Effects of Organic Cation Structure on Behavior of Organobentonites

This study quantified the effects of four quaternary ammonium cations (TMA, DTMA, HDTMA, and BTEA) on the mechanical properties of a modified Wyoming bentonite. TMA, DTMA, and HDTMA are organic cations with alkyl chains of lengths 1, 10, and 16 carbons, while BTEA has a benzyl ring attached. Substitution of the organic cations onto the bentonite surfaces altered the soil's behavior by: (1) reducing the soil's specific gravity, with the specific gravity decreasing linearly as the size of the exchanged organic cation was increased; (2) dramatically reducing the soil's liquid limit (from 458% for the unmodified bentonite to 52–65% for the organoclays); (3) reducing the compressibility of the clays (from a compression index of 5.5 for the unmodified bentonite to compression indices of 0.47–0.59 for the organoclays); (4) reducing the swell potential (Cs = ∼1.5 for bentonite compared with 0.035–0.083 for the organoclays); and (5) increasing the direct shear friction angle (7° for unmodified bentonite compared with 34–37° for the organoclays).