As you're starting with a dry material then laser diffraction (Mastersizer) will be the preferred technique. For further background see this webinar (free registration required):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

Methodology for laser diffraction is given in:

PST & BDAS - An acronym approach to laser diffraction method development

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190815PST

Hi!

The high polydispersity and sedimentation observed in your 0.5% w/w nanoclay suspensions, despite sonication, likely stem from incomplete exfoliation and heterogeneity in particle size distribution, common in montmorillonite and bentonite dispersions. Since DLS and zeta potential measurements require stable, monodisperse suspensions, working only with supernatants may bias results toward smaller, more dispersed particles, underrepresenting the bulk sample behavior. To improve stability and measurement accuracy, consider lowering the concentration (e.g., to 0.05–0.1% w/w), adjusting pH (e.g., to ~4 or ~10 depending on application), and using dispersants like sodium hexametaphosphate or tetrasodium pyrophosphate to prevent agglomeration. Extended probe sonication under controlled temperature with intermittent pulsing (to avoid heating) can aid delamination. Additionally, centrifuging at low speed to remove only coarse aggregates prior to DLS, rather than relying solely on supernatants, may provide a more representative and measurable colloidal fraction.