In situ investigation of the particle adsorption layer properties of positively and negatively charged silica nanoparticles adsorbed to pristine and aminolipid-laden liquid interfaces

Abstract

The self-assembly and confinement of particles in combination with surfactants to gas/liquid and liquid/liquid interfaces can result in complex, two- and three-dimensional configurations commonly found in various natural and industrial processes. A fundamental understanding into the physicochemical aspects related to the dynamics of adsorption and equilibration of particle-laden interfaces are essential for the fabrication or destabilization of dispersed systems and reconfigurable devices. The decrease in interfacial area upon particle adsorption is accompanied by a change in interfacial energy, which in turn is thermodynamically related to the interfacial tension. The macroscopic observable change in interfacial tension can be related to the interfacial concentration of adsorbed particles if a suitable (thermodynamic) model exists. This has been mostly limited to adsorption of single components owing to the potential interactions between multiple components that can potentially alter the particle wettability.

The selected system consists of hydrophilic aminated-silica particles dispersed in the aqueous phase, while the oil-soluble aminolipid is dispersed in the immiscible oil. Consequently, it is expected that particle-surfactant interactions are limited owing to the similarly charged amino-groups and their interactions are limited to the interfacial region. Therefore, their separate and combined effects on the equilibrium interfacial tension can be treated as additive elements to the overall interfacial free energy of the system. The analytical additivity model indicated that the 80 nm particles adsorb to the surfactant-laden interface at all investigated surfactant concentrations and compete with the surfactants for interfacial coverage. Additionally, the wettability of the hydrophilic particles does not change in the presence of the lipids, except for the highest investigated lipid concentration.

To substantiate these findings, adsorption of smaller negatively and positively charged silica particles (around 19 nm in diameter) in combination with the aminolipid were studied using in situ synchrotron-based X-ray reflectometry combined with dynamic interfacial tensiometry. The negatively charged particles only adsorb when the aminolipid is present, which is indicative for synergistic adsorption. The positively charged particles readily adsorb independently to the pure oil/water, but compete with the aminolipid and reversibly desorb with increasing lipid concentrations, indicative for competitive adsorption. Applying the additivity model in the latter case indicates that an electrostatic exclusion zone exists around the adsorbed particles, which prevents adsorption of lipid molecules in this area.

Nanoparticle wettability at a liquid interface, characterized through their contact angle or immersion depth, is generally extrapolated from contact angles of macroscopic sessile drops on planar substrates. These planar surfaces are often assumed to be physical and chemical equivalents to the adsorbed particles. This approach is investigated in detail where chemical equivalency is assumed when the zeta potential of the particles and substrates is identical, and substantiated with physical topographic analysis. Advancing and receding contact angles are measured on the planar substrates and the contact angles of the particles are obtained from X-ray reflectometry. Generally, the receding contact angles on the smooth surfaces provides a reasonable estimate of nanoparticle wetting properties, but cannot predict adsorption barriers as is the case for the negatively charged silica particles.