Symmetric C–C Stretching Mode Splitting versus CH₂-Chain Conformation Order in Sodium Montmorillonite Modified by Cetyltrimethylammonium Bromide

Abstract

Exploiting Raman spectroscopy and computational modeling, for the first time, we report and explain an interesting phenomenon in clay modified by cetyltrimethylammonium bromide. A splitting of the CH(2)-chain's symmetric C-C stretching Raman mode found at ~1128 cm(-1) in cetyltrimethylammonium bromide into two bands at 1128 and 1139 cm(-1) in clay modified by cetyltrimethylammonium bromide is observed. We demonstrate that this splitting appears if two types of trans-segments with nonequivalent lengths and terminal groups coexist in the CH(2)-chain of the alkylammonium ion embedded into the clay interlayer space. We report Raman experimental evidence for a CH(2)-chain bending within the clay galleries, resulting in the symmetric C-C stretching band splitting, as was also suggested by computational modeling. Noteworthy, we postulate that this unique behavior based on CH(2)-chain bending provides a general understanding of conformation reorganization and switching within long CH(2)-chain molecules confined within modified clay interlayer galleries. For all modifier concentrations, we show that the intercalated cetyltrimethylammonium ions exist in a liquid-like state, consisting mainly of trans conformations (~86%) of two types in approximately equal proportions. Moreover, we demonstrate that the integral Raman intensity ratio I(1295)(CH(2))/I(705)(clay) provides a rapid nondestructive quantification of the relative content of alkylammonium ions in modified clays. These results demonstrate that a simple direct monitoring of specific modifier-dependent interlayer conformational states is possible, which is of great importance for a tunable fabrication of modified clays-based nanocomposites with desired properties.

Raman Spectroscopy Study of Structurally Uniform Hydrogenated Oligomers of α-Olefins

“…The Raman-active vibrations, whose frequencies depend on a chain length of alkanes and related substances (fatty acids, alkenes, alkylammonium compounds, bromoalkanes, etc. ), belong to the longitudinal acoustic mode (LAM), disorder LAM (D-LAM), and symmetric C–C stretching mode [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ].…”

“…The relation between the wavenumber of the symmetric C–C stretching mode, further the C–C mode, and the chain length is more complex [ 7 , 18 , 19 , 20 , 21 ] as compared with that for the LAMs.…”

“…Here ν ∞ is the wavenumber of this mode for a hypothetical infinite trans -chain, β = s /π ac , where a is a repetition period in the alkyl chain, s is the velocity of a light wave through the trans -chain, and c is the velocity of light in a vacuum. This formula excellently describes the relation between the C–C mode wavenumber and the total number of carbon atoms in the molecule for linear alkanes [ 18 , 19 ]. However, experimental studies showed that the C–C mode wavenumber is strongly influenced by “perturbations” in the alkyl chain, such as a double C=C bond in alkenes [ 20 ], a heavy end group, or a chain branching [ 19 , 21 ].…”

“…This formula excellently describes the relation between the C–C mode wavenumber and the total number of carbon atoms in the molecule for linear alkanes [ 18 , 19 ]. However, experimental studies showed that the C–C mode wavenumber is strongly influenced by “perturbations” in the alkyl chain, such as a double C=C bond in alkenes [ 20 ], a heavy end group, or a chain branching [ 19 , 21 ]. As a consequence, breaking of this law is observed for different substances, containing alkyl chains, such as the hydrogenated 1-hexene oligomers [ 7 ], alkyl ammonium compounds [ 19 ], and trans -alkenes [ 20 ].…”

“…However, experimental studies showed that the C–C mode wavenumber is strongly influenced by “perturbations” in the alkyl chain, such as a double C=C bond in alkenes [ 20 ], a heavy end group, or a chain branching [ 19 , 21 ]. As a consequence, breaking of this law is observed for different substances, containing alkyl chains, such as the hydrogenated 1-hexene oligomers [ 7 ], alkyl ammonium compounds [ 19 ], and trans -alkenes [ 20 ]. For instance, the observed C–C mode wavenumber of single-chain alkyl ammonium compounds increases with the increasing n [ 19 ], in clear contradiction with the law (2).…”