shown by SAXS that addition of salts, such as NaCl, to POM aqueous solutions leads to the screening of electrostatic repulsions between the POM anions . A salt induced screening effect was also previously observed between non‐ionic micelles decorated by POMs .…”

“…In a previous study it was shown by SAXS that addition of salts, such as NaCl, to POM aqueous solutions leads to the screening of electrostatic repulsions between the POM anions . A salt induced screening effect was also previously observed between non‐ionic micelles decorated by POMs . In the following, the screening of electrostatic repulsions in the PW–(EO) x mixtures was investigated for a fixed PW concentration of 25 m m by adding 100 m m NaCl, which is sufficient to screen most of the electrostatic repulsions.…”

“…The methodology developed here, which is based on the electrostatic screening between pre‐formed organic‐inorganic EO x –PW nanoassemblies, can be extended to other kind of water soluble oligomers that interact with POMs in solution. For example the POMs, PW and silicotungstate (SiW), have been found previously to also interact with sugar moieties by adsorbing at surfaces (water‐micelle or water‐air surfaces) covered by sugar based surfactants . Such a general strategy to build hybrid materials which is based on a self‐assembly approach in water has the advantages to avoid the time consuming multi‐step synthesis of organic‐inorganic POM building blocks as well as the use of toxic polar solvents, such as acetonitrile or DMSO typically employed to dissolve both POMs and organic building blocks.…”

“…Recently, the strong propensity of the α‐Keggin based POMs, silico‐ and phospho‐tungstate (α‐SiW 12 O 40 4− and α‐PW 12 O 40 3− ), to adsorb on neutral soft interfaces covered with polar organic moieties, such as sugar and ethylene oxide (EO) groups, was highlighted . POMs were indeed shown to adsorb at the micellar surface of, for example, tetraethyleneglycol mono‐octyl ether (C 8 EO 4 ) and octyl beta‐glucoside (C 8 G 1 ), without disrupting the micellar assembly.…”

Jenseits von Ladungsausgleich: Gegenkationen in der Polyoxometallat‐Chemie

“…[PW 12 O 40 ] 3− , q/M=0.25; [P 2 W 18 O 62 ] 6− , q/M=0.33), während die Nb V ‐Spezies [Nb 6 O 19 ] 8− größere q/M‐Verhältnise (hier: 1.33) aufweisen. Zusammengefasst bilden POMs also so genannte chaotrope Anionen, die das Wasserstoffbrückennetz in wässriger Lösung unterbrechen und so zum Ausfällen von Makromolekülen wie Polymeren, neutralen Tensiden und Proteinen führen. Entsprechend ist wegen der Anwendungsbreite von POMs auf die Solvatationsenergien von POM und Kation zu achten, weil das Wechselspiel zwischen Solvatationsenergie und Gitterbildungsenergie des entsprechenden POM‐Kation‐Salzes die Löslichkeit der Spezies kontrolliert.…”