Dodecyl pyridinium alkali metals ion exchange on muscovite mica

Model muscovite with high cation exchange capacity was prepared, the surfac e of which was saturated with a single species of alkali metal ions.. The k inetics and thermodynamics of the exchange reaction of the 1-dodecylpyridin ium ion (NDP+) with alkali metal ions onto that substrate were studied. The exchange rate depended on the type of alkali metal ions present on the sur face because of their different affinities to mica However, in all cases th e reaction was fast at the beginning and about 50% of the ions: were exchan ged within one hour; then the reaction rate decreased and equilibrium was o nly reached after several hours. This was attributed to a rate-determining rearrangement step in which the alkyl chains rearrange to adopt a dense pac king. The reactivity of the alkali metal ions was in the order Li+ > Na+ > K+ > Rb+, Cs+, and in the case of K+, Rb+, and Cs+, equilibrium was only re ached after 72 h. The lithium and sodium ions were exchanged almost quantit atively until a saturation value was nearly reached, while the K+, Rb+, and Cs+ exchange isotherms were less steep. The equilibrium constants (K) as w ell as the ion exchange capacity (S) were calculated by least-squares fits. Since K is infinite for quantitative exchange and decreases asymptotically upon deviation from this ideal behavior, the high K values (>10) of the ND P+/Li+ and Na+ exchange cannot be accurately determined. K ranges between 1 and 3 for the NDP+/K+, Rb+, and Cs+ exchange. The affinity of NDP+ to musc ovite was similar or slightly higher than that of K+, Rb+, and Cs+, but was much higher than that of Li+ and Na+. The presence of oxonium ions in wate r did not strongly influence the exchange reaction on delaminated mica, as in the case of mica sheets, due to its high CEC. (C) 1999 Academic Press.