Symmetric C-C stretching mode splitting versus CH2-chain conformation order in sodium montmorillonite modified by cetyltrimethylammonium bromide.

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 spectroscopic characterization of the interlayer structure of Na+-montmorillonite clay modified by ditetradecyl dimethyl ammonium bromide.

Raman spectroscopy has been applied for the rapid and nondestructive monitoring of the interlayer structure of sodium montmorillonite (MMT) clay modified by ditetradecyl dimethyl ammonium (DDA+) bromide. This work demonstrates that a detailed analysis of Raman spectra in the fingerprint region (600-1600 cm(-1)), in combination with model simulation, allows one to distinguish different conformational states of DDA+ in the interlayer space of the modified clay, namely, a liquidlike state but rich in trans conformers, disordered conformational states, and a crystallike conformation appearing at increasing modifier content. These conformations differ in the angle between their alkyl chains, the relative content of trans and gauche conformers and the relative length of trans segments. The shape and width of the Raman band at 1300 cm(-1) and the peak intensity ratio I1088/I1064 can be used for a qualitative analysis of the ratio of gauche/trans conformers. The integral intensity ratios I*1064/I*1300 and I*1300/I*705 help to determine the proportion of trans conformers and the content of the modifier in the clay, respectively, thereof providing quantitative characterization of the modified clay (conformational reorganization and modifier content). Noteworthy, the transition from a liquidlike to crystal-like conformation is further supported by the splitting of the symmetric C-C stretching Raman band of the trans segments within the alkyl chains at 1133 cm(-1) (liquidlike conformation) into two modes at 1124 and 1135 cm(-1) corresponding to two parallel trans chains of nonequivalent lengths (crystal-like conformation).