Synergistic and Competitive Adsorption of Hydrophilic Nanoparticles and Oil-Soluble Surfactants at the Oil–Water Interface

Abstract

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.

Tracking of Nanoparticle Diffusion at a Liquid–Liquid Interface Adsorbed by Nonionic Surfactants

“…In this system, Φ of surfactants is 71.3% (Table ). Because D s at this condition is comparable to that at a pristine interface, it is proposed that the number of surfactants in the vicinity of a nanoparticle is small due to a repulsive interaction between the nanoparticle and surfactant . By considering the types of interactions that may be relevant for the system involving nanoparticles and surfactants at a nonaqueous liquid interface, we found that the repulsion was phenomenologically consistent with Coulombic effects in nonaqueous solutions. , …”

Assessment of nanoparticle immersion depth at liquid interfaces from chemically equivalent macroscopic surfaces

“…The synchrotron reflectivity measurements (XRR) at the decane-water interface were performed at the Liquid Interface Scattering Apparatus (LISA) diffractometer at the P08 beamline, PETRA III for its delivery of X-rays with high brilliance (DESY, Hamburg, Germany) [47,48] and are described in detail in our recent publication [49]. Briefly, a 25 keV collimated X-ray beam with a size of 0.05 x 0.4 mm (vertical x horizontal direction) probes the gas/liquid surface and liquid/fluid interface while varying the incident angles (α i ), without moving the sample, to obtain a reflectivity profile as a function of the momentum transfer q = 4πsin(α i )/λ, where λ = 0.496 Å.…”

“…The NPs self-assemble at a quiescent air/water surface and oil/water interface, with an immersion depth that can be determined in situ from high-resolution X-ray reflectivity (XRR) data. This discussion is based on a dataset and analytical approach that was also used in our recent publication which focuses on an in-depth discussion of particle assembly at the oil/water interface in presence of oil-soluble surfactants [49]. The purity of the particle dispersions (0.1 w/w% at aqueous pH ~ 5.8) and decane are tested with time-resolved interfacial tension measurements before the XRR tests (Supplementary Section S3, Fig.…”

“…The preference of the positively charged particles to adsorb at the interface can be linked to the negative charge of a pristine air/water surface or immiscible oil/water interface, while the negatively charged particles are electrostatically repelled [25,73,74], and to their inherent surface wetting properties [19]. The raw data is fitted with electron density (ED) profiles using the REFLEX software [50] to simulate the XRR data based on Parratt's recursive formalism [75] using a 3-slab model along with the interface normal and the corresponding EDs are fitted against a physically realistic model to obtain adsorption parameters (Supplementary Section S4, Table S4a) [49]. The EDs normalized against water (ρw) in the insets of Fig.…”

Synergistic Effect of Nanofluids and Surfactants on Heavy Oil Recovery and Oil-Wet Calcite Wettability

“…TiO 2 nanoparticles could effectively improve surfactant-enhanced oil recovery [ 13 ]. Furthermore, the use of surfactants in heavy oil recovery has also shown advantages [ 14 , 15 ]. However, there are few studies on the possible synergistic effect of using both nanofluids and surfactants in heavy oil recovery, and the mechanism is unclear.…”