Colloidal aggregation and structural assembly of aspect ratio variant goethite (α-FeOOH) with nC60 fullerene in environmental media.

Environmental mobility of C 60 fullerene can be significantly affected in the presence of naturally abundant α-FeOOH. However, α-FeOOH vary significantly in sizes, shapes and associated properties that can greatly influence the fate and transport of C 60 fullerene in environmental media. Therefore, colloidal hetero-association between well crystallized low aspect (L Asp ) α-FeOOH and nC 60 fullerene may differ substantially to weakly crystallized high-aspect (H Asp ) counterpart. In contrast to L Asp α-FeOOH, inherent crystal defects and surface charge generation in H Asp α-FeOOH facilitated strong Coulombic attraction and aggregation with fullerene in acidic pH. However, L Asp α-FeOOH demonstrated subtle entropic depletion mediated interaction with fullerene prevalent in hard rods. Humic acid (HA) encapsulation of H Asp α-FeOOH substantially blocked fullerene attachment. Minute enhancement in colloidal stability was detected for HA-coated H Asp α-FeOOH and fullerene mixture to HA-coated H Asp α-FeOOH alone. To investigate the interfacial assembly of α-FeOOH with fullerene “ in situ” differential interference contrast (DIC) microscopic investigations were employed. This study showed significantly different interface behavior of the binary mixtures of fullerene and H Asp α-FeOOH NPs, and L Asp particles. On air-water interface, bare H Asp α-FeOOH displayed liquid crystalline packing. However, addition of fullerene to H Asp α-FeOOH suspension at pH5 produced closed-loop polygonal and circular ring structures. Head-to-tail alignment of magnetic dipoles as well as fullerene hydrophobicity facilitated such assembly formation. “ Ex situ ” AFM investigation revealed further the presence of magnetically derived ring structure which asserts that the formed “ in situ ” ensembles were not transient, hence, may abate fullerene transport through environmental interfaces. Barring hydrophobicity assisted attachment of fullerene to L Asp α-FeOOHs, the absence of any close-packed structures may unlikely abate fullerene transport as envisaged in case of H Asp α-FeOOH. Thus, aspect ratio variation and associated material properties of naturally abundant α-FeOOH may significantly impact fullerene transport through environmental media. [ABSTRACT FROM AUTHOR]