Preparation of Janus film for fog water collection via layer-by-layer assembling of nanocellulose and nanochitin on PLA

In order to explore the possibility of natural carbohydrate polymers as a biodegradable and sustainable fog water harvesting material, this work proposed an efficient substrate (hydrophobic)-transition layer (amphoteric)-coating (hydrophilic) sandwich spin-coating strategy to form all biomass-based Janus film. The oxalic acid hydrolyzed nanochitin (OAChN) was applied as a transition layer that enabled successful spin-coating of the hydrophilic nanocellulose (TEMPO-oxidized cellulose nanofiber, TOCN) and nanochitin (partially deacetylated chitin nanofibers, DEChN) on the hydrophobic polylactic acid (PLA) film substrate. In which a layer-by-layer (LBL) assembling of TOCN (carboxyl-rich negative surface charge) and DEChN (amino-rich positive surface charge) was designed to form a thickness and surface property controllable polysaccharide coating on PLA. The finally formed PLA-OAChN-TOCN/DEChN (LBL) film showed hydrophilic and hydrophobic heteromeric faces at the opposite sides and thus had improved fog water collection capacity of 90.85 mg·cm-2·h-1 (30 layers of TOCN/DEChN spin-coated on PLA), which was 276 % higher than the pure PLA film. The transition layer engaged sandwich spin-coating strategy, together with LBL assembling method proposed in this study provided a feasible fabrication of all biomass-based fog water collectors (FWC) that could contribute to alleviating water shortage.