Morphological properties of vermiculite particles in size-selected fractions obtained by sonication

The objective of this study is to determine the morphological properties of vermiculite particles obtained by sonication followed by separation into different size fractions (0.1–0.2, 1–2 and 10–20 μm). These size fractions were chosen because they represent the wide range of particle sizes encountered in natural media. As previously reported, the crystal-chemistry of this vermiculite from Santa Olalla (Huelva, Spain) was found to be unaffected by the sonication process, although a loss of crystallinity and an increase in structural disorder might have occurred for the finest fraction. Using low-pressure argon adsorption analyses and a suite of microscopy techniques (i.e., atomic force, optical and transmission electron microscopy), the geometrical properties of the particles, including the distribution of the basal and lateral surfaces and aspect ratios, were obtained. Good agreement was observed between the gas adsorption experiments when applying the derivative isotherm summation procedure and the atomic force or optical microscopy analyses. However, the surface area was overestimated by transmission electron microscopy, which was attributed to the sample preparation technique. All of these data demonstrated that the particle aspect ratios are similar for all fractions, and the finest particles tend to show slightly more elongated surface morphologies than the coarser particles. Finally, a comparison of the distribution of lateral, basal and interlayer adsorption sites to the experimental cation-exchange capacity (CEC) showed that only the basal and interlayer sites are available in the conditions of CEC analysis. Based on these findings, the pertinence of using these size fractions as models to better understand the interactions between swelling clay minerals and water/solutes is discussed.