Accurately Tuning the Charge on Giant Polyoxometalate Type Keplerates through Stoichiometric Interaction with Cationic Surfactants

Abstract

We report an approach of exploring the interaction between cationic surfactants and a type of structurally well-defined, spherical "Keplerate" polyoxometalate (POM) macroanionic molecular clusters, {Mo72V30}, in aqueous solution. The effectiveness of the interaction can be determined by monitoring the size change of the "blackberry" supramolecular structures formed by the self-assembly of {Mo72V30} macroions, which is determined by the effective charge density on the macroions. Long-chain surfactants (CTAB and CTAT) can interact with {Mo72V30} macroions stoichiometrically and lower their charge density. Consequently, the blackberry size decreases continuously with increasing surfactant concentration in solution. On the other hand, for short-chain surfactants (e.g., OTAB), a larger fraction of surfactants exist as discrete chains in solution and do not strongly interact with the macroions. This approach shows that a controllable amount of suitable surfactants can accurately tune the charge on large molecular clusters.

Surfactant-encapsulated polyoxometalate building blocks: controlled assembly and their catalytic properties

“…48 Liu, Kegel, Müller, and coworkers investigated that the size of blackberry assemblies is highly controllable either by optimizing the solvent contents or by tuning the charge density of the POM giant anion. [55][56][57][58][59] Covalent grafting of an organic group onto POM clusters has drawn tremendous attention in the last few years. Many researchers including Wei's, Gouzerh's, Cronin's, Peng's, Hill's and Bonchio's research groups, among others, have reported several covalently modified POM hybrid clusters.…”

Inhomogeneous Distribution of Cationic Surfactants around Anionic Molecular Clusters

“…When CTAB was added to {Mo 72 V 30 }, stable supramolecular structures formed (Note: from current point of view, the “blackberry” structure we suggested previously could also be vesicles formed by amphiphilic surfactant–clusters complexes.) even up to the CTAB/{Mo 72 V 30 } molar ratio of 10 (Table S1, Supporting Information) and no inhomogeneous distribution of cationic surfactants was observed. Given that all other conditions are similar, the most probable reason that leads to the different behaviors of {Mo 72 V 30 } and U 60 clusters would be the surface charge density.…”

Properties of shell-like assemblies of weakly charged Keplerate-type polyoxometalates

“…Moreover, the model predicts that at constant e, i.e., in the same solvent, the size of the shell R Ã / r À2=3 . If surfactant is used to regulate the charges on the POMs [14], then a model describing adsorption of surfactant on the chargeable groups of the POM may be needed to relate the charge density with the concentration of the surfactant. For the condition of constant charge density to be valid, the maximum permissible value of surface potential should satisfy the following condition:…”

“…Recently Kistler et al [14] investigated the effect of addition of surfactants to {Mo 72 V 30 } POMs and reported the size of the shell as a function of concentration of surfactant. They used cationic surfactants such as cetyltrimethylammonium bromide (CTAB), cetyltrimethyltetradecylammonium chloride (CTAT), octyltrimethylammonium bromide (DTAB) and octyltrimethylammonium bromide (OTAB).…”

“…To induce the formation of shell, the charge density on the POMs is regulated by either using mixture of solvents [3], or by varying the pH [13], or by the addition of oppositely charged surfactants [14]. These studies showed that charge density is an important parameter in controlling the radius of the shell.…”