Probing the Mechanism of Targeted Delivery of Molecular Surfactants Loaded into Nanoparticles after Their Assembly at Oil–Water Interfaces

Abstract

A targeted and controlled delivery of molecular surfactants at oil–water interfaces using the directed assembly of nanoparticles, NPs, is reported. The mechanism of NP assembly at the interface and the release of molecular surfactants is followed by laser scanning confocal microscopy and surface force spectroscopy. The assembly of positively charged polystyrene NPs at the oil–water interface was facilitated by the introduction of carboxylic acid groups in the oil phase (e.g., by adding 1 wt % stearic acid to hexadecane to produce a model oil). The presence of positively charged NPs consistently lowers the stiffness of the water–oil interface. The effect is lessened, when the NPs are present in a solution of NaCl or deionized water at pH 2, consistent with a less dense monolayer of NPs at the interface in the last two systems. In addition, the NPs reduce the interfacial adhesion (i.e., the “stickiness” of the interface or, put differently, the pull-off force experienced by the atomic force microscopy (AFM) tip during retraction). After the assembly, the NPs can release a previously loaded cargo of surfactant molecules, which then facilitate the formation of a much finer oil–water emulsion. As a proof of concept, we demonstrate the release of octadecyl amine, ODA, that has been incorporated into the NPs prior to the assembly. The release of ODA causes the NPs to detach from the interface altering the interfacial properties and leads to finer oil droplets. This approach can be exploited in applications in several fields ranging from pharmaceutical and cosmetics to hydrocarbon recovery and oil-spill remediation, where a targeted and controlled release of surfactants is wanted.

Fine-Tuning the Surface and Interfacial Chemistry of Silica Nanoparticles to Control Stability in High Ionic Strength Electrolytes and Modulate Assembly at Oil–Water Interfaces

“…We use crude oil and, in some cases, model oil as substrates for interfacial studies. Both of these systems bear permanent negative surface charges in water at neutral pH (ζ-potential = −52 ± 3 mV and −34 ± 5, respectively) due to the presence of deprotonated carboxylic acid groups. ,, These permanent charges differ from the charges developed at a neat hydrocarbon–water interface, which are due to the adsorption of hydroxyl groups on the hydrocarbon surface from water. , …”

“…Manipulating the properties of liquid–liquid interfaces and specifically oil–water interfaces using nanoparticles (so-called Pickering emulsions) has attracted widespread attention due to both scientific interest as well as use in practical applications. − Assembly of nanoparticles of various chemistries and dimensionalities at the oil–water interfaces stabilizes the emulsion and offers the possibility to synthesize materials with controlled behavior by fine-tuning interfacial properties. ,, By varying the chemistry and dimensionality of the nanoparticles, the adsorption energy and thus the interfacial properties can be modulated and controlled. , …”

“…13−23 Assembly of nanoparticles of various chemistries and dimensionalities at the oil−water interfaces stabilizes the emulsion and offers the possibility to synthesize materials with controlled behavior by fine-tuning interfacial properties. 18,24,25 By varying the chemistry and dimensionality of the nanoparticles, the adsorption energy and thus the interfacial properties can be modulated and controlled. 18 Nanoparticle assembly and thus interfacial properties can be fine-tuned by controlling the pH, 27−29 electric and magnetic fields, 30−32 or combinations thereof.…”

“…Both of these systems bear permanent negative surface charges in water at neutral pH (ζ-potential = −52 ± 3 mV and −34 ± 5, respectively) due to the presence of deprotonated carboxylic acid groups. 24,41,55 These permanent charges differ from the charges developed at a neat hydrocarbon−water interface, which are due to the adsorption of hydroxyl groups on the hydrocarbon surface from water. 41,56 The interfacial tension, IFT, as a function of time for crude oil in a 1 wt % suspension of S 1,2 in DI water is shown in Figure 3a.…”

Synthesis of Raspberry-like Nanoparticles via Surface Grafting of Positively Charged Polyelectrolyte Brushes: Colloidal Stability and Surface Properties

“…Force–separation curves were collected and analyzed. The stiffness of the samples was calculated from the linear part of the retrace curve, while the adhesion was obtained from the minima of the same curve …”