Driven by the potential applications of ionic liquids (ILs) in many emerging electrochemical technologies, recent research efforts have been directed at understanding the complex ion ordering in these systems, to uncover novel energy storage mechanisms at IL/electrode interfaces. Here, we discover that surface-active ionic liquids (SAILs), which contain amphiphilic structures inducing self-assembly, exhibit enhanced charge storage performance at electrified surfaces. Unlike conventional nonamphiphilic ILs (NAILs), for which ion distribution is dominated by Coulombic interactions, SAILs exhibit significant and competing van der Waals interactions owing to the nonpolar surfactant tails, leading to unusual interfacial ion distributions. We reveal that at an intermediate degree of electrode polarization SAILs display optimal performance, because the low-charge-density alkyl tails are effectively excluded from the electrode surfaces, whereas the formation of nonpolar domains along the 2 surface suppresses undesired overscreening effects. This work represents a crucial step towards understanding the unique interfacial behavior and electrochemical properties of amphiphilic liquid systems showing long-range ordering, and offers insights into the design principles for high-energydensity electrolytes based on spontaneous self-assembly behavior. Research interest in ionic liquids (ILs) as electrolytes for energy devices stems from several unique properties such as low volatility and flammability, as well as high electrochemical stability 1-5. An understanding of the molecular-level interactions between ILs and electrified interfaces is crucial for optimization of device performance 6. For instance, interfacial IL layers at charged surfaces govern the electric double layer (EDL) structure, a key factor determining the device energy density 2,4,6,7. The EDL structure with ILs is drastically different from that in aqueous and organic electrolytes 8-10 ; the complex ion ordering in ILs exhibits many subtleties, and remains an active area of debate 11-14. Here we present the first detailed investigation into electrocapacitive characteristics and fundamental EDL structures of an emerging IL class based on surface-active agents, or surface-active ILs (SAILs) 13,15-19. Our study reveals a novel material design principle for enhancing charge storage owing to the self-assembled nanostructures in amphiphilic liquids, and introduces a class of liquids with long-range ordering, having broad implications for diverse fields, ranging from interfacial science 20,21 to energy technologies 22,23. SAILs are inherently amphiphilic, and can self-assemble into nanostructures composed of distinct polar and nonpolar domains 13,15-19. Most previous studies on the IL EDL structure and IL-based energy devices focus on non-amphiphilic ILs (NAILs) where neither ion is based on a classical surfactant structure 6,24. Whereas nanostructuring was observed under confinement for some NAILs where one of the ions, usually the cation, bears moderate to long chai...
This article analyzes how the individual structural elements of surfactant molecules affect surface properties, in particular, the point of reference defined by the limiting surface tension at the aqueous cmc, γcmc. Particular emphasis is given to how the chemical nature and structure of the hydrophobic tails influence γcmc. By comparing the three different classes of surfactants, fluorocarbon, silicone, and hydrocarbon, a generalized surface packing index is introduced which is independent of the chemical nature of the surfactants. This parameter ϕcmc represents the volume fraction of surfactant chain fragments in a surface film at the aqueous cmc. It is shown that ϕcmc is a useful index for understanding the limiting surface tension of surfactants and can be useful for designing new superefficient surfactants.
Infections resulting from bacterial biofilm formation on the surface of medical devices are challenging to treat and can cause significant patient morbidity. Recently, it has become apparent that regulation of surface nanotopography can render surfaces bactericidal. In this study, poly(ethylene terephthalate) nanocone arrays are generated through a polystyrene nanosphere-mask colloidal lithographic process. It is shown that modification of the mask diameter leads to a direct modification of centre-to-centre spacing between nanocones. By altering the oxygen plasma etching time it is possible to modify the height, tip width and base diameter of the individual nanocone features. The bactericidal activity of the nanocone arrays was investigated against Escherichia coli and Klebsiella pneumoniae. It is shown that surfaces with the most densely populated nanocone arrays (center-to-center spacing of 200 nm), higher aspect ratios (>3) and tip widths <20 nm kill the highest percentage of bacteria (∼30%).
'Black silicon' (bSi) samples with surfaces covered in nanoneedles of varying length, areal density and sharpness, have been fabricated using a plasma etching process. These nanostructures were then coated with a conformal uniform layer of diamond using hot filament chemical vapour deposition to produce 'black diamond' (bD) surfaces. The effectiveness of these bSi and bD surfaces in killing Gram-negative (E. coli) and Gram-positive (S. gordonii) bacteria was investigated by culturing the bacteria on the surfaces for a set time and then measuring the live-to-dead ratio. All the nanostructured surfaces killed E. coli at a significantly higher rate than the respective flat Si or diamond control samples. The length of the needles was found to be less important than their separation, i.e. areal density. This is consistent with a model for mechanical bacteria death based on the stretching and disruption of the cell membrane, enhanced by the cells motility on the surfaces. In contrast, S. gordonii were unaffected by the nanostructured surfaces, possibly due to their smaller size, thicker cell membrane and/or their lack of motility.
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