Intercalation of montmorillonites with a mixture of intercalates has not been studied extensively. The objective of the present investigation was to study the effects of phosphonium-based intercalate mixtures on the properties (organic loading and basal spacing) of montmorillonite. These phosphonium-intercalated montmorillonites are promising candidates as high-temperature stable nanofillers for application in clay polymer nanocomposites.

Two salts with different cationic heads and chain lengths were mixed in varying molar ratios and the mixtures were intercalated into the interlayer space of montmorillonite. Two sets were chosen based on the chain length and the cationic head-group structure of the two intercalated salts (referred to hereafter as set 1 and set 2). The resultant intercalated montmorillonite was characterized by thermogravimetric analysis, X-ray diffraction, and transmission electron microscopy. The organic loading of the intercalated montmorillonite increased with the proportion of longer carbon-chain intercalate in the mixture. The intensity of the characteristic XRD peak of each intercalate varied with the mole fraction percent of that intercalate in the solution mixture. No marked synergistic effect of the intercalate mixture on the basal spacing and organic loading properties of the intercalated montmorillonite was observed — the proportional influence of individual components was found to be more prominent.

A comprehensive study of the intercalation of organo-phosphonium salts into Thai bentonite (Mt) was conducted to investigate the influence of the molecular structures of organic moieties, including chain types (alkyl vs. aryl), chain length, and structural symmetry, on their intercalation. A series of quaternary phosphonium salts with systematically varied molecular structures (tetraphenyl phosphonium, TPP-Br; tetrabutyl phosphonium, TBP-Br; tetraoctyl phosphonium, TOP-Br; methyl triphenyl phosphonium, MTPP-Br; and butyl triphenyl phosphonium, BTPP-Br) was intercalated into Mt via an ion-exchange reaction. From thermogravimetric analysis results, tetrabutyl phosphonium-modified Mt (TBP) with shorter alkyl chain length began to decompose at a slightly lower temperature (263 vs. 351°C), yet showed comparable thermal stability (i.e. maximum decomposition temperature) at 470°C, compared to tetraoctyl phosphonium-modified Mt (TOP). Aryl phosphonium-modified Mt (TPP) showed a higher thermal decomposition temperature (576 vs. 470°C) than those of alkyl phosphonium-modified Mts (TBP and TOP). Introducing short alkyl chains into the aryl phosphonium moiety (MTPP, BTPP) caused a slight decrease in thermal decomposition temperature, but an increase in cation loadings of their modified Mts (71 and 73%, respectively). X-ray diffraction analysis showed that the flexibility of alkyl chains in TBP yielded smaller increases in basal spacing, i.e. lower degree of intercalation, compared to the rigid aryl structure in TPP. Increasing chain length resulted in greater basal spacing in alkyl phosphonium-modified Mts (1.67 nm. in TBP vs. 2.46 nm. in TOP). Such an effect, however, was less significant in aryl phosphonium-modified Mt.