Quantitative Analysis of Montmorillonite Platelet Size by Atomic Force Microscopy

Abstract

This work focuses on the use of atomic force microscopy (AFM) for quantitative analysis of the size of suspended, exfoliated platelet materials. In particular, we study a model materialmontmorillonite (MMT) clayand use tapping-mode AFM with image analysis software to quantify the distributions of MMT platelet thickness and aspect ratio. X-ray diffraction (XRD), transmission electron microscopy (TEM), and AFM confirm that, upon dispersion in water, MMT particles exfoliate into stable platelet suspensions, as is well-known. We use dry weight analysis, dynamic light scattering (DLS), and AFM to assess the impact of sample preparation procedures for exfoliation and obtaining high-quality AFM images. Unexfoliated MMT and other contaminant particles can be removed by low-speed centrifugation; however, large exfoliated platelets are increasingly removed as centrifugal acceleration increases. Dilution of MMT suspensions with acetone prior to deposition onto mica leads to high-quality AFM images with many isolated platelets. Quantitative analysis of the data from many AFM images shows that the distribution of MMT platelet thickness is narrow and centered near 1.0 nm. The distribution of platelet aspect ratio closely follows a log-normal distribution, which has not been previously reported in the literature for MMT. We find that the average aspect ratio measured by AFM correlates linearly with the effective spherical particle diameter measured by DLS.