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In this study, we show that hydrophilic nanoparticles can readily desorb from liquid−liquid interfaces in the presence of surfactants that do not change the wettability of the particles. Our observations are based on a simple theoretical approach to assess the number of adsorbed particles at the surfactant-laden liquid−liquid interface. We test this approach by studying the interfacial self-assembly of equally charged particles and lipids dissolved in separate immiscible phases. Hence, we investigate the interfacial adsorption of aminated silica particles (80 nm) and octadecylamine to the decane/water interface by interfacial tension measurements, which are supplemented by interfacial rheology of the adsorbed interfacial films, scanning electron microscopy images of Langmuir−Blodgett films, and measurements of the three-phase contact angle of the particle surface in the presence of surfactants. The measurements show that particles adsorb at 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. Comparing the adsorption energies of one particle and of the lipids as a function of the particle contact angle provides an estimate of the tendency for interfacial adsorption of particles from which the particle coverage can be assessed. Based on these findings, equally charged particles and lipids show a competitive behavior at the interface determined by the bulk surfactant concentration and the attachment energies of the particles at the interface. This leads to a simple mechanistic model demonstrating that particles can readily desorb from the interface due to direct displacement by surfactants, which are loosely adsorbed at the oil-facing particle side. This mechanism critically lowers the otherwise high interfacial energy barrier against particle desorption, which otherwise would lead to virtually irreversible particle attachment at the interface.

Fundamental insights into the interplay and self-assembly of nanoparticles and surface-active agents at the liquid−liquid interface play a pivotal role in understanding the ubiquitous colloidal systems present in our natural surroundings, including foods and aquatic life, and in the industry for emulsion stabilization, drug delivery, or enhanced oil recovery. Moreover, wellcontrolled model systems for mixed interfacial adsorption of nanoparticles and surfactants allow unprecedented insights into nonideal or contaminated particle-stabilized emulsions. Here, we investigate such a model system composed of hydrophilic, negatively, and positively charged silica nanoparticles and the oil-soluble cationic lipid octadecyl amine with in situ synchrotronbased X-ray reflectometry, which is analyzed and discussed jointly with dynamic interfacial tensiometry. Our results indicate that negatively charged silica nanoparticles only adsorb if the oil−water interface is covered with the positively charged lipid, indicating synergistic adsorption. Conversely, the positively charged nanoparticles readily adsorb on their own, but compete with octadecyl amine and reversibly desorb with increasing concentrations of the lipid. These results further indicate that with competitive adsorption, an electrostatic exclusion zone exists around the adsorbed particles. This prevents the adsorption of lipid molecules in this area, leading to a decreased surface excess concentration of surfactants and unexpectedly high interfacial tension.

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