Stable & re-dispersible polyacryloyl hydrazide-Ag nanocomposite Pickering emulsions

Freeze drying and re-dispersibility of oil-in-water (o/w) emulsions is important from the perspective of storage, transportation and usability. A set of stable and re-dispersible o/w emulsions using polyacryloyl hydrazide (PAHz) capped Ag nanoparticles (NP) as the stabilizer is reported in which the NP size (D_avg) and PAHz concentration collectively controlled the stability and re-dispersibility of the emulsion system. O/w emulsions prepared using different concentrations of PAHz (0.05-0.25 g mL^-1) and sizes of Ag NPs (10-25 nm) were analyzed by DLS, IFT, contact angle, SEM, and rheological studies. The emulsion stabilized by 0.05 g mL^-1 of PAHz and Ag NPs (D_avg ≈ 20 nm, 5 wt% PAHz-Ag NPs) was unstable against coalescence, exhibited maximum oil leakage during freeze-drying and lacked re-dispersibility. The stability of the Pickering emulsions was inversely proportional to the D_avg of the Ag NPs. The presence of Ag NPs possessing D_avg ≈ 10 nm and 0.25 g mL^-1 of PAHz in the aqueous phase (25 wt% PAHz-Ag NPs) stabilized the Pickering emulsions for up to 30 days without any sign of creaming and the oil powders (oil content ≈ 98%) obtained after freeze drying exhibited adequate re-dispersibility in aqueous media. In addition, the emulsion stabilized by 25 wt% PAHz-Ag NPs showed maximum recovery of the viscosity value after re-dispersion and exhibited similar shear-thinning behavior to that of the original sample. Furthermore, the trends of moduli vs. frequency for the re-dispersed samples were similar to that of the original samples suggesting that the structural arrangements between Ag NPs and oil droplets were least affected by the drying process. Thus, we conclude that o/w emulsions stabilized by PAHz-Ag NPs can be a potential alternative to produce stable oil powders or gels for industrial applications.