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1. Atomically resolved interfacial water structures on crystalline hydrophilic and hydrophobic surfaces

   https://doi.org/10.1039/d1nr00351h  

   Uhlig, Manuel R.; Benaglia, Simone; Thakkar, Ravindra; Comer, Jeffrey; Garcia, Ricardo (March 2021, Nanoscale)

   null (Ed.)

   Hydration layers are formed on hydrophilic crystalline surfaces immersed in water. Their existence has also been predicted for hydrophobic surfaces, yet the experimental evidence is controversial. Using 3D-AFM imaging, we probed the interfacial water structure of hydrophobic and hydrophilic surfaces with atomic-scale spatial resolution. We demonstrate that the atomic-scale structure of interfacial water on crystalline surfaces presents two antagonistic arrangements. On mica, a common hydrophilic crystalline surface, the interface is characterized by the formation of 2 to 3 hydration layers separated by approximately 0.3 nm. On hydrophobic surfaces such as graphite or hexagonal boron nitride (h-BN), the interface is characterized by the formation of 2 to 4 layers separated by about 0.5 nm. The latter interlayer distance indicates that water molecules are expelled from the vicinity of the surface and replaced by hydrocarbon molecules. This creates a new 1.5–2 nm thick interface between the hydrophobic surface and the bulk water. Molecular dynamics simulations reproduced the experimental data and confirmed the above interfacial water structures. 

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2. Structures and Dynamics of Complex Guest Molecules in Confinement, Revealed by Solid-State NMR, Molecular Dynamics, and Calorimetry

   https://doi.org/10.3390/molecules29071669  

   Haro_Mares, Nadia B; Döller, Sonja C; Wissel, Till; Hoffmann, Markus; Vogel, Michael; Buntkowsky, Gerd (April 2024, Molecules)

   This review gives an overview of current trends in the investigation of confined molecules such as water, small and higher alcohols, carbonic acids, ethylene glycol, and non-ionic surfactants, such as polyethylene glycol or Triton-X, as guest molecules in neat and functionalized mesoporous silica materials employing solid-state NMR spectroscopy, supported by calorimetry and molecular dynamics simulations. The combination of steric interactions, hydrogen bonds, and hydrophobic and hydrophilic interactions results in a fascinating phase behavior in the confinement. Combining solid-state NMR and relaxometry, DNP hyperpolarization, molecular dynamics simulations, and general physicochemical techniques, it is possible to monitor these confined molecules and gain deep insights into this phase behavior and the underlying molecular arrangements. In many cases, the competition between hydrogen bonding and electrostatic interactions between polar and non-polar moieties of the guests and the host leads to the formation of ordered structures, despite the cramped surroundings inside the pores. 

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3. Vapor-phase grafting of functional silanes on atomic layer deposited Al2O3

   https://doi.org/10.1116/6.0002364  

   Rozyyev, Vepa; Shevate, Rahul; Pathak, Rajesh; Murphy, Julia G.; Mane, Anil U.; Sibener, S. J.; Elam, Jeffrey W. (May 2023, Journal of Vacuum Science & Technology A)

   Fundamental studies are needed to advance our understanding of selective adsorption in aqueous environments and develop more effective sorbents and filters for water treatment. Vapor-phase grafting of functional silanes is an effective method to prepare well-defined surfaces to study selective adsorption. In this investigation, we perform vapor phase grafting of five different silane compounds on aluminum oxide (Al2O3) surfaces prepared by atomic layer deposition. These silane compounds have the general formula L3Si–C3H6–X where the ligand, L, controls the reactivity with the hydroxylated Al2O3 surface and the functional moiety, X, dictates the surface properties of the grafted layer. We study the grafting process using in situ Fourier transform infrared spectroscopy and ex situ x-ray photoelectron spectroscopy measurements, and we characterize the surfaces using scanning electron microscopy, atomic force microscopy, and water contact angle measurements. We found that the structure and density of grafted aminosilanes are influenced by their chemical reactivity and steric constraints around the silicon atom as well as by the nature of the anchoring functional groups. Methyl substituted aminosilanes yielded more hydrophobic surfaces with a higher surface density at higher grafting temperatures. Thiol and nitrile terminated silanes were also studied and compared to the aminosilane terminated surfaces. Uniform monolayer coatings were observed for ethoxy-based silanes, but chlorosilanes exhibited nonuniform coatings as verified by atomic force microscopy measurements. 

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4. Effects of Ionic Strength on Arsenate Adsorption at Aluminum Hydroxide–Water Interfaces

   https://doi.org/10.3390/soils2010001  

   Xu, Tingying; Catalano, Jeffrey G (March 2018, Soil Systems)

   Adsorption processes at mineral–water interfaces control the fate and transport of arsenic in soils and aquatic systems. Mechanistic and thermodynamic models to describe this phenomenon only consider inner-sphere complexes but recent observation of the simultaneous adsorption of inner- and outer-sphere arsenate on single crystal surfaces complicates this picture. In this study, we investigate the ionic strength-dependence of the macroscopic adsorption behavior and molecular-scale surface speciation of arsenate bound to gibbsite and bayerite. Arsenate adsorption decreases with increasing ionic strength on both minerals, with a larger effect at pH 4 than pH 7. The observed pH-dependence corresponds with a substantial decrease in surface charge at pH 7, as indicated by zeta-potential measurements. Extended X-ray absorption fine structure (EXAFS) spectroscopy finds that the number of second shell Al neighbors around arsenate is lower than that required for arsenate to occur solely as an inner-sphere surface complex. Together, these observations demonstrate that arsenate displays macroscopic and molecular-scale behavior consistent with the co-occurrence of inner- and outer-sphere surface complexes. This demonstrated that outer-sphere species can be responsible for strong adsorption of ions and suggests that environments experiencing an increase in salt content may induce arsenic release to water, especially under weakly acidic conditions. 

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5. Fluorinated Surfactant Adsorption on Mineral Surfaces: Implications for PFAS Fate and Transport in the Environment

   https://doi.org/10.3390/surfaces3040037  

   Alves, Anthony V.; Tsianou, Marina; Alexandridis, Paschalis (December 2020, Surfaces)

   null (Ed.)

   Fluorinated surfactants, which fall under the class of per- and polyfluoroalkyl substances (PFAS), are amphiphilic molecules that comprise hydrophobic fluorocarbon chains and hydrophilic head-groups. Fluorinated surfactants have been utilized in many applications, e.g., fire-fighting foams, paints, household/kitchenware items, product packaging, and fabrics. These compounds then made their way into the environment, and have been detected in soil, fresh water, and seawater. From there, they can enter human bodies. Fluorinated surfactants are persistent in water and soil environments, and their adsorption onto mineral surfaces contributes to this persistence. This review examines how fluorinated surfactants adsorb onto mineral surfaces, by analyzing the thermodynamics and kinetics of adsorption, and the underlying mechanisms. Adsorption of fluorinated surfactants onto mineral surfaces can be explained by electrostatic interactions, hydrophobic interactions, hydrogen bonding, and ligand and ion exchange. The aqueous pH, varying salt or humic acid concentrations, and the surfactant chemistry can influence the adsorption of fluorinated surfactants onto mineral surfaces. Further research is needed on fluorinated surfactant adsorbent materials to treat drinking water, and on strategies that can modulate the fate of these compounds in specific environmental locations. 

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