(183e) A Colloidal Stability Model for Fuel Driven Dissipative Assembly of Hydrophobic Colloids and Nanoparticles

Dissipative assembly utilizes fuel-driven reactions to assemble transient nanostructures far from equilibrium, which autonomously disassemble when the fuel reaction is exhausted. Despite its ubiquitous nature in biology, e.g. assembly of microtubules in cells, synthetic systems that enable programming nanoparticle assembly in both time and space are rare due to a lack of design rules and poor understanding of particle scale assembly mechanisms. Prior work has utilized chemical fuels to temporarily convert hydrophilic, charged surface ligands on colloids and nanoparticles into uncharged hydrophobic groups that induce assembly over a time scale of several hours. After the fuel is exhausted hydrolysis back reactions return the hydrophobic ligands to the their original hydrophilic state over a time scale of several hours. Depending on experimental conditions (e.g. ionic strength, fuel concentration, particle surface chemistry, particle size) the colloids either form irreversible aggregates or they will disassemble into single particles after fuel is exhausted. Recent experimental work has shown that the initial fuel to ligand ratio is a critical parameter that mediates whether particles disassemble after the fuel reactions are extinguished. For a given nanoparticle and fuel system, establishing the fuel to ligand ratio for dissipative assembly requires significant experimentation as there is currently no quantitative model capable of predicting assembly lifetime or determining conditions that will enable reversible assembly.

Here we demonstrate a coupled reaction kinetic and colloidal stability model for dissipative nanoparticle assembly (Figure 1). The model combines a reaction kinetic model for fuel-ligand reactions along with a colloidal stability model based on pairwise interparticle interactions. The reaction kinetic model determines the time-dependent concentration of charged hydrophilic surface ligands and reveals that ligands are rapidly converted to uncharged hydrophilic groups, which reach a peak concentration within 1 hour of adding the chemical fuel. We utilize the time-dependent concentration of charged ligands to calculate the time dependent surface charge of the nanoparticles. The pairwise interparticle interaction potential is then calculated as a function of time based on the surface charge. The model shows that for fuel concentrations that lead to dissipative assembly, a secondary minimum emerges about an hour after fuel is added, which causes loose aggregates to form. Fuel to ligand ratio is observed to control the emergence of the secondary minimum and an energy barrier to irreversible aggregation, both of which are required for dissipative assembly. The model predicts assembly and disassembly kinetics with a two-stage colloidal aggregation model to determine the rate constants for assembly, disassembly, and irreversible aggregation and show this model can estimate the lifetime of colloidal assemblies formed during dissipative assembly. The model is compared to experiments on micron-scale polystyrene colloids.