Is Bigger Always Better? Why Large Clay Crystals May Wreck Their Potential for Barrier Coatings

The development of high-performance nanocomposite barrier coatings hinges on the efficient exploitation of nanosheet geometry and dispersion. While Cussler’s model predicts extreme barrier improvement with increasing aspect ratio of nanosheets, experimental values frequently fall short. In this study, we identify and characterize smectic liquid crystalline domains─termed “accordions”─as critical structural defects within liquid crystalline suspensions of high aspect ratio synthetic hectorite. These vertically oriented structures represent defects penetrating otherwise lamellar, cofacially aligned nanosheet domains and thus act as gas diffusion pathways, significantly reducing barrier performance. We develop an ion-exchange strategy using NH4+ to selectively eliminate these accordions via interstratification, yielding double stacks that can be subsequently redelaminated into monolayers. Despite a reduction in nanosheet diameter during this procedure that is expected to hamper the barrier improvement factor, in reality the resulting coatings demonstrate a 36-fold lower oxygen permeability, confirming the dominant role of accordion-type defects as permeation pinholes. These findings highlight a previously overlooked structural origin of limited barrier enhancement and provide a general route to suppress defect formation in 2D material-based barrier films.