The dissolution of hectorite: In-situ, real-time observations using atomicforce microscopy

The dissolution of individual hectorite (a trioctahedral smectite) particle s has been observed at molecular scales in acidic aqueous solution with ato mic force microscopy (AFM). A new sample preparation technique was used to attach nanometer-sized hectorite particles to a mica substrate. The reactiv e surface area of individual hectorite particles was identified and its cha nge during a dissolution experiment was quantified. The dissolution of hect orite under pH 2 conditions takes place exclusively at the edge surfaces. I n contrast, the basal surface is completely unreactive within the investiga ted time scale of several hours. The short edges of the hectorite laths wer e found to react somewhat more quickly than the long edges. The edge surfac e area represents 1.5-3.3% of the total surface area. The total surface are a has been determined from the actual particle dimensions derived from AFM data to be 730 m(2)/g. The dissolution rate normalized to the reactive edge surface area (ESA) has been determined to be 7.3 x 10(-9) mol hectorite/(m (2)-s), which represents a total surface area (TSA) normalized dissolution rate of 1.9 x 10(-10) mol hectorite/(m2-s). The ESA/TSA ratio increases by about 15% within 1 h exposure to a pH 2 aqueous solution at 22 degrees C.