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1. Insights into the behavior of nonanoic acid and its conjugate base at the air/water interface through a combined experimental and theoretical approach

   https://doi.org/10.1039/D0SC02354J  

   Luo, Man; Wauer, Nicholas A.; Angle, Kyle J.; Dommer, Abigail C.; Song, Meishi; Nowak, Christopher M.; Amaro, Rommie E.; Grassian, Vicki H. (October 2020, Chemical Science)

   null (Ed.)

   The partitioning of medium-chain fatty acid surfactants such as nonanoic acid (NA) between the bulk phase and the air/water interface is of interest to a number of fields including marine and atmospheric chemistry. However, questions remain about the behavior of these molecules, the contributions of various relevant chemical equilibria, and the impact of pH, salt and bulk surfactant concentrations. In this study, the surface adsorption of nonanoic acid and its conjugate base is quantitatively investigated at various pH values, surfactant concentrations and the presence of salts. Surface concentrations of protonated and deprotonated species are dictated by surface-bulk equilibria which can be calculated from thermodynamic considerations. Notably we conclude that the surface dissociation constant of soluble surfactants cannot be directly obtained from these experimental measurements, however, we show that molecular dynamics (MD) simulation methods, such as free energy perturbation (FEP), can be used to calculate the surface acid dissociation constant relative to that in the bulk. These simulations show that nonanoic acid is less acidic at the surface compared to in the bulk solution with a p K a shift of 1.1 ± 0.6, yielding a predicted surface p K a of 5.9 ± 0.6. A thermodynamic cycle for nonanoic acid and its conjugate base between the air/water interface and the bulk phase can therefore be established. Furthermore, the effect of salts, namely NaCl, on the surface activity of protonated and deprotonated forms of nonanoic acid is also examined. Interestingly, salts cause both a decrease in the bulk p K a of nonanoic acid and a stabilization of both the protonated and deprotonated forms at the surface. Overall, these results suggest that the deprotonated medium-chain fatty acids under ocean conditions can also be present within the sea surface microlayer (SSML) present at the ocean/atmosphere interface due to the stabilization effect of the salts in the ocean. This allows the transfer of these species into sea spray aerosols (SSAs). More generally, we present a framework with which the behavior of partially soluble species at the air/water interface can be predicted from surface adsorption models and the surface p K a can be predicted from MD simulations. 

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2. Ionic Liquid-Mediated Interfacial Polymerization for Fabrication of Reverse Osmosis Membranes

   https://doi.org/10.3390/membranes12111081  

   Verma, Nisha; Chen, Lexin; Fu, Qinyi; Wu, Skyler; Hsiao, Benjamin S. (November 2022, Membranes)

   This study revealed the effects of incorporating ionic liquid (IL) molecules: 1-ethyl, 1-butyl, and 1-octyl-3-methyl-imidazolium chlorides with different alkyl chain lengths, in interfacial polymerization (IP) on the structure and property (i.e., permeate-flux and salt rejection ratio) relationships of resulting RO membranes. The IL additive was added in the aqueous meta-phenylene diamine (MPD; 0.1% w/v) phase, which was subsequently reacted with trimesoyl chloride (TMC; 0.004% w/v) in the hexane phase to produce polyamide (PA) barrier layer. The structure of resulting free-standing PA thin films was characterized by grazing incidence wide-angle X-rays scattering (GIWAXS), which results were correlated with the performance of thin-film composite RO membranes having PA barrier layers prepared under the same IP conditions. Additionally, the membrane surface properties were characterized by zeta potential and water contact angle measurements. It was found that the membrane prepared by the longer chain IL molecule generally showed lower salt rejection ratio and higher permeation flux, possibly due to the inclusion of IL molecules in the PA scaffold. This hypothesis was supported by the GIWAXS results, where a self-assembled surfactant-like structure formed by IL with the longest aliphatic chain length was detected. 

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3. Reduced Biofilm Formation at the Air–Liquid–Solid Interface via Introduction of Surfactants

   https://doi.org/10.1021/acsbiomaterials.0c01691  

   Chen, Pengyu; Lang, Jiayan; Donadt, Trevor; Yu, Zichen; Yang, Rong (March 2021, ACS Biomaterials Science & Engineering)

   null (Ed.)

   Reduced biofilm formation is highly desirable in applications ranging from transportation to separations and healthcare. Biofilms often form at the three-phase interface where air, liquid, and solid coexist due to the close proximity to nutrients and oxygen. Reducing biofilm formation at the triple interface presents challenges because of the conflicting requirements for hydrophobicity at the air−solid interface (for selfcleaning properties) and for hydrophilicity at the liquid−solid interface (for reduced foulant adhesion). Meeting those needs simultaneously likely entails a dynamic surface, capable of shifting the surface energy landscape in response to wetting conditions and thus enabling hydrophobicity in air and hydrophilicity in water. Here, we designed a facile approach to render existing surfaces resistant to biofilm formation at the triple interface. By adding trace amounts (∼0.1 mM) of surfactants, biofilm formation of Pseudomonas aeruginosa (known to form biofilm at the triple interface) was reduced on all surfaces tested, ranging from hydrophilic to hydrophobic, polar to nonpolar. That reduced fouling was not a result of the known antimicrobial effects. Instead, it was attributed to the surface-adsorbed surfactants that dynamically control surface energy at the triple interface. To further understand the effect of surfactant−surface interactions on biofilm reduction, we systematically varied the surfactant charge type and surface properties (surface energy and charge). Electrostatic interactions between surfactants and surfaces were identified as an influential factor when predicting the relative fouling reduction upon introduction of surfactants. Nevertheless, biofilm formation was reduced even on the charge-neutral, fluorinated surface made of poly(1H, 1H, 2H, 2H-perfluorodecyl acrylate) by more than 2-fold simply via adding 0.2 mM dodecyl trimethylammonium chloride or 0.3 mM sodium dodecyl sulfate. Given its robustness, this strategy is broadly applicable for reducing fouling on existing surfaces, which in turn improves the cost-effectiveness of membrane separations and mitigates contaminations and nosocomial infections in healthcare. 

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4. Functionalization of Metakaolin with Non-Ionic Surfactants: Swelling and Pozzolanic Reactivity

   Luo, Dayou; Wei, Jianqiang (October 2023, International Congress on the Chemistry of Cement 2023 (ICCC2023))

   Metakaolin (MK) has been widely used in modifying cement and designing high-performance concrete, while the role of this alumino-silicate mineral has not been fully exploited due to its low reaction degree, especially at high-volume incorporations. To enhance the pozzolanic reactivity, functionalization of MK particles with two non-ionic surfactants, namely polyoxyethylene (9) nonylphenylether (PONPE9) and t-octyl phenoxy poly ethoxyethanol (TX100), are investigated in this study under a hypothesis that the intercalations of the surfactants into MK’s interlayer space can trigger changes in structure and properties. The dry MK particles were mixed with aqueous solutions with two surfactant concentrations to reach two surfactant loadings in MK at its 1.0 and 6.0 cation exchange capacity (CEC). The surfactant uptake behavior of MK and its influence on the hygroscopic swelling, pozzolanic reactivity, and dissolution behavior in simulated cement pore solution were characterized. The results indicate that, compared with TX100, PONPE9 can be absorbed by MK more easily. After functionalization at 1.0 and 6.0 CEC, MK exhibited surfactant mass fractions of 1.85% and 3.81% for TX100, and 1.95% and 5.39% for PONPE9, respectively. The intercalation of surfactants resulted in an up to 28.6% increase in the swell index of MK when absorbing water. A more robust aluminum and silicon dissolution behavior in the simulated cement pore solution was observed from the functionalized MK. Increases in reaction heat and lime consumption capacity were obtained in the MK-lime blends indicating the enhanced pozzolanic reactivity of MK after functionalization and paving a path to enhance the role of MK in future sustainable concrete design. 

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5. A single parameter can predict surfactant impairment of superhydrophobic drag reduction

   https://doi.org/10.1073/pnas.2211092120  

   Temprano-Coleto, Fernando; Smith, Scott M.; Peaudecerf, François J.; Landel, Julien R.; Gibou, Frédéric; Luzzatto-Fegiz, Paolo (January 2023, Proceedings of the National Academy of Sciences)

   Recent experimental and computational investigations have shown that trace amounts of surfactants, unavoidable in practice, can critically impair the drag reduction of superhydrophobic surfaces (SHSs), by inducing Marangoni stresses at the air–liquid interface. However, predictive models for realistic SHS geometries do not yet exist, which has limited the understanding and mitigation of these adverse surfactant effects. To address this issue, we derive a model for laminar, three-dimensional flow over SHS gratings as a function of geometry and soluble surfactant properties, which together encompass 10 dimensionless groups. We establish that the grating lengthgis the key geometric parameter and predict that the ratio between actual and surfactant-free slip increases withg². Guided by our model, we perform synergistic numerical simulations and microfluidic experiments, finding good agreement with the theory as we vary surfactant type and SHS geometry. Our model also enables the estimation, based on velocity measurements, of a priori unknown properties of surfactants inherently present in microfluidic systems. For SHSs, we show that surfactant effects can be predicted by a single parameter, representing the ratio between the grating length and the interface length scale beyond which the flow mobilizes the air–water interface. This mobilization length is more sensitive to the surfactant chemistry than to its concentration, such that even trace-level contaminants may significantly increase drag if they are highly surface active. These findings advance the fundamental understanding of realistic interfacial flows and provide practical strategies to maximize superhydrophobic drag reduction. 

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