Characterization of Freeze-Dried Core–Shell Nanoparticles Prepared via Gelatin–Acacia Complex Coacervation: A Study on Particle Formation upon Freezing

Abstract

Core–shell nanoparticles were prepared using a technique based on complex coacervation, and their characteristics were investigated after subsequent freeze drying. An oil-in-water emulsion was stabilized with a gelatin–acacia solution, and the pH of the solution was adjusted to a selected value with acetic acid. The solutions were frozen at a selected freezing rate and subsequently freeze dried. This is a strategic approach to produce particles upon freezing in order to tune their properties by controlling the processing parameters. It was found that particles with mean diameters in the range of 100–400 nm were successfully prepared under all of the selected conditions; however, the size distributions were highly dependent on the values of the variables employed. The efficiency of encapsulation of β-carotene dissolved in the oil phase was significantly affected by the specific processing parameters. Interestingly, the ratio between the surface and inner loads could be tuned by selecting the appropriate conditions. Furthermore, the freezing protocol affected the mass transfer properties of the shell membranes at certain pH values. Complexation of the gelatin and acacia during freezing was confirmed using in situ small-angle X-ray scattering analysis (SAXS) with oil-free gelatin–acacia solutions. A structural change on freezing of the solution was detected at a subzero temperature, and the structural features were found to be dependent on the acidity of the solution and the cooling rate during freezing.

Keywords:

• Coacervation
• Encapsulation
• Freeze drying
• Nanoparticles
• SAXS analysis