Nanoparticle Adsorption at Fluid Interfaces

Xiaoqing Hua

Nanoparticles (NPs) adsorption can add functionality (e.g., catalytic, optical, rheological) to fluid interfaces. NPs can also stabilize fluid interfaces to allow for complex droplet shapes, bijels, and buckled microstructures. Both NPs and surfactants adsorption decreases interfacial tension and stabilizes foams and emulsions. However, unlike surfactants, NP adsorption at fluid interfaces is generally irreversible because their adsorption energy is much larger than the thermal energy, kT. However, the distinctions between a surfactant and a NP becomes blurred as the dimensions of NPs decrease. For example, adsorption of sub-10 nm NPs can be reversible, and dynamic equilibrium can be achieved between a bulk phase and the interface. Therefore, the objective of this dissertation is to uncover the fundamental differences between NPs and surfactants as they adsorb at fluid interfaces. As the distinguishing features between a surfactant and a NP start to vanish, can NPs act as both a particle and a surfactant? In particular, what are the driving forces for NP adsorption and how do they affect the surface pressure? What are the dynamics of NP adsorption to fluid interfaces? To answer these questions, a material system (ion-pair gold NPs) is selected to show that NPs can achieve dynamic equilibrium between the bulk aqueous phase and its interface with toluene. To demonstrate reversibility of the adsorption process, pendant drop measurements are employed where adsorbed NPs in equilibrium with the bulk phase undergo both mechanical perturbations (compression/expansion) and chemical perturbations (dilution). After perturbations the same equilibrium states are recovered, indicating that reversible dynamic equilibrium is established between NPs at the interface and in the bulk phase. With demonstration of a reversible system, a thermodynamically consistent framework is developed to describe the relationship between the bulk NP concentration, adsorbed amount at the interface, and surface pressure. The thermodynamic treatment shows why the Gibbs adsorption isotherm valid for surfactant adsorption does not provide a relationship between surface pressure and adsorbed amount for NPs. Instead the particle-laden interface needs to be treated as a composite material where contributions from both the solid and fluid area in the Helmholtz energy are incorporated. The increase in surface pressure due to particle adsorption originates from both

1 the fluid (surface activity) and the wetting (area replacement) of the particles at the interface. The treatment highlights the fundamental differences between the measured surface pressure caused by particle adsorption compared to the one measured from compression of the interface. The proposed framework is then tested with the adsorption of ion-pair gold NPs. Independent measurements of the surface pressure and adsorbed amount yields a linear relationship between the surface pressure and surface excess. Such a linear relationship is to be expected if wetting (area replacement) is the main driving force for particle adsorption (while surface activity contributions are negligible). A surface-active species, tetrapentyl ammonium ion, is then introduced to the system. It is found that when NP and surfactants are present, the surface pressure has two contributions: wetting of the NPs at the interface, as well as surface activity (surfactant-surfactant and surfactant-NP interactions). The proposed model decouples the contributions to the total surface pressure originating from wetting and surface activity. Finally, the thermodynamic model is combined with the Ward-Tordai model traditionally employed to describe surfactant adsorption dynamics, to successfully predict diffusion-limited adsorption of NPs to the oil-water interface. These findings elucidate the effects of NP adsorption on interfacial properties. The framework described in this thesis provides a general approach that can be adapted to other experimental systems, and to make predictions as part of guiding experimental designs.

2