Solid Solution Formation in Amphiphilic Organic−Inorganic Clay Heterostructures

Abstract

Amphiphilic mixed ion smectite clays have been prepared by the reaction of inorganic ion exchanged forms of synthetic fluorohectorite with [C16H33EBu3+] onium ions, where E is N or P. Regardless of the initial inorganic exchange cation (Li+, Na+, Ca2+, Ba2+, Al3+, Ce3+), heterostructures with regularly stacked galleries of organic and inorganic ions were formed at fQ = 0.50, where fQ is the fraction of surfactant onium ions. The stagelike segregation of cations into separate galleries is attributable to the steric restrictions of the headgroup and to hydrophobic interactions between the alkyl groups of the onium ions. The same 1:1 mixed ion intercalates were formed by ion redistribution reactions of the homoionic end-member clays, indicating their thermodynamic stability. Decreasing the fraction of onium ions from fQ = 0.50 to 0.33 resulted in the displacement of up to 30% of the onium ions (C16H33PBu3+) in the organic galleries by inorganic cations (Na+). The same gallery stacking sequence as in a 1:1 mixed ion heterostructure was observed, signifying that up to 30% of the onium ions in the organic galleries can be replaced by inorganic cations to form solid solutions. This alloylike mixing of gallery organic and inorganic ions appears to be unique among metal oxide intercalation compounds. Phase segregation of the parent end-member clays was observed at compositions outside the heterostructure composition range. By combining the hydrophobic and hydrophilic properties of organic and inorganic clays into a single heterostructured phase, we expect these amphiphilic materials to find applications as versatile ion exchangers, sorbents, and nanoparticle reagents.