Nanomaterial (NM) aggregation is a key process
determining their environmental, fate behavior an
d effects. Nanomaterials are typically engineere
d to remain kinetically stable; however, in environmental an
d toxicological me
dia, NMs are prone to aggregation. The aggregation kinetics of NM is typically quantifie
d by measuring their attachment efficiency (α) an
d critical coagulation concentration (CCC). Several stu
dies measure
d α an
d CCC for Ag NMs with a major focus on investigating the effects of ionic strength, ion valency an
d natural organic matter, with few stu
dies investigating other environmental factors such as light an
d dissolve
d oxygen an
d none investigating the effect of
particle size, buffer type an
d concentration, or surface
coverage by capping agent. A survey of recent research articles reporting CCC values for Ag NMs reveals substantial variation in experimental con
ditions an
d particle stability with CCC values of monovalent an
d divalent counterions covering a wi
de range (ca
. 25 to infinity for monovalent counterions an
d 1.6 to infinity for
divalent counterions).
d="sp0015">Here, we rationalize the differences in the CCC values for Ag NMs based on the variability in the experimental conditions, which includes NM and medium physicochemical properties. Capping agents determines NM stability mechanism with citrate, sodium dodecyl sulfate (SDS), and alginate stabilizing NM by electrostatic mechanism; whereas polyvinylpyrrolidone (PVP), casein, dextrin, tween, branched polyethyleneimine (BPEI), and Gum Arabic stabilizing NMs by steric mechanisms. The CCC values for Ag NMs with different capping agents follow the order citrate ∼ alginate ∼ SDS < casein < dextrin < PVP < tween < branched polyethyleneimine (BPEI) ∼ gum Arabic. For charge stabilized Ag NMs, the CCC increases with the decrease in NM size and buffer concentration and decreases with light irradiation. For sterically stabilized PVP-Ag NMs, the CCC increases with the coating concentration/surface coverage and completely coated Ag NMs do not undergo aggregation even at high salt concentrations.