Quantifying Biolipid (Rhamnolipid) Effects on the Aggregation Behavior of Engineered Nanoparticles

Predicting nanoscale material stability in aqueous systems is essential to accurately model particle fate and transport in the environment. Such stability is not only a function of particle surface chemistry and ionic strength and type, but can also be strongly affected by common aqueous constituents including natural organic matter (NOM), proteins, and lipids, among other macromolecules. Of these, biological surfactants, when present, have been hypothesized to play a significant, interfacial role with regard to nanoparticle stability, mobility and thus ultimate fate. Specifically, the role(s) of rhamnolipid(s), which are some of the most common naturally occurring biosurfactants, remains unclear. To address this knowledge gap, aggregation dynamics of 8 nm monodispersed iron oxide (nano)particles (IONPs) with cationic and anionic surface chemistries were evaluated in presence of monorhamnolipid (monoRL) and dirhamnolipid (diRL), two amphiphilic glycolipids excreted by Pseudomonas aeruginosa, among other bacteria. Results demonstrate that IONP surface charge, RL type (i.e. mono- vs. dirhamnolipid), and concentration govern particle stability. Further, water chemistry (considering monovalent and divalent ions) plays a key role in these processes and outcomes. RLs at higher concentrations (above CMCmonoRL = 20.9, CMCdiRL = 10.1 mg of OC/L) adsorbed strongly on anionic IONPs. For these, the critical coagulation concentration (CCC) of anionic IONPs increased from 700 mM to 1500 mM in the presence of DiRL. RLs also strongly sorbs on IONP with a positive surface charge (at concentrations < CMC). Positively charged IONPs aggregated at intermediate concentrations (~CMC) of monoRL and diRL, and then effectively re-stabilized at higher concentrations (1.5 – 2 CMC) due to (NP) surface RL bilayer formation. For RL coated IONPs, three distinct aggregation regimes were identified as a function of electrolyte concentration (1-2000 mM), for which positively charged IONPs do not follow typical DLVO-based particle interaction theory.