pH-dependent aggregation of tannic acid: Insights from molecular dynamics simulations

Colloidal fouling of polymeric membranes is still a limiting factor in the use of filtration for polyphenol recovery. In this context, understanding the self-aggregation mechanisms of tannic acid (TA) is of great importance. In this study, molecular dynamics simulations were performed to investigate the effect of pH on TA self-aggregation. The results show a non-monotonic relationship between pH and TA self-aggregation, with different mechanisms observed for different pH conditions. At an intermediate pH, the moderate deprotonation of TAs is associated with electrostatic repulsion between the molecules, resulting in the formation of aggregates composed of a small number of TAs. Then, at lower pH, fully protonated TA molecules promote the aggregation of a larger number of TAs because of the reduced electrostatic repulsion. Conversely, at higher pH, despite the increased negative charge of TAs leading to important electrostatic repulsion, the formation of cationic TA-H3O+-TA bridges favours aggregation, enhancing the number of TAs per aggregate in comparison to an intermediate pH. Finally, analysis of the size and density of the aggregates shows that at higher pH less dense aggregates are formed, because of repulsion between molecules, resulting in colloid particles of larger size compared to aggregates formed from the same number of molecules at low pH. These results provide valuable insights into the pH-dependent mechanisms of TA self-aggregation, which are crucial for adjusting operating conditions to control membrane fouling and for designing effective filtration processes.