Adsorption dynamics of polymeric nanoparticles at an air-water interface with addition of surfactants

Hypothesis: The unusual observation that addition of sodium dodecylsulfate surfactant to an aqueous nanoparticle dispersion slows down the decrease of air:water interfacial tension is attributed to the combined interactions of the nanoparticle with surfactant and surfactant at the air:water interface. Such dynamics are controlled by electrostatic interactions. Experiments: The study of dynamics is achieved using the maximum bubble pressure measurement of surface tension from 0.1 s to 30 s. The NPs are assembled by Flash NanoPrecipitation with 5 kDa polyethylene glycol coronas, and cores of polystyrene, polydimethylsiloxane, or polycaprolactone. Anionic (sodium dodecylsulfate), cationic (cetyltrimethylammonium bromide), and non-ionic (decaethylene glycol monododecyl ether) surfactants are employed over concentration 10⁻⁴ to 10⁻² mM. The zeta potentials of the NPs are measured with surfactants. Electrostatic repulsion between charged NPs and interface is calculated, as well as the adsorption energy. Findings: This is the first report to quantitatively explain the effect of surfactants on the dynamics of NP assembly at an interface. An electrostatic energy barrier slows the adsorption kinetics for NPs when the NPs have the same charge as the interface. Increasing ionic strength of the solution reduces the electrostatic barrier. Decreasing interactions between the NP core material and the surfactant reduces the barrier. Our findings offer new insights into understanding of NP interfacial self-assembly dynamics in a complex environment.