MEASURING THE LAYER CHARGE OF DIOCTAHEDRAL SMECTITE BY O-D VIBRATIONAL SPECTROSCOPY

摘要

Layer charge (LC) is a fundamental property of smectite but its measurement remains challenging and tedious to apply on a high-throughput basis. The present study demonstrates that the position of a sharp, high-energy O-D stretching band of adsorbed D2O (nu O-D, at similar to 2686-2700 cm(-1)), determined by infrared spectroscopy, correlates with LC and provides a simple method for its measurement. Twenty nine natural dioctahedral smectites and 14 reduced-charge montmorillonites with LC determined previously by different methodologies were saturated with D2O and examined by attenuated total reflectance infrared spectroscopy (ATR-IR). The samples included smectites in Mg, Ca, Na, Li, K, and Cs forms and covered the full range of the smectite LC (0.2 to 0.6 e per formula unit). Statistically significant correlations were found between nu O-D and LC values determined with each of the two main methods of LC determination: the structural formula method (R-2 = 0.96, sigma = 0.02, similar to 0.2 < LC < 0.6) and the alkylammonium method (R-2 = 0.92, sigma = 0.01, 0.27 < LC < 0.37). These correlations were based on Li- and Na-saturated smectites, respectively, but other cationic forms can be employed provided that the exchangeable cations are of sufficiently high hydration enthalpy (e.g. Mg2+ or Ca2+, but not K+ or Cs+). The new method is fast, low-cost, implemented easily in laboratories equipped with ATR-FTIR, and applicable to samples as small as similar to 5 mg.