Molecular Simulation of the Interlayer Structure and the Mobility of Alkyl Chains in HDTMA⁺/Montmorillonite Hybrids

Abstract

Molecular simulation techniques are used to find the basal spacing of organoclay on the basis of the energy minimum, using the canonical NVT ensemble. Then, the interlayer structure and mobility of alkyl chains are explored based on the interlayer atomic density profiles. Besides the basic lateral-monolayer arrangement, lateral-bilayer accompanied by partial a pseudo-trilayer and a transition structure between the two basic lateral models are observed. The later provides an excellent explanation about the reflection at 16 Å on XRD patterns in the literature. The atomic density profiles reveal that nitrogen atoms show stronger layering behavior than carbon atoms do. Our simulation demonstrates that the molecular mobility of the confined alkyl chains decreases from lateral-monolayer to lateral-bilayer with the increase of the intercalated surfactant. This is in accordance with the suggestion deduced from experiments. Furthermore, our simulation indicates that the mobility of the alkyl chains strongly depends on the surfactant arrangement rather than the surfactant packing density.

The influence of alkyl chain length on surfactant distribution within organo-montmorillonites and their thermal stability

“…In the case of NM-C18-2.0CEC, half of the surfactant was cations while the other half was in the form of ionic pairs. It has been shown that the ammonium head groups of surfactant cations were close to clay mineral surface due to the coulomb interaction in previous simulations [15][16][17]. The same phenomenon was found in the present simulation.…”

“…In the case of NM-C18-S-4.0, the montmorillonite interlayer spaces are dramatically expanded from 1.5 nm (original Ca-montmorillonite) to 4.10 nm (NM-C18-S-4.0) and a paraffin bilayer arrangement of the intercalated surfactants was adopted, providing big space for adopting surfactant molecules (ionic pairs). However, in the case of NM-C8-S-4.0, the basal spacing is similar to that of the used Ca-montmorillonite, indicating a lateral monolayer arrangement of the intercalated surfactant [8,11,16]. In this case, the space for adopting surfactant molecules (ionic pairs) is limited.…”

“…Various structural models of organoclays have been proposed by using experimental technologies (e.g. XRD, FTIR, NMR, and TEM) [8][9][10][11][12][13][14] and molecular modelling methods [15][16][17]. And their thermal stabilities have been examined by using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) [13,[18][19][20][21][22][23][24][25], TGA combined with infrared spectroscopy and mass spectrometry (TGA-FTIR-MS) [26,27], and TGA combined with pyrolysis/ GC-MS [28].…”

Expansion characteristics of organo montmorillonites during the intercalation, aging, drying and rehydration processes: Effect of surfactant/CEC ratio

“…In the following drying process, the relative intensity of the first reflection is shown to continually decrease in the first 6 h. Finally only one reflection appears after 6 h drying, and this reflection also shifts to smaller basal spacing value as the drying process continues. After 12 h drying, the basal spacing value of OMt0.8 appears at 1.68 nm, which means CTMA can also adopt a transition structure between lateral-monolayer and lateral-bilayer [39]. The interesting result shown in Fig.…”

“…After 12 h drying the sample shows a basal spacing value of 1.62 nm. According to He et al [39], this should correspond to a transition structure between lateral-monolayer and lateral-bilayer for CTMA. In the rehydration process, OMt0.6 displays two reflections again at ∼2.70 nm and ∼1.90 nm, respectively.…”