Nanoscale Effects on the Surfactant Adsorption and Interface Charging in Hexadecane/Water Emulsions

Abstract

Nanoscale properties at interfaces play a key role in the colloidal stability of emulsions and other soft matter materials where physical properties need to be controlled from the nano to macroscopically visible length scales. Our molecular level understanding of oil− water interfaces arises mostly from results at extended interfaces and the common view that emulsions are stabilized by a large number of surfactant molecules at the droplet's interface which, however, has been recently challenged. In this work, we show that the particle size and the curvature of oil droplets at the nanoscale is of great importance for the interface adsorption of dodecyl sulfate surfactants and possible counterion condensation at the charged hexadecane−water interface. Using second-harmonic scattering, we have studied the surface charge of oil droplets in nanoemulsions where we systematically varied the particle size R between 80 and 270 nm and demonstrate that the surface charge density σ changes drastically with size: For sizes >200 nm, σ is similar to what can be expected at flat extended interfaces, while σ is dramatically reduced by almost an order of magnitude when the particle size of the oil droplet is 80 nm. Using a theoretical approach that considers counterion condensation, we quantify the nanoscale effects on the change in surface charge with particle size and find excellent agreement with our experimental result. Modeling of the experimental results also implies that the charge per particle remains constant and depends on a critical balance of surfactant adsorption and ion condensation.

General approach to surface-accessible plasmonic Pickering emulsions for SERS sensing and interfacial catalysis

“…The fact that the addition of a substantial number of AuNPs to the water-oil interface did not generate any obvious changes to the size and morphology of the emulsions also suggested that there was already sufficient room at the water-oil interface of the CNT-stabilized emulsions to accommodate the additional AuNPs. This assembly model is consistent with previous reports which showed that only a sub-monolayer coverage of the particulate stabilizers is required to form stable Pickering emulsions 39,40 . In contrast, the stability of the Pickering emulsions was found to be less sensitive to the change in the concentration of CNT stabilizers.…”

Effects of surfactant adsorption on the wettability and friction of biomimetic surfaces

“…The charge density for the medium bleached hair surface is very similar to that measured for an SDS monolayer at the hexadecane–water interface (−37.9 μC cm −2 ). 84…”

“…The charge density for the medium bleached hair surface is very similar to that measured for an SDS monolayer at the hexadecane-water interface (À37.9 mC cm À2 ). 84 We also determined the root-mean-square (RMS) roughness of our model surfaces by obtaining a discrete time-averaged topography map of the surface. The surface was discretized by treating the coarse-grained beads as hemispheres with a radius equivalent to half the Lennard-Jones potential energy minimum s/2 and using a grid spacing of Dx = 0.071 nm À1 .…”

Boosting Catalytic Selectivity through a Precise Spatial Control of Catalysts at Pickering Droplet Interfaces

“…We notice that the halogen group of halonitrobenzenes and haloanilines is less polar while the nitro or amino group is polar, which makes it possible for both molecules to orient at oil−water interfaces. 49,50 With such a preferential molecular orientation, oil−water interfaces may regulate molecular adsorption patterns on the surfaces of metal NPs located in the inner interfacial layer. Thereupon, selective hydrogenation of p-chloronitrobenzene (p-CNB) was selected as a model reaction to investigate the impact of oil−water interfaces on the catalytic performances of metal NPs.…”