Hydrocarbon chain conformation in an intercalated surfactant monolayer and bilayer |
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Authors: | N V Venkataraman and S Vasudevan |
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Institution: | (1) Department of Inorganic and Physical Chemistry, Indian Institute of Science, 560 012 Bangalore, India;(2) Solid State and Structural Chemistry Unit, Indian Institute of Science, 560 012 Bangalore, India |
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Abstract: | Cetyl trimethyl ammonium (CTA) ions have been confined within galleries of layered CdPS3 at two different grafting densities. Low grafting densities are obtained on direct intercalation of CTA ions into CdPS3 to give Cd0.93PS3(CTA)0.14. Intercalation occurs with a lattice expansion of 4.8 ? with the interlamellar surfactant ion lying flat forming a monolayer.
Intercalation at higher grafting densities was effected by a two-step ion-exchange process to give Cd0.83PS3(CTA)0.34, with a lattice expansion of 26.5 ?. At higher grafting densities the interlamellar surfactant ions adopt a tilted bilayer
structure.13C NMR and orientation-dependent IR vibrational spectroscopy on single crystals have been used to probe the conformation and
orientation of the methylene ‘tail’ of the intercalated surfactant in the two phases. In the monolayer phase, the confined
methylene chain adopts an essentially all-trans conformation with most of the trans chain aligned parallel to the gallery
walls. On lowering the temperature, molecular plane aligns parallel, so that the methylene chain lies flat, rigid and aligned
to the confining surface. In the bilayer phase, most bonds in the methylene chain are in trans conformation. It is possible
to identify specific conformational sequences containing a gauche bond, in the interior and termini of the intercalated methylene.
These high energy conformers disappear on cooling leaving all fifteen methylene units of the intercalated cetyl trimethyl
ammonium ion in trans conformational registry at 40 K. |
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Keywords: | Intercalation layered compound surfactant assembly conformation |
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