STAGING OF ORGANIC AND INORGANIC GALLERY CATIONS IN LAYERED SILICATE HETEROSTRUCTURES

Heterostructured fluorohectorite clays with regularly alternating inte rlayers of inorganic and organic exchange cations have been prepared t hrough ion exchange reaction of the sodium clay in aqueous suspension with half an equivalent of alkylammonium ions, In order to elucidate t his staging-like behavior of the intercalates, the exchange process wa s examined for a series of onium ion surfactants of the type [CnH((2n1))N(CmH(2m+1))(3)](+). One set of onium ions varied the alkyl chain l ength (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 c hain length remained fixed (n = 16), Two fundamental factors, namely, alkyl chain length and head group size determined staging behavior, Th e surfactant alkyl chain length determined the extent of Na+ replaceme nt by onium ions, Relatively little Na+ exchange (similar to 10%) occu rred for the short chain onium ions with n = 4, m = 4 and n = 6, m = 4 , The replacement of Na+ by onium ions became more favorable as the al kyl chain length was increased to n = 8 or 10, For very hydrophobic su rfactants with n greater than or equal to 12, onium ion uptake was ess entially 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 in tercalates, For m greater than or equal to 3 the head group footprint matched the clay surface charge density and precluded the mixing of or ganic and inorganic exchange cations within the same gallery, The segr egation of the hydrophobic organic cations and the hydrophilic inorgan ic cations on the internal and external surfaces of two-nanolayer tact oids during the ion exchange process was proposed as the pathway for s taging behavior in the heterostructured intercalates, The smaller foot prints of trimethyl and triethyl onium ion head groups (m = 1, 2) allo wed for comingling of the exchange ions on single-nanolayer tactoids a nd the formation of phase - segregated organic and sodium ion clays up on stacking of the tactoids. (C) 1998 Academic Press.