The performance of nonionic surfactants is mediated by the interfacial interactions at the solid–liquid interface. Here we applied sum frequency generation (SFG) vibrational spectroscopy to probe the molecular structure of the silica–nonionic surfactant solution interface in situ, supplemented by quartz crystal microbalance with dissipation monitoring (QCM-D) and molecular dynamics (MD) simulations. The combined studies elucidated the effects of nonionic surfactant solution concentration, surfactant composition, and rinsing on the silica–surfactant solution interfacial structure. The nonionic surfactants studied include ethylene-oxide (EO) and butylene oxide (BO) components with different ratios. It was found that the CH groups of the surfactants at the silica–surfactant solution interfaces are disordered, but the interfacial water molecules are ordered, generating strong SFG OH signals. Solutions with higher concentrations of surfactant lead to a slightly higher amount of adsorbed surfactant at the silica interface, resulting in more water molecules being ordered at the interface, or a higher ordering of water molecules at the interface, or both. MD simulation results indicated that the nonionic surface molecules preferentially adsorb onto silanol sites on silica. A surfactant with a higher EO/BO ratio leads to more water molecules being ordered and a higher degree of ordering of water molecules at the silica–surfactant solution interface, exhibiting stronger SFG OH signal, although less material is adsorbed according to the QCM-D data. A thin layer of surfactants remained on the silica surface after multiple water rinses. To the best of our knowledge, this is the first time the combined approaches of SFG, QCM-D and MD simulation techniques have been applied to study nonionic surfactants at the silica–solution interface, which enhances our understanding on the interfacial interactions between nonionic surfactants, water and silica. The knowledge obtained from this study can be helpful to design the optimal surfactant concentration and composition for future applications.
Polymer-encapsulated cobalt phthalocyanine (CoPc) is a model system for studying how polymer–catalyst interactions in electrocatalytic systems influence performance for the CO2 reduction reaction. In particular, understanding how bulk electrolyte and proton concentration influence polymer protonation and in turn how the extent of polymer protonation influences catalytic activity and selectivity is crucial to understanding polymer–catalyst composite materials. We report a study of the dependence of bulk pH and electrolyte concentration on the fractional protonation of poly(4-vinylpyridine) and related polymers with both electrochemical and spectroscopic evidence. In addition, we show that the fractional protonation of the polymer is directly related to both the activity of the catalyst and the reaction selectivity for the CO2 reduction reaction over the competitive hydrogen evolution reaction. Of particular note is that the fractional protonation of the film is related to electrolyte concentration, which suggests that the transport of counterions plays an important role in regulating proton transport within the polymer film. These insights suggest that electrolyte concentration and pH play an important role in the electrocatalytic performance for polymer–catalyst composite systems, and these influences should be considered in both experimental preparation and analysis.
As the world becomes increasingly globalized, the propensity of pandemics, such as COVID-19, increases. The United States Pandemic and All-Hazards Preparedness and Advancing Innovation Act of 2019 (PAHPAI) details the federal response to a health crisis including pandemics. The PAHPAI has hindered the nation’s response to COVID-19 due to its lack of emphasis on diagnostic testing (Burr 2019). Rapid testing is critical to slowing the spread of this disease. Ample testing will identify infected populations and will allow communities to take necessary precautions such as staying home and avoiding contact with others. Widespread shelter in place would not be necessary to control the spread of the virus, therefore reducing the economic impact of the pandemic. We propose Congress amends the PAHPAI to institute an improved testing response for future pandemics.
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