Transverse Layer Rigidity in Vermiculite Pillared by Mixed Alkylammonium Ions.

The mixed alkylammonium ion pillared clay system [ (CH_3)_4 N ^+] _{x}[ (CH_3)_3NH^+] _ {1-x}-vermiculites (x = 0-0.96) has been shown to be suitable for quantitatively describing the transverse layer rigidity of the host clay layers by observing the relationship between gallery height and the fraction of pillaring rm (CH_3)_4N^+ ions occupying the gallery surfaces. The mixed ion [ (CH _3)_4 N^+] _{x }[ (CH_3)_3NH^+] _{1-x}-vermiculites were prepared by the partial exchange of rm (CH_3) _3NH^+ ions in [ (CH_3)_3NH^+]-vermiculite with the appropriate amounts of rm (CH_3) _4N^+ ions. Very poor agreement was obtained between the X -ray diffraction peak positions of the observed and the calculated 001 reflections based on the Hendricks-Teller and Reynolds models for ion segregation. However, the observed patterns were in good agreement with the Bragg model for uniform ion mixing within the galleries. Also, a one-dimensional Patterson synthesis of the mixed ion system showed vector distributions corresponding to a unique d-spacing. In addition, the experimentally observed x dependence of the normalized d-spacings rm (d_{n}(x) = (d(x) - d(0))/(d(1) - d(0))) of the [ (CH_3) _4 N^+] _{x}[ (CH_3)_3NH^+] _ {1-x}-vermiculites deviated strongly from the expected behavior for an ion-segregated system. Thus, the diffraction data are consistent with the random distribution of both ions within each gallery, as opposed to ion-demixed intercalates in which the two ions occupy separate galleries. A rapid rise in d_{rm n}(x) with increasing x was observed near the threshold value of x_{rm t} = 0.2 and the maximum possible d _{rm n}(x) value of 1.0 was observed near an x value of 0.6. A model which related d_{rm n}(x) to the layer rigidity parameter p, the binding energy difference between the gallery sites and the non-gallery(defect) sites, and the ratio of the two sites yielded an excellent fit to the experimental d_{rm n} (x). A layer rigidity parameter p = 8, a binding energy difference of 2.5 Kcal/mole, and the defect site ratio of 33% were determined for the vermiculite host layers.