Near-infrared spectroscopy as an effective tool for monitoring the conformation of alkylammonium surfactants in montmorillonite interlayers

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• 作者：Jana Madejová ; ; ^{jana.madejova@savba.sk" class="auth_mail" title="E-mail the corresponding author} ; Ľudmila Sekerá ; ková ; ^{ludmila.sekerakova@savba.sk" class="auth_mail" title="E-mail the corresponding author} ; Valé ; ria Bizovská ; ^{valeria.bizovska@savba.sk" class="auth_mail" title="E-mail the corresponding author} ; Michal Slaný ; ^{michal.slany@savba.sk" class="auth_mail" title="E-mail the corresponding author} ; Ľubo&scaron ; Jankovič ^{lubos.jankovic@savba.sk" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Organo-montmorillonite ; Alkylammonium ; Conformation ; Chain length ; Near infrared spectroscopy
• 刊名：Vibrational Spectroscopy
• 出版年：2016
• 出版时间：May 2016
• 年：2016
• 卷：84
• 期：Complete
• 页码：44-52
• 全文大小：1984 K

文摘

Application of near-infrared (NIR) spectroscopy to probing the arrangement of trimethylalkylammonium cations in montmorillonite interlayers has been demonstrated. Detailed analysis of the mid-IR (MIR) and NIR spectra of montmorillonite from Jelšový Potok (JP, Slovakia) saturated with surfactants with varying alkyl chain length (even numbers of carbon atoms from C6 to C18) was performed to show the advantages of the NIR region in characterizing surfactant conformations. The position of the ν_as(CH₂), (∼2930–2920 cm^−1), ν_s(CH₂) (∼2860–2850 cm^−1), 2ν_as(CH₂) (∼5810–5785 cm^−1), (ν + δ)_as(CH₂) (∼4340–4330 cm^−1) and (ν + δ)_s(CH₂) (∼4270–4250 cm^−1) signals was used as an indicator of the gauche/trans conformer ratio. For all bands, a shift toward lower wavenumber on increasing the alkyl chain length from 6 to 18 carbons suggests a transition from disordered liquid-like to more ordered solid-like structures of the surfactants. The magnitude of the shift was significantly higher for 2ν_as(CH₂) (28 cm^−1) than for ν_as(CH₂) (8 cm^−1) or ν_s(CH₂) (10 cm^−1), showing the NIR region to be a useful tool for examining this issue. Comparison of the IR spectra of crystalline alkylammonium salts and the corresponding organo-montmorillonites demonstrated a confining effect of montmorillonite layers on surfactant ordering. For each alkyl chain length the CH₂ bands of the organo-montmorillonites appeared at higher wavenumbers than for the unconfined surfactant, thus indicating a higher disorder of the alkyl chains. The wavenumber difference between corresponding samples was always higher in the NIR than in the MIR region. All these findings show NIR spectroscopy to be useful for conformational studies.