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We investigate the formation/re-dissociation mechanisms of hybrid complexesmade from negatively charged PAA2k coated {\gamma}-Fe2O3 nanoparticles (NP) andpositively charged polycations (PDADMAC) in aqueous solution in the regime ofvery high ionic strength (I). When the building blocks are mixed at large ionicstrength (1M NH4Cl), the electrostatic interaction is screened and complexationdoes not occur. If the ionic strength is then lowered down to a targeted ionicstrength Itarget, there is a critical threshold Ic = 0.62 M at whichcomplexation occurs, that is independent on the charge ratio Z and the pathwayused to reduce salinity (drop-by-drop mixing or fast mixing). If salt is addedback up to 1M, the transition is not reversible and persistentout-of-equilibrium aggregates are formed. The lifetimes of such aggregatesdepends on Itarget: the closer Itarget to Ic is, the more difficult it is todissolve the aggregates. Such peculiar behavior is driven by the innerstructure of the complexes that are formed after desalting. When Itarget is farbelow Ic, strong electrostatic interactions induce the formation of dense,compact and frozen aggregates. Such aggregates can only poorly reorganizefurther on with time, which makes their dissolution upon resalting almostreversible. Conversely, when Itarget is close to Ic more open aggregates areformed due to weaker electrostatic interactions upon desalting. System can thusrearrange with time to lower its free energy and reach more stableout-of-equilibrium states which are very difficult to dissociate back uponresalting, even at very high ionic strength.

The desalting/salting pathway: a route to form metastable aggre-gates with tuneable morphologies and lifetimes