Smectite in bentonite: Near infrared systematics and estimation of layer charge

The facile and accurate measurement of the layer charge of dioctahedral smectite has been demonstrated recently by means of the O-D method, an infrared-based technique capable of recording the exact position of a dangling O-D (OH) bond stretch of interlayer water. The present work attempts to transfer this know-how to the near-infrared (NIR) spectra of natural bentonites in anticipation of even higher throughput capabilities. For this reason, the NIR systematics of ~70 bentonites in both natural and Na-exchanged form have been analyzed against the mid-infrared-derived layer charges of the same samples. The as determined layer charges were not biased by the nature of the interlayer cations (Ca2+, Mg2+, Na+). A linear correlation between the (ν+δ) combination modes of H2O and Al2OH at ~5250 and 4525cm−1, respectively, was established, confirmed on a broader collection of bentonite samples, and discussed in terms of interlayer and layer chemistry. The (ν+δ) H2O wavenumber was found to correlate linearly with layer charge, but the slope was dependent on interlayer cation due to the cation-dependence of the bending δ Η2Ο fundamental mode. Additionally, PLS chemometric modeling of the NIR spectra was employed successfully to provide layer charge predictive algorithms, desensitized from the effect of interlayer cation (Ca2+, Mg2+, Na+).