Classical Molecular Dynamics Simulation of Glyonic Liquids: Structural Insights and Relation to Conductive Properties

Rhamnolipids are biosurfactants that have obtained wide industrial and environmental interests with their biodegradability and great surface activity. Besides their important roles as surfactants, they are found to function as a new type of glycolipid-based protic ionic liquids (ILs)glyonic liquids (GLs). GLs are reported to have impressive physicochemical properties, especially superionic conductivity, and it was reported in experiments that specific ion selections and the fraction of water content have a strong effect on the conductivity. Also, the shape of the conductivity curve as a function of water fraction in GLs is interesting with a sharp increase first and a long plateau. We related the conductivities to the three-dimensional (3D) networks composed of −OH inside the GLs utilizing classical molecular dynamics (MD) simulations. The amount and size of these networks vary with both ion species and water fractions. Before reaching the first hydration layer, the −OH networks with higher projection/box length ratios indicate better conductivity; after reaching the first hydration layer and forming continuous structures, the conductivity retains with more water molecules participating in the continuous networks. Therefore, networks are found to be a qualitative predictor of actual conductivity. This is explained by the analysis of the atomic structures, including radial distribution function, fraction free volume, anion conformations, and hydrogen bond occupancies, of GLs and their water mixtures under different chemical conditions

Thickness, Composition, and Molecular Structure of Residual Thin Films Formed by Forced Dewetting of Ag from Glycerol/D₂O Solutions

The thickness, composition, and interfacial molecular structure of residual thin films retained on the surface of polycrystalline Ag substrates after being forcibly dewet from glycerol/D₂O solutions are investigated using contact angle measurements, ellipsometry, and polarization modulation-infrared reflection–absorption spectroscopy (PM-IRRAS). Residual film thicknesses are rationalized on the basis of the relevant long-range van der Waals and structural forces leading to residual film formation along with the interfacial glycerol and D₂O structure. Unique interfacial composition, wherein glycerol preferentially segregates to the residual film interfaces, is substantiated by PM-IRRAS. Thus, the residual films possess composition and molecular structure that differ from those of bulk solution. Specifically, in the thinnest residual films, glycerol interacts strongly with the Ag substrate, leading to glycerol that is more ordered than the bulk liquid that coexists with bulk-like D₂O. In thicker residual films, the glycerol mole fraction is still enhanced relative to the bulk solution, but both ordered and liquid-like glycerol species are observed along with D₂O that is more strongly hydrogen-bonded than in the bulk. The creation of residual films by forced dewetting and their interrogation by spectroscopic methods are thus demonstrated to represent a powerful approach for characterizing interfacial liquid molecular structure near solid surfaces but beyond the first monolayer under ambient conditions

Reaction of Thin Films of Solid-State Benzene and Pyridine with Calcium

The reaction between small organic molecules and low work function metals is of interest in organometallic, astronomical, and optoelectronic device chemistry. Here, thin, solid-state, amorphous benzene and pyridine films are reacted with Ca at 30 K under ultrahigh vacuum with the reaction progress monitored by Raman spectroscopy. Although both films react with Ca to produce product species identifiable by their vibrational spectroscopic signatures, benzene is less reactive with Ca than pyridine. Benzene reacts by electron transfer from Ca to benzene producing multiple species including the phenyl radical anion, the phenyl radical, and the benzyne diradical. Pyridine initially reacts along a similar electron transfer pathway as indicated by the presence of the corresponding pyridyl radical and pyridyne diradical species, but these pyridyl radicals are less stable and subject to further ring-opening reactions that lead to a complex array of smaller molecule reaction products and ultimately amorphous carbon. The elucidation of this reaction pathway provides insight into the reactions of aromatics with Ca that are relevant in the areas of catalysis, astrochemistry, and organic optoelectronics

Flow Field Penetration in Thin Nanoporous Polymer Films under Laminar Flow by Förster Resonance Energy Transfer Coupled with Total Internal Reflectance Fluorescence Microscopy

Polymer–fluid interfaces are used widely in a variety of applications, including separations, which require exposure of the polymer to dynamic flow conditions. Despite the ubiquity of such interfaces, the importance of convective mass transport within the near-interface region of a polymer is a fundamental process that is still poorly defined. As a step toward better defining mass transport behavior within the near-interface portion of a polymer, in this work, a new application of a spectroscopic method based on the combination of Förster resonance energy transfer (FRET) and total internal reflectance fluorescence microscopy (TIRFM) is reported that allows quantification of the penetration depth of a laminar flow field (i.e., the slip length) in a densely grafted, thin poly­(<i>N</i>-isopropylacrylamide) (pNIPAM) film as a model polymer system. Specifically, decay curves from FRET of an acceptor with a donor attached at the substrate surface are fit to a combined Taylor–Aris–Fickian mass transport model to extract apparent linear diffusion coefficients of acceptor molecules for different flow rates. Apparent diffusion coefficients range from 1.9 × 10^–12 to 9.1 × 10^–12 cm²/s for near-surface flow linear velocities ranging from 192 to 2952 μm/s. This increase in apparent diffusion coefficient with fluid flow rate suggests increasing contributions from convective mass transport that are indicative of flow field penetration into the polymer film. The depth of penetration of the flow field is estimated to range from ∼6% of the polymer film thickness in a good solvent at ∼192 μm/s to ∼60% of the film thickness at ∼2952 μm/s. Thus, flow field penetration into polymer thin films, with its concomitant contributions from convective mass transport within the near-interface region of the polymer, is demonstrated and quantified experimentally

Signature Vibrational Bands for Defects in CVD Single-Layer Graphene by Surface-Enhanced Raman Spectroscopy

We report the observation of signature vibrational bands in the frequency region between 900 and 1600 cm^–1 for defects in single-layer graphene (SLG) using surface Raman spectroscopy in ultrahigh vacuum. Vapor deposition of Ag leads to the formation of surface nanoparticles that migrate to defects in the SLG, leading to surface-enhanced Raman scattering (SERS) of the graphene G and 2D bands as well as new vibrational modes ascribed to native defects. Many of the new spectral bands of these native defects are similar, although not identical, to those predicted previously for −C₂ defects. These new bands are observed in addition to bands more commonly observed for defective graphene that are attributed to the D, G*, D+G, and 2D′ modes. The defects observed in these SLG films are not believed to result from the Ag deposition process but are postulated to be formed during the graphene CVD growth process. These defects are then made visible by postdeposition of Ag due to SERS

Molecular Dynamics Simulation of the Oil Sequestration Properties of a Nonionic Rhamnolipid

A detailed molecular dynamics simulation study is presented on the behavior of aggregates composed of the nonionic monorhamnolipid α-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10) and decane in bulk water. A graph theoretical approach was utilized to characterize the size and composition of the many aggregates generated in our simulations. Overall, we observe that the formation of oil in Rha-C10-C10 aggregates is a favorable process. Detailed analysis on the surfactant/oil aggregate shows that larger aggregates are stable. The shape and size of the aggregates are widely distributed, with the majority of the aggregates preferring ellipsoidal or cylindrical structures. Irrespective of the decane concentration in the system, we did not observe free decane in any of the simulations. Further insights into the binding energy of decane were carried out using free-energy perturbation calculations. The results showed that the trapped decane molecules provide stability to the Rha-C10-C10 aggregates of size <i>N</i> = 50 which are shown to be unstable in our previous study and allow for the growth of larger aggregates than pure Rha-C10-C10 in water. The density profile plots show that decane molecules encapsulated inside the aggregate preferred to remain closer to the center of mass. This study points to the feasibility of using this biosurfactant as an environmental remediation agent

Structural Properties of Nonionic Monorhamnolipid Aggregates in Water Studied by Classical Molecular Dynamics Simulations

Molecular dynamics simulations were carried out to investigate the structure and stabilizing factors of aggregates of the nonionic form of the most common congener of monorhamnolipids, α-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10), in water. Aggregates of size ranging from 5 to 810 monomers were observed in the simulation forming spherical and ellipsoidal structures, a torus-like structure, and a unilamellar vesicle. The effects of the hydrophobic chain conformation and alignment in the aggregate, role of monomer···monomer and monomer···water H-bonds, and conformations of monomers in the aggregate were studied in detail. The unilamellar vesicle is highly stable due to the presence of isolated water molecules inside the core adding to the binding energy. Dissociation of a monomer from a larger micellar aggregate is relatively easy compared to that from smaller aggregates as seen from potential of mean force calculations. This analysis also shows that monomers are held more strongly in aggregates of Rha-C10-C10 than the widely used surfactant sodium dodecyl sulfate. Comparisons between the aggregation behavior of nonionic and anionic forms of Rha-C10-C10 are presented

Molecular Dynamics Simulation of the Oil Sequestration Properties of a Nonionic Rhamnolipid

A detailed molecular dynamics simulation study is presented on the behavior of aggregates composed of the nonionic monorhamnolipid α-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10) and decane in bulk water. A graph theoretical approach was utilized to characterize the size and composition of the many aggregates generated in our simulations. Overall, we observe that the formation of oil in Rha-C10-C10 aggregates is a favorable process. Detailed analysis on the surfactant/oil aggregate shows that larger aggregates are stable. The shape and size of the aggregates are widely distributed, with the majority of the aggregates preferring ellipsoidal or cylindrical structures. Irrespective of the decane concentration in the system, we did not observe free decane in any of the simulations. Further insights into the binding energy of decane were carried out using free-energy perturbation calculations. The results showed that the trapped decane molecules provide stability to the Rha-C10-C10 aggregates of size <i>N</i> = 50 which are shown to be unstable in our previous study and allow for the growth of larger aggregates than pure Rha-C10-C10 in water. The density profile plots show that decane molecules encapsulated inside the aggregate preferred to remain closer to the center of mass. This study points to the feasibility of using this biosurfactant as an environmental remediation agent

A PM-IRRAS Investigation of Monorhamnolipid Orientation at the Air–Water Interface

The rhamnolipid biosurfactants have been considered as possible “green” alternatives to synthetic surfactants due to their greater compatibility with the environment and excellent surface active properties. In order to understand the molecular orientation of rhamnolipids at the air–water interface, a new monorhamnolipid with two octadecyl chains, Rha–C18–C18, has been studied at the air–water interface with polarization modulated-infrared reflection absorption spectroscopy (PM-IRRAS). Since rhamnolipids possess a carboxylic acid, and hence exhibit pH-dependent properties, their water surface orientation is studied in solutions of pH 2, 5, and 8. Rhamnolipids have also been reported to form strong complexes with Pb²⁺; thus, the effect of the presence of Pb²⁺ on molecular orientation at the interface is also investigated. PM-IRRA spectra indicate an increase in alkyl chain order and a decrease in alkyl chain tilt angle as the surface pressure of the monolayer increases, with pH-independent tilt angles ranging from 63° to 45°. Molecular modeling using Spartan provides insight into the cause of this large tilt angle as being due to the nature of the monorhamnolipid packing at the air–water interface as dictated by its large hydrophilic headgroup

PM-IRRAS Determination of Molecular Orientation of Phosphonic Acid Self-Assembled Monolayers on Indium Zinc Oxide

Self-assembled monolayers (SAMs) of phosphonic acids (PAs) on transparent conductive oxide (TCO) surfaces can facilitate improvement in TCO/organic semiconductor interface properties. When ordered PA SAMs are formed on oxide substrates, interface dipole and electronic structure are affected by the functional group properties, orientation, and binding modes of the modifiers. Choosing octylphosphonic acid (OPA), F₁₃-octylphosphonic acid (F₁₃OPA), pentafluorophenyl phosphonic acid (F₅PPA), benzyl phosphonic acid (BnPA), and pentafluorobenzyl phosphonic acid (F₅BnPA) as a representative group of modifiers, we report polarization modulation-infrared reflection–absorption spectroscopy (PM-IRRAS) of binding and molecular orientation on indium-doped zinc oxide (IZO) substrates. Considerable variability in molecular orientation and binding type is observed with changes in PA functional group. OPA exhibits partially disordered alkyl chains but on average the chain axis is tilted ∼57° from the surface normal. F₁₃OPA tilts 26° with mostly tridentate binding. The F₅PPA ring is tilted 23° from the surface normal with a mixture of bidentate and tridentate binding; the BnPA ring tilts 31° from normal with a mixture of bidentate and tridentate binding, and the F₅BnPA ring tilts 58° from normal with a majority of bidentate with some tridenate binding. These trends are consistent with what has been observed previously for the effects of fluorination on orientation of phosphonic acid modifiers. These results from PM-IRRAS are correlated with recent results on similar systems from near-edge X-ray absorption fine structure (NEXAFS) and density functional theory (DFT) calculations. Overall, these results indicate that both surface binding geometry and intermolecular interactions play important roles in dictating the orientation of PA modifiers on TCO surfaces. This work also establishes PM-IRRAS as a routine method for SAM orientation determination on complex oxide substrates