The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the waterand often the water molecules themselvesto detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular-and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.
There are three unsolved problems in thin film lubrication (TFL) since it was proposed 20 years ago, i.e., the determination of the type of molecules that can enter the contact region efficiently during sliding, the orientation of molecules in the contact region, and the effect of solid surfaces on the liquid molecular orientation in TFL. In order to answer the first two questions, an in situ measurement system comprising a self-designed Raman microscopy and relative optical interference intensity (ROII) system was set up to study the molecular behaviors. A variety of binary mixtures were used as lubricants in the test, and the concentration distribution profile and orientation of the additive molecules in TFL were characterized. The molecular behavior was determined via a combination of shearing, confinement, and surface adsorption. Furthermore, the difference in molecular polarity resulted in different competing effect of surface adsorption and intermolecular interaction, the influence of which on molecular behavior was discussed. Polar additive molecules interacted with the steel surface and exhibited an enrichment effect in the Hertz contact region when added into a nonpolar base oil. No enrichment effect was observed for nonpolar molecules that were added into the nonpolar base oil and polar molecules added into polar base oil. The enrichment of additive molecules enhanced the film-forming ability of the lubricant and resulted in a reduction in the friction coefficient of up to 61%. The orderly arrangement of the additive molecules was another reason for the friction-reducing. A binary multilayer model was proposed to illuminate the molecular behavior in the TFL, and the model was supported by contrary experiment results in elastohydrodynamic lubrication. This research may aid in understanding the nanoscale lubrication mechanism in TFL and the development of novel liquid lubricants.
Cysteine-based polyzwitterionic brushes have been prepared
via
a two-step route. First, poly(allyl methacrylate) (PAMA) brushes have
been grown from the surface of silicon substrates using surface-initiated
atom transfer radical polymerization. The obtained PAMA brushes with
free pendant vinyl groups were further modified via radical thiol-ene
addition reaction to attach l-cysteine moieties. Surface
ζ potential investigations on pH-responsiveness of these poly(cysteine
methacrylate) (PCysMA) brushes confirm their zwitterionic character
at intermediate pH range, while at pH values either below pH 3.50
or above pH 8.59, they exhibit polyelectrolyte character. Under acid
(pH < 3.50) or base (pH > 8.59) conditions, they possess either
cationic or anionic character, respectively. In the zwitterionic region,
these PCysMA brushes show positive surface ζ potential in the
presence of Pb(CH3COO)2 solutions of various
concentrations. The results are in line with microscopic investigations
using anomalous X-ray reflectivity (AXRR) carried out along the absorption
edge of Pb2+ ions. When the photon energies were varied
around the absorption L3 edge of lead (13037 eV), the Pb2+ concentration normal to the silicon substrates, as a function of
depth inside PCysMA brushes, could be revealed at the nanoscale. Both
ζ potential and AXRR measurements confirm the enrichment of
Pb2+ ions inside PCysMA brushes, indicating the potential
of PCysMA to be used as a water purification material.
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