Interlayer structure of organically modified montmorillonites: Effect of surfactant loading
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The surfactant cetyltrimethylammonium ion (CTA+) was confined within the galleries of montmorillonite (MMT) to obtain a series of organo-montmorillonites (C16-MMTs) through an ion-exchange intercalation reaction. The C16-MMT formed a single precipitate layer when CTA+ loading was 18.3 wt% but stratified at high loadings. The conformational disorder increased with increasing CTA+ loading. The upper precipitate was characterized by a larger gallery height and a higher surfactant loading in comparison with lower precipitate. The confined methylene chains adopted a lateral monolayer with a small percentage of conformation freedoms at CTA+ loading of 18.3 wt%. The intercalated methylene chains were arranged either in a lateral monolayer or in a tilted interdigitated bilayer at CTA+ loading of 24.7 wt% while in either a tilted interdigitated bilayer or a lateral bilayer at high CTA+ loadings. The different arrangements of methylene chains intercalated in the MMT galleries are believed to be the reason for the stratification.
I. INTRODUCTION
Layered solids have been attracting considerable interest as an ideal model for studying of confined molecules and polymer melts over the past decades.1–9 They are also involved in a variety of industrial applications such as electronics, nonlinear optics, cosmetic, catalysis, and water treatment.3,4 In most of these applications, intercalation of surfactant molecules converts the essentially hydrophilic internal surface within an inorganic layered solid into an organophilic one, which renders the surface energy and leads the materials more compatible with non-polar polymers especially.10–12 The gallery structure of organo-layered solids has been extensively investigated using a variety of technologies13–32 and more recently, computer simulations.33–36 The intercalated alkyl chains might adopt a paraffin-type monolayer,7,16 a lateral monolayer,23 or a lateral bilayer,27,37,38 depending on the packing density,23 the temperature and the chain length of methylene “tail”.39 Therefore, alkyl chains in galleries might behave as solid-like, liquid-crystalline, or liquidlike. 7 Alkylammonium surfactants of different chain length exhibit multi-layer density distributions in layered silicates of different cation exchange capacity (CEC).33,35 Understanding of the interlayer structure of the organo-layered
solids provides guidelines for designing materials with better properties and wider applications. Nevertheless, it still remains unclearly in the molecular arrangement, the packing density, the interchain interaction as well as their dependence on the available space per molecule. As a commonly used layered silicate, montmorillonite (MMT) consists of two silica tetrahedral sheets sandwiching an edge-shared octahedral sheet of either aluminum or magnesium hydroxide.2 Stacking of the layers (thickness ≈0.96 nm7) leads to an interlayer or gallery between two adjacent layers, which can accommodate cationic organic molecules via Coulombic interaction. Intercalation of cationic molecules results in
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