Staging of Organic and Inorganic Gallery Cations in Layered Silicate Heterostructures

Abstract

Heterostructured fluorohectorite clays with regularly alternating interlayers of inorganic and organic exchange cations have been prepared through ion exchange reaction of the sodium clay in aqueous suspension with half an equivalent of alkylammonium ions. In order to elucidate this staging-like behavior of the intercalates, the exchange process was examined for a series of onium ion surfactants of the type [CnH(2n+1)N(CmH(2m+1))3]+. One set of onium ions varied the alkyl chain length (n=4–22), while maintaining a constant onium head group size (m=4). A second set varied the head group size (m=1–5), while the chain length remained fixed (n=16). Two fundamental factors, namely, alkyl chain length and head group size determined staging behavior. The surfactant alkyl chain length determined the extent of Na+replacement by onium ions. Relatively little Na+exchange (∼10%) occurred for the short chain onium ions withn=4,m=4 andn=6,m=4. The replacement of Na+by onium ions became more favorable as the alkyl chain length was increased ton=8 or 10. For very hydrophobic surfactants withn≥12, onium ion uptake was essentially quantitative, affording 1:1 mixed Na+–onium ion intercalates. The “footprint” or area covered by the onium ion head group on the interlayer surface controlled heterostructure formation for the 1:1 intercalates. Form≥3 the head group footprint matched the clay surface charge density and precluded the mixing of organic and inorganic exchange cations within the same gallery. The segregation of the hydrophobic organic cations and the hydrophilic inorganic cations on the internal and external surfaces of two-nanolayer tactoids during the ion exchange process was proposed as the pathway for staging behavior in the heterostructured intercalates. The smaller footprints of trimethyl and triethyl onium ion head groups (m=1,2) allowed for comingling of the exchange ions on single-nanolayer tactoids and the formation of phase–segregated organic and sodium ion clays upon stacking of the tactoids.