Hydrophobicity of Self-Assembled Monolayers of Alkanes: Fluorination, Density, Roughness, and Lennard-Jones Cutoffs

Organic Field-Effect Transistors for Interfacial Chemistry: Monitoring Reactions on SAMs at the Solid-Liquid Interface

Chemical modification of self-assembled monolayers (SAMs) at the solid-liquid interface can effectively impart SAMs with desired functions on demand. However, appropriate methods to monitor organic reactions at the solid-liquid interface have not yet been established. Therefore, this perspective introduces an extended-gate type organic field-effect transistor (EG-OFET)-based detector to monitor chemical reactions at the interface between SAMs on the extended-gate electrode of the OFET and an aqueous solution containing reactants. The EG-OFET is operated by applying gate voltages, enabling the monitoring of organic reactions on the extended-gate electrode through changes in transistor characteristics. Leveraging its amplification ability, the EG-OFET enables the sensitive detection of slight differences in product properties accompanied by variations in the charge and/or dipole moment of the SAM caused by chemical reactions at the interface. This perspective summarizes strategies, including those combined with chemometrics and microfluidic technologies, for monitoring irreversible and reversible chemical reactions at the solid-liquid interface.

Infrared Spectroscopic Signatures of the Fluorous Effect Arise from a Change of Conformational Dynamics

Per- and polyfluoroalkyl substances (PFAS) are synthetic compounds widely employed in society due to their chemical inertness. These substances accumulate in the environment, from where they enter human bodies, leading to harmful effects like cancer. PFAS exhibit omniphobic properties, which often cause them to separate from both aqueous and organic phases, forming a fluorous phase. Yet, the physical nature of this fluorous effect is unknown. Here, we shed light on the fluorous effect by analyzing the infrared absorption spectra of perfluorinated and semifluorinated alkanes in various solvents. We find that specific bands in the C-F stretching vibrational region exhibit selective behaviors in fluorous and nonfluorous environments. In a fluorous environment, these bands undergo significant broadening, and the asymmetric CF3 stretching bands decrease in intensity. Using static density functional theory calculations and force-field molecular dynamics simulations, we decipher the underlying molecular mechanisms: The decrease in absorption intensities is related to the intermolecular vibrational coupling of the perfluoroalkyl chains, while an acceleration of conformational changes in the carbon backbone of the molecules causes the observed band broadening. Given the high specificity of the reported spectral changes to a fluorous environment, bands in the C-F stretching range can serve as spectroscopic markers for the fluorous phase, facilitating the study of PFAS aggregation. Such knowledge can lead to the rational design of absorber materials for PFAS, which are aimed at mitigating their environmental impact.

Applicability of the thermodynamic and mechanical route to Young's equation for rigid and flexible solids: A molecular dynamics simulations study of a Lennard-Jones system model

The wetting properties of a liquid in contact with a solid are commonly described by Young's equation, which defines the relationship between the angle made by a fluid droplet onto the solid surface and the interfacial properties of the different interfaces involved. When modeling such interfacial systems, several assumptions are usually made to determine this angle of contact, such as a completely rigid solid or the use of the tension at the interface instead of the surface free energy. In this work, we perform molecular dynamics simulations of a Lennard-Jones liquid in contact with a Lennard-Jones crystal and compare the contact angles measured from a droplet simulation with those calculated using Young's equation based on surface free energy or surface stress. We analyze cases where the solid atoms are kept frozen in their positions and where they are allowed to relax and simulate surfaces with different wettability and degrees of softness. Our results show that using either surface free energy or surface stress in Young's equation leads to similar contact angles but different interfacial properties. We find that the approximation of keeping the solid atoms frozen must be done carefully, especially if the liquid can efficiently pack at the interface. Finally, we show that to correctly reproduce the measured contact angles when the solid becomes soft, the quantity to be used in Young's equation is the surface free energy only and that the error committed in using the surface stress becomes larger as the softness of the solid increases.

Weavable, Reconfigurable Triboelectric Ferrofluid Fiber for Early Warning

As communication technologies have become omnipresent, the prevalence of electromagnetic field (EMF) exposures poses possible health risks, particularly to vulnerable groups such as pregnant women. In response, we introduce a triboelectric ferrofluid fiber (TFF) that moves in response to EMF, thereby generating charge in a way that is self-powered. The TFF is flexible, stretchable (470%), and can be woven into fabrics. The TFF utilizes a soft-contact (ferrofluid-silicon rubber fiber) triboelectric core layer to enhance its sensitivity to EMF, enabling it to detect even minor electromagnetic fluctuations, such as those from cell phone typing. By integrating hydrogel electrodes that offer conductivity and minimal electromagnetic interference shielding, the TFF's sensitivity to magnetic fields is further amplified. Moreover, its open-circuit voltage output is increased by 50% compared to the conventional electrodes. Building on this technology, we designed a smart fabric for environmental early warning and potential real-time pulse monitoring, specifically tailored for the safety and healthcare needs of vulnerable groups. Finally, we developed a sensing and communication apparel (SCA) by integrating TFF into the apparel and exploring its capabilities in a wireless transmission of warning signals and long-distance NFC functionality.

Investigation and insights on the on-demand generation of monodispersed emulsion droplets from a floating capillary-based open microfluidic device

Simple and stable generation of monodispersed droplets with volume from picolitre to nanoliter is one of the key factors in high-throughput quantitative microreactors for chemical and biomedical applications. In this work, an efficient method that could realize simple manipulating microflow with a broad operation window for preparing monodispersed droplets with controllable diameter is developed. The microfluidic device is constructed by inserting a capillary with an oblique angle (α) into the continuous phase, named a floating capillary-based open microfluidic device (FCOMD). The transition of droplet-generating mode between dripping and jetting can be achieved by changing capillary number and α. A computational model based on the volume-of-fluid/continuum-surface-force method to explain the controllability of α on the droplet formation regime and droplet breakage, verifying the synergistic effect of ΔP and Fb, facilitates the droplet pinching. A descending order of Pn of capillary with different α is that 45° > 30° > 15° > 60° > 75°, leading to the same order of generated droplet's D. When compared with the traditional capillary co-flow device, the generating throughput of the integrated FCOMD obtained by integrating different numbers of capillaries is at least ten times. Moreover, water in oil, oil in water double-emulsion, colloidal dispersed droplets, and liquid crystal droplets with diameters ranging from 25 to 800 μm are prepared on-demand by the FCOMD, indicating the universality of the microfluidic device. Thus, the FCOMD shows the features of simplicity, practicability, and flexibility, offering valuable guidance for generating controllable droplets with wide size change and showing a great potential application in material science, foods, pharmaceuticals, and cosmetics.

Modeling Water Interactions with Graphene and Graphite via Force Fields Consistent with Experimental Contact Angles

Accurate simulation models for water interactions with graphene and graphite are important for nanofluidic applications, but existing force fields produce widely varying contact angles. Our extensive review of the experimental literature reveals extreme variation among reported values of graphene-water contact angles and a clustering of graphite-water contact angles into groups of freshly exfoliated (60° ± 13°) and not-freshly exfoliated graphite surfaces. The carbon-oxygen dispersion energy for a classical force field is optimized with respect to this 60° graphite-water contact angle in the infinite-force-cutoff limit, which in turn yields a contact angle for unsupported graphene of 80°, in agreement with the mean of the experimental results. Interaction force fields for finite cutoffs are also derived. A method for calculating contact angles from pressure tensors of planar equilibrium simulations that is ideally suited to graphite and graphene surfaces is introduced. Our methodology is widely applicable to any liquid-surface combination.

Roughness-Mediated SI-Fe0CRP for Polymer Brush Engineering toward Superior Drag Reduction

Surface-initiated iron(0)-mediated controlled radical polymerization (SI-Fe0CRP) with low toxicity and excellent biocompatibility is promising for the fabrication of biofunctional polymer coatings. However, the development of Fe(0)-based catalysts remains limited by the lower dissociation activity of the Fe(0) surface in comparison to Cu(0). Here, we found that, by simply polishing the Fe(0) plate surface with sandpaper, the poly(methacryloyloxy)ethyl trimethylammonium chloride brush growth rate has been increased significantly to 3.3 from 0.14 nm min-1 of the pristine Fe(0) plate. The excellent controllability of roughness-mediated SI-Fe0CRP can be demonstrated by customizing multicompartment brushes and triblock brushes. Furthermore, we found that the resulting polymer brush coatings exhibit remarkably low water adhesion (0.097 mN) and an outstanding drag reduction rate of 52% in water. This work provides a promising strategy for regulating the grafting rate of polymer brushes via SI-Fe0CRP for biocompatible marine drag reduction coatings.

Conditions for the stable adsorption of lipid monolayers to solid surfaces

Lipid monolayers are ubiquitous in biological systems and have multiple roles in biotechnological applications, such as lipid coatings that enhance colloidal stability or prevent surface fouling. Despite the great technological importance of surface-adsorbed lipid monolayers, the connection between their formation and the chemical characteristics of the underlying surfaces has remained poorly understood. Here, we elucidate the conditions required for stable lipid monolayers nonspecifically adsorbed on solid surfaces in aqueous solutions and water/alcohol mixtures. We use a framework that combines the general thermodynamic principles of monolayer adsorption with fully atomistic molecular dynamics simulations. We find that, very universally, the chief descriptor of adsorption free energy is the wetting contact angle of the solvent on the surface. It turns out that monolayers can form and remain thermodynamically stable only on substrates with contact angles above the adsorption contact angle, θads. Our analysis establishes that θads falls into a narrow range of around 60∘-70∘ in aqueous media and is only weakly dependent on the surface chemistry. Moreover, to a good approximation, θads is roughly determined by the ratio between the surface tensions of hydrocarbons and the solvent. Adding small amounts of alcohol to the aqueous medium lowers θads and thereby facilitates monolayer formation on hydrophilic solid surfaces. At the same time, alcohol addition weakens the adsorption strength on hydrophobic surfaces and results in a slowdown of the adsorption kinetics, which can be useful for the preparation of defect-free monolayers.

Quantitative evaluation of perfluorinated alkanethiol molecular order on gold surfaces

Self-assembled monolayers (SAMs) of perfluoroalkanethiols [CF3(CF2)xCH2CH2SH (x = 3, 5, 7, and 9)] on gold were characterized by x-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), and static time-of-flight secondary ion mass spectrometry (ToF-SIMS). Perfluoroalkanethiols of several chain lengths were synthesized using a known hydride reduction method for transforming commercially available perfluoroalkyliodides to corresponding perfluoroalkanethiols. This strategy provides improved product yields compared to other known routes based on hydrolysis from the common thioacetyl perfluoroalkyl intermediate. Angle-dependent XPS analysis revealed that CF3(CF2)xCH2CH2SH (x = 5, 7, and 9; F6, F8, and F10, respectively) SAMs on gold exhibited significant enrichment of the terminal CF3 group at the outer monolayer surface with the sulfur present as a metal-bound thiolate located at the monolayer-gold interface. XPS of the CF3(CF2)3CH2CH2SH (F4) monolayer revealed a thin film with a significant (>50%) amount of hydrocarbon contamination consistent with poorly organized monolayers, while the longest thiol (F10) showed XPS signals attributed to substantial ordering and anisotropy. ToF-SIMS spectra from all four SAMs contained molecular ions representative of the particular perfluorinated thiol used to prepare the monolayer. NEXAFS methods were used to determine degrees of ordering and average tilt for molecules comprising monolayers. The SAMs prepared from the longest (F10) thiols exhibited the highest degree of ordering with the molecular axis nearly perpendicular to the gold surface. The degree of ordering decreased significantly with decreasing length of the perfluorocarbon tail.

Synthesis and Linker-Controlled Self-Assembly of Dendritic Amphiphiles with Branched Fluorinated Tails

Amphiphiles containing fluorinated segments tend to aggregate in the aqueous solution into structure of lower curvature than their hydrocarbon analogs due to their larger diameter. A benefit of supramolecular structures incorporating fluorine moieties is their high electron density, which can be viewed in cryo-TEM with better contrast than their hydrogenated forms. A modular approach has been developed for the synthesis of a new family of nonionic branched amphiphiles consisting of oligoglycerol units (G2) as the hydrophilic part and a branched fluorinated (F27) hydrophobic part. The design of this hydrophobic moiety allows to achieve a higher fluorine density than the previously used straight-chain perfluoroalkanes. Two different chemical approaches, amide, and triazole, are used to link the hydrophilic and hydrophobic segments. In addition, the aggregation behavior is investigated by dynamic light scattering (DLS) and cryo-TEM. The measurements prove the formation of multivesicular (MVVs) and multilamellar (MLVs) vesicles as well as smaller unilamellar vesicles. Further, the cell viability test proves the low cell toxicity of these nanoarchitectures for potential biomedical applications.

Computing the Work of Solid-Liquid Adhesion in Systems with Damped Coulomb Interactions via Molecular Dynamics: Approaches and Insights

Recently, the dry-surface method [Langmuir2016, 31, 8335−8345] has been developed to compute the work of adhesion of solid–liquid and other interfaces using molecular dynamics via thermodynamic integration. Unfortunately, when long-range Coulombic interactions are present in the interface, a special treatment is required, such as solving additional Poisson equations, which is usually not implemented in generic molecular dynamics software, or as fixing some groups of atoms in place, which is undesirable most of the time. In this work, we replace the long-range Coulombic interactions with damped Coulomb interactions, and explore several thermal integration paths. We demonstrate that regardless of the integration path, the same work of adhesion values are obtained as long as the path is reversible, but the numerical efficiency differs vastly. Simple scaling of the interactions is most efficient, requiring as little as 8 sampling points, followed by changing the Coulomb damping parameter, while modifying the Coulomb interaction cutoff length performs worst. We also demonstrate that switching long-range Coulombic interactions to damped ones results in a higher work of adhesion by about 10 mJ/m² because of slightly different liquid molecule orientation at the solid–liquid interface, and this value is mostly unchanged for surfaces with substantially different Coulombic interactions at the solid–liquid interface. Finally, even though it is possible to split the work of adhesion into van der Waals and Coulomb components, it is known that the specific per-component values are highly dependent on the integration path. We obtain an extreme case, which demonstrates that caution should be taken even when restricting to qualitative comparison.

Surface Roughness Explains the Observed Water Contact Angle and Slip Length on 2D Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) is a two-dimensional (2D) material that is currently being explored in a number of applications, such as atomically thin coatings, water desalination, and biological sensors. In many of these applications, the hBN surface comes into intimate contact with water. In this work, we investigate the wetting and frictional behavior of realistic 2D hBN surfaces with atomic-scale defects and roughness. We combine density functional theory calculations of the charge distribution inside hBN with free energy calculations using molecular dynamics simulations of the hBN-water interface. We find that the presence of surface roughness, but not that of vacancy defects, leads to remarkable agreement with the experimentally observed water contact angle of 66° on freshly synthesized, uncontaminated hBN. Not only that, the inclusion of surface roughness predicts with exceptional accuracy the experimental water slip length of ∼1 nm on hBN. Our results underscore the importance of considering realistic models of 2D materials with surface roughness while modeling nanomaterial-water interfaces in molecular simulations.

Critical Packing Density of Water-Mediated Nonstick Self-Assembled Monolayer Coatings

Nanoparticle-mineral surface interactions are relevant in many biological and geological applications. We have previously studied nanoparticle coatings based on closely packed bicomponent polyol-fluoroalkane self-assembled monolayers (SAMs) that can have tunable stickiness on calcite surfaces by changing the compositions of fluoroalkanes in SAMs, where the coatings show nonstick properties if fluoroalkanes can effectively perturb hydration layers on calcite surfaces. However, when applying coatings on nanoparticles, it can be challenging to predict the maximum achievable coating density. Here, we study how would water-mediated SAM-calcite interactions change with different SAM coating densities. Molecular dynamics simulations show that compositionally repulsive, closely packed polyol-fluoroalkane SAMs become adhesive to calcite surfaces with decreasing coating densities. Our modeling shows that this results from the collapsing of fluoroalkanes into the voids of SAMs, where fluoroalkanes can no longer perturb hydration layers on calcite surfaces. Interestingly, we find that the nonstick-stick transition occurs when the volume fractions of the voids on SAMs are greater than the volume fractions of hydrophilic coating molecules.