STRUCTURE OF MONTMORILLONITE CO-INTERCALATED WITH OCTADECYLAMINE AND STEARIC ACID

M. Pospíšil¹, P. Čapková¹

¹Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, 12116 Prague 2, Czech Republic

One way to find new suitable precursors for nanocomposite materials based on polymer/clay is modifying the interlayer space of clays, especially silicates like montmorillonite, with different types of organic molecules. In this work stearic acid and octadecylamine species were used for intercalation of Na-montmorillonite. Na-montmorillonite was co-intercalated by two different ways. The first one three powder substances: octadecylamine (ODA), stearic acid (STA) and Na-montmorillonite (MMT) were mixed with molar ratio 1:1:2, carefully homogenized and heated at 80 °C for one hour. The second one Na-MMT was firstly intercalated with ODA to the stage with basal spacing 33 Å and subsequently the powder mixture of ODA-MMT intercalate and STA was heated at 80°C for one hour. The molar ratio of the individual components was the same. Stearic acid itself does not intercalate into Na-montmorillonite.

Structure analysis was carried out using combination of experimental methods like X-ray powder diffraction and IR spectroscopy with molecular modeling (force field calculations) in Cerius² modeling environment. Results of structure analysis revealed the chemical reaction of guest compounds leading to the formation of octadecylammonium stearate (ODST). This reaction may occur even before the intercalation out of the interlayer space of montmorillonite. The presence of ODST in the samples was clearly confirmed by IR spectroscopy and X-ray diffraction.

Present result showed that both ways of co-intercalation leads to the similar result. In the interlayer space of montmorillonite the formation of ODST salt is created. This structure of Na-montmorillonite intercalated with ODST does not create a stable arrangement in the interlayer space. Intercalated Na-MMT exhibits highly disordered structure with a maximum basal spacing within the range 27-39 Å. This structure is very unstable and exposure to ambient conditions has two consequences. First of all intercalate is gradually decomposed, that means part of ODST escapes from the interlayer space and the second this deintercalation is accompanied by the rearrangement of the interlayer guest structure and probable formation of the Na-stearate and octadecylammonium cations in the interlayer space. Molecular modeling showed that STA has stronger interaction with Na-cations, than with octadecylammonium cations and formation of Na-stearate in the interlayer space is highly probable. The formation of Na-stearate in the interlayer space is accompanied with rising of octadecylammonium cations compensating the layer charge. The consequence of these changes is the rearrangement of polar head-groups and gradual rearrangement of the guest structure in the interlayer space. As a result of the deintercalation we obtain a mixture of ODST salt, which escapes from the interlayer space, and the intercalated Na-MMT, where the guests are: ODST salt, Na-stearate and octadecylammonium cations. This structure of intercalate is characterized by powder diffraction pattern after 4 days in ambient conditions, where one can see the small peak at 48 Å corresponding to the ODST salt, and peak at 28 Å corresponding to the liquid like arrangement of the ODST and Na-stearate and peak at 18 Å corresponding to the MMT intercalated with octadecylammonium. Anyway the character of the interlayer structure, i.e. the monolayer liquid like arrangement of the chains does not promise the easy exfoliation required for polymer-clay nanocomposite precursor.