Induction of Superhydrophobicity in a Cellulose Substrate by LbL Assembly of Covalently Linked Dual-Sized Silica Nanoparticles Layers

Micro/nano texturized oxidized cellulose membranes (MNOCM) were constructed by layer-by-layer (LbL) assembly in which a base cellulose film was modified by covalent linkages to amino-functionalized silica nanoparticles (amino-SiO2 NPs, 260 nm diameter) and epoxy-functionalized silica nanoparticles (epoxy-SiO2 NPs, 30 nm diameter). The amino-SiO2 NPs grafted onto the MNOCM surface through a standard amidation reaction between the amino groups of the SiO2 NPs and the carboxyl groups of the MNOCM surface in the presence of EDC and NHS consequently forming a first layer of large (260 nm) nanoparticles; subsequently, it was reacted with smaller (30 nm) epoxy-SiO2 NPs. Continuous repetitions of these alternating sized silica NPs through a standard LbL approach lead to a highly micro/nano-texturized MNOCM film as shown by SEM, which was ultimately sealed with a layer of hydrophobic PFOTES (1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane). Although the wettability of MNOCM was no longer hydrophilic, it was found that at five layers deep of NPs, it became superhydrophobic as evidenced by a water contact angle of 151° ± 2° and slide angle of 4°. The change in wettability was attributed to increases in final LbL layer surface roughness induced by the sufficient LbL layering of alternating sizes of NPs akin to what is observed in a lotus leaf surface. It was also noted that these superhydrophobic-MNOCM materials displayed good self-cleaning.