(400d) Accelerated Acoustic Prediction of Aging in Nanoemulsions

The stability of thermoresponsive materials, particularly nanoemulsions, is important in maintaining long-lasting product performance. However, traditional accelerated aging methods such as heating cannot be used to extrapolate their shelf life due to the innate temperature sensitivity of the material. We report the acoustic aging of oil-in-water thermoresponsive nanoemulsions containing 20 vol% polydimethylsiloxane droplets (diameter 2a = 38.3 nm ± 22%, viscosity η = 5 cP) dispersed in an aqueous phase including 200 mM sodium dodecyl sulfate acting as a surfactant and 33 vol% polyethylene glycol diacrylate (molecular weight M_n = 700 g mol⁻¹) as a thermal gelator. Microemulsions were prepared by dropwise addition of oil droplets to the continuous phase followed by ultrasonication at 35% power to produce thermoresponsive nanoemulsions (gelation temperature T_gel = 38.6°C). An ice bath was used during the ultrasonication process to keep the temperature below T_gel. Acoustic aging was conducted using a setup comprising a piezoelectric actuator, operating at a frequency of 20 kHz across five different acoustic power settings ranging from 0.2 W to 11.6 W. Samples were subjected to acoustic aging for a duration of one hour. Dynamic light scattering showed that for acoustically aged samples at waiting times between 40 and 60 days, droplets grew to sizes that were statistically larger than that obtained from ambient and heat aging. Dynamic measurements of the droplet size indicate that the optimal acoustic power to accelerate droplet growth was at 1.2 W and 3.6 W. Since the system is density matched, the acoustic energy is likely speeding up coalescence and flocculation processes, underscoring acoustic energy as a novel method to accelerate aging phenomena in nanoemulsions and offering promising avenues for optimization strategies in formulation applications.