The charge of wettable illite-smectite surfaces measured with the O-D method

Abstract The aim of the present study was to test the applicability of the O-D method, a newly developed spectroscopic layer charge measurement technique, to measure the charge present on wettable surfaces of mixed-layer illite-smectite (I-S). The O-D method employs the exact position of the sharp, high frequency O-D stretching band of adsorbed D2O at 2685–2700 cm−1, which was found to correlate linearly with the layer charge for dioctahedral smectites. The O-D method has therefore potential to measure the charge present on the wettable surfaces of I-S crystallites, i.e. surfaces of smectite monolayers and external surfaces of illite fundamental particles. The O-D method was found to be applicable to mixed-layer I-S. The I-S' surface charge was measured for 41 well-documented I-S samples coming from bentonites and hydrothermal deposits, and representing smectite contents between 8 and 100%. The measured charge averaged 0.47 ± 0.03 e− per formula unit and was very narrowly distributed irrespectively of smectite content or sample origin. Such a result is close to earlier estimates based on chemical analyses. It was concluded that the O-D method measures only the surface charge of fundamental particles, without the influence of the illitic charge from the K-occupied interlayer. No difference in the surface charge among samples with and without smectite monolayers was observed, which is in agreement with the polar model of illite fundamental particles and confirms the smectitic nature of their basal surfaces. On the other hand, a systematic increase of surface charge upon decreasing fraction size was noticed for ultrafine fractions. This trend suggests a low polarity of illite fundamental particles at the initial stages of nucleation.