Particle Size

The analysis of the 3-D response surface plots for Z-Ave, D50, and D90 (Figure 3A–C) revealed that as the emulsification speed increased, the particle size decreased. Regarding the sonication amplitude, a direct effect on particle size was not observed. However, when these parameters increased together, the particle size decreased. Z-Ave, D50, and D90 values were higher for 11,000 rpm and lower for 13,400 rpm. The increase of the emulsification speed allowed the formation of a stable rivastigmine-loaded NLC formulation with uniform particle size distribution [11]. Increasing the sonication amplitude results in higher ultrasonic wave energy with a consequent increase in the shear cavitation forces, leading to the breakdown of the emulsion oil droplets to nanometric sizes [46,51].

The lowest and highest Z-Ave (295 and 152 nm), D50 (57 and 80 nm), and D90 (180 and 220 nm) values were observed when 13,400 rpm and 85% amplitude were applied. Thus, 13,400 rpm with 85% amplitude were set as the desired conditions for the ultrasound technique.
