Exploring the effect of experimental conditions on the synthesis and stability of alginate–gelatin coacervates

Abstract

Alginate–gelatin coacervation has been studied by considering different experimental parameters, such as gelatin preheating, pH, alginate–gelatin ratio and their respective concentrations, and salt effect. Results were assessed in terms of size and polydispersion via dynamic light scattering, electrostatic charge in the surface by zeta potential measurements, electrostatic interaction forces by static light scattering, stability by turbidimetry and viscoelastic and pseudoplastic behavior by rheology (oscillatory and statistical analysis). According to the results, gelatin structure has to be previously modified to induce the proper interactions with a subsequent pH reduction. Specifically, stable coacervates (according to turbidimetry and dynamic light scattering) with a size of 300–600 nm and a polydispersion lower than 0.25 were obtained after preheating the gelatin at 37°C and with a subsequent pH reduction until 4–5 for an alginate–gelatin ratio between 1:4 and 1:6. However, different experimental conditions promote an unsuccessful coacervation, obtaining always precipitates and/or coacervates with a wider particle size distribution. Furthermore, in order to study the effect of the temperature on the coacervates, different cooling–heating cycles were applied on them over a week, showing the stability of the thermo‐reversible coacervates for almost 5 days. Also, the interactions were characterized via static light scattering, analyzing the second virial coefficient. Moreover, rheological oscillatory results can be used to identify a proper coacervation due to the increase of the storage modulus. However, no significant changes were observed with statistical analysis due to the highly diluted character of the precursor solutions. These results highlighted how a proper combination of different experimental conditions, mainly temperature to promote a partial gelatin unraveling as well as pH reduction, is required to successfully produce coacervates. Finally, salt effect was proven to induce precipitation when NaCl was increasingly added to solutions of stable coacervates.