Long-range Intermolecular Forces Research Articles

Preparation of structural colors from lignin: Improving the homogeneity between different raw materials by solvent precipitation fractionation

Structural color materials prepared by lignin colloidal spheres (LCSs) have attracted great interest due to their biocompatibility, biodegradability, multifunctionality, and ability to weaken incoherent scattering. However, the lignin raw materials originating from different biomass resources have distinct heterogeneity leading to a big difference in the size of resultant LCSs via self-assembly, which is a major challenge for the preparation of stable structural colors. In this work, we employ the solvent precipitation method to fractionate the crude lignin samples obtained from corncob, softwood, hardwood, and bamboo, respectively. After solvent precipitation fractionation, the self-assembly behaviors of different lignin samples become similar. The LCSs prepared by fractionated lignin samples show quite similar sizes (212, 202, 215, and 210 nm) compared with the LCSs prepared by crude lignin samples (341, 83, 128, and 253 nm). The quantitative atomic force microscope force spectroscopy demonstrates the improved homogeneity of LCSs prepared by fractionated lignin samples is attributed to similar long-ranged intermolecular forces in self-assembly. By employing LCSs as building blocks, the lignin-based pigments with stable structural colors are successfully prepared. The presented colors are independent of the biomass resource types, which benefits the product stability and large-scale production of lignin-based structural color materials.

The multicomponent diffuse-interface model and its application to water/air interfaces

Fundamental properties of the multicomponent diffuse-interface model (DIM), such as the maximum entropy principle and conservation laws, are used to explore the basic interfacial dynamics and phase transitions in fluids. Flat interfaces with monotonically changing densities of the components are proved to be stable. A liquid layer in contact with oversaturated but stable vapour is shown to either fully evaporate or eternally expand (depending on the initial perturbation), whereas a liquid in contact with saturated vapour always evaporates. If vapour is bounded by a solid wall with a sufficiently large contact angle, spontaneous condensation occurs in the vapour. The external parameters of the multicomponent DIM – e.g. the Korteweg matrix describing the long-range intermolecular forces – are determined for the water/air combination. The Soret and Dufour effects are shown to be negligible in this case, and the interfacial flow, close to isothermal.

Monodispersed Lignin Colloidal Spheres with Tailorable Sizes for Bio-Photonic Materials.

Lignin colloidal spheres (LCSs) are promising biomaterials for application in drug storage and delivery, pollutant adsorption, and ultraviolet protection due to their biocompatibility, amphiphilicity, and conjugated structure. However, wide size distribution of LCSs greatly limits their performances, especially in many precise and advanced applications. Herein, the fabrication of monodispersed LCSs with tailorable sizes ranging from the nanoscale to microscale is reported. Lignin raw materials are first fractionated by solvent extraction, and then the lignin fraction is used to fabricate monodispersed LCSs by solvent/antisolvent self-assembly. The underlying mechanism for the formation of monodispersed LCS is primarily ascribed to the improved homogeneity of long-range intermolecular forces, especially the electrostatic forces and hydrophobic forces, between lignin molecules. Moreover, by manipulating the short-range order of LCSs, an innovative application of lignin as bio-photonic materials with tunable structural colorations (e.g., red, green, or blue) is demonstrated. This work not only provides deep insight and an effective strategy to eliminate the serious inhomogeneity of LCSs, but also makes lignin resources have great potential as biodegradable and biocompatible photonic materials in diverse advanced optical application fields such as photonic devices, anti-counterfeiting labels, and structural color pigments.

Experimental evidence for long-distance electrodynamic intermolecular forces.

Both classical and quantum electrodynamics predict the existence of dipole-dipole long-range electrodynamic intermolecular forces; however, these have never been hitherto experimentally observed. The discovery of completely new and unanticipated forces acting between biomolecules could have considerable impact on our understanding of the dynamics and functioning of the molecular machines at work in living organisms. Here, using two independent experiments, on the basis of different physical effects detected by fluorescence correlation spectroscopy and terahertz spectroscopy, respectively, we demonstrate experimentally the activation of resonant electrodynamic intermolecular forces. This is an unprecedented experimental proof of principle of a physical phenomenon that, having been observed for biomacromolecules and with long-range action (up to 1000 Å), could be of importance for biology. In addition to thermal fluctuations that drive molecular motion randomly, these resonant (and thus selective) electrodynamic forces may contribute to molecular encounters in the crowded cellular space.

Effective removal of water-soluble methylated arsenic contaminants with phosphorene oxide nanoflakes: A DFT study

This work explores by density functional theory the ability of oxidized phosphorene nanoflakes (PhosO) to simultaneously remove toxic water-soluble methylarsenicals from contaminated waters. We focused on the microscopic understanding of the PhosO-Methylarsenic complexes in solution, i.e., structure, stability, and intermolecular driving forces. In this way, adsorption energies and conformations, binding and energy decomposition analyses (ALMO-EDA), implicitly/explicitly solvated structures, and competitive adsorption with coexisting molecules in solution afford deep insights into the selectivity adsorption and interaction mechanisms. The PhosO nanoflakes form inner-sphere surface complexes with methylarsenicals in solution, even with enhanced adsorption stability compared to intrinsic phosphorene and without the competition of water molecules for adsorption sites. The inner-sphere surface adsorption of trivalent methylarsenicals is driven by electrostatic and charge-transfer (orbital) intermolecular forces. Surface complexation of pentavalent methylarsenicals occurs by a balanced contribution of orbital and long-range intermolecular forces, while anionic contaminants also show a high stabilization via extra polarization effects. PhosO nanoflakes turn convenient to recycle via simple treatment with alkaline eluents because of the increased affinity with hydroxide anions in solution compared with intrinsic phosphorene. Conceptually understanding the adsorption properties of phosphorene oxide towards hazardous methylarsenicals provides a valuable framework for new developments in future water and wastewater treatment technologies with adsorbents of large surface area, high adsorption capacity/selectivity, and easy recovery.

Stereo-dynamical effects in chemi-ionization reactions of atmospheric O2 and N2 molecules promoted by collisions with Ne*(3P2,0) atoms

This letter focuses on general aspects of the stereodynamics of chemi-ionization reactions, triggered by collisions of O2 and N2 molecules with metastable neon atoms. Basic characteristics of the long-range intermolecular forces involved, some of chemical and others of physical origin, suggest that the weakly bound precursor state, determining the reaction transition state structure and stability, is rather different. Fundamental reaction selectivities are clarified, highlighting differences in the: i) optical potential whose real and imaginary parts must be interdependent; ii) polarizability, quadrupolar moments and orbital representation of the molecules involved, which allow to lay the foundations for future more in-depth investigations.

Rate enhancement in collisions of sulfuric acid molecules due to long-range intermolecular forces

Abstract. Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical noninteracting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We studied the statistics of collisions of sulfuric acid molecules in a vacuum using atomistic molecular dynamics simulations. We found that the effective collision cross section of the H2SO4 molecule, as described by an optimized potentials for liquid simulation (OPLS). OPLS all-atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid. As a consequence, the actual collision coefficient is enhanced by a factor of 2.2 at 300 K compared with kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard-sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, based on the attractive part of the atomistic intermolecular potential of mean force.

Direct detection of polar structure formation in helium nanodroplets by beam deflection measurements.

Long-range intermolecular forces are able to steer polar molecules submerged in superfluid helium nanodroplets into highly polar metastable configurations. We demonstrate that the presence of such special structures can be identified, in a direct and determinative way, by electrostatic deflection of the doped nanodroplet beam. The measurement also establishes the structures' electric dipole moments. In consequence, the introduced approach is complementary to spectroscopic studies of low-temperature molecular assembly reactions. It is enabled by the fact that within the cold superfluid matrix the molecular dipoles become nearly completely oriented by the applied electric field. As a result, the massive (tens of thousands of helium atoms) nanodroplets undergo significant deflections. The method is illustrated here by an application to dimers and trimers of dimethyl sulfoxide (DMSO) molecules. We interpret the experimental results with ab initio theory, mapping the potential energy surface of DMSO complexes and simulating their low temperature aggregation dynamics.

Mapping Depletion of Lubricant Films on Antibiofouling Wrinkled Slippery Surfaces.

Slippery liquid infused porous surfaces (SLIPS) have recently gained a lot of attention because of their wide range of applications. We recently showed that SLIPS with most of their surface depleted of lubricant, as little lubricant as 0.02 ± 0.01 μL cm-1, were effective against marine biofouling. Characterization of the depletion and configuration of the immobilized liquid layer on SLIPS is crucial to optimizing their performance. Previous attempts at mapping lubricant thickness have been diffraction limited or indirectly measured thickness. Here, we use atomic force microscopy meniscus force measurements to directly map lubricant thickness with nanoscale resolution on wrinkled surfaces made from Teflon and poly(4-vinylpyridine) (P4VP). Using this method, we show that SLIPS are easily depleted and are effectively heterogeneous surfaces, where the majority of the surface is a thick lubricating layer stabilized by capillary forces and part nanothin layer stabilized long-range intermolecular forces. We found that the depleted silicone oil thickness on the tops of nonwettable (Teflon) wrinkles is approx. 5 nm, close to but greater than the minimum measurable thickness of approx. 3 nm. The silicone oil thickness on the tops of wettable (P4VP) wrinkles is approx. 15 nm. Surfaces in this state still show antibiofouling properties and thus show that a thick lubricating layer is not necessary for all favorable properties of SLIPS.

The finite-size effect in thin liquid crystal systems

Effects of surface ordering in liquid crystal systems confined between cell plates are of great theoretical and experimental interest. Liquid crystals introduced in thin cells are known to be strongly stabilized and ordered by cell plates. We introduce a new theoretical method for analyzing the effect of surfaces on local molecular ordering in thin liquid crystal systems with planar geometry of the smectic layers. Our results show that, due to the interplay between pair long-range intermolecular forces and nonlocal, relatively short-range, surface interactions, both orientational and translational orders of liquid crystal molecules across confining cells are very complex. In particular, it is demonstrated that the SmA, nematic, and isotropic phases can coexist. The phase transitions from SmA to nematic, as well as from nematic to isotropic phases, occur not simultaneously in the whole volume of the system but begin to appear locally in some regions of the LC sample. Phase transition temperatures are demonstrated to be strongly affected by the thickness of the LC system. The dependence of the corresponding shifts of phase transition temperatures on the layer number is shown to exhibit a power law character. This new type of scaling behavior is concerned with the coexistence of local phases in finite systems. The influence of a specific character of interactions of molecules with surfaces and other molecules on values of the resulting critical exponents is also analyzed.

The Selective Role of Long-Range Forces in the Stereodynamics of Ion-Molecule Reactions: The He+ +Methyl Formate Case From Guided-Ion-Beam Experiments.

Long-range intermolecular forces play a crucial role in controlling the outcome of ion-molecule chemical reactions, such as those determining the disappearance of organic or inorganic "complex" molecules recently detected in various regions of the interstellar medium due to collisions with abundant interstellar atomic ions (e.g. H+ and He+ ). Theoretical treatments, for example, based on simple capture models, are nowadays often adopted to evaluate the collision-energy dependence of reactive cross sections and the temperature dependent rate coefficients of many ion-molecule reactions. The obtained results are widely used for the modelling of phenomena occurring in different natural environments or technological applications such as astrophysical and laboratory plasmas. Herein it is demonstrated, through a combined experimental and theoretical investigation on a prototype ion-molecule reaction (He+ +methyl formate), that the dynamics, investigated in detail, shows some intriguing features that can lead to rate coefficients at odds with the expectations (e.g. Arrhenius versus anti-Arrhenius behaviour). Therefore, this study casts light on some new and general guidelines to be properly taken into account for a suitable evaluation of rate coefficients of ion-molecule reactions.

Effect of surface tension on the behavior of adhesive contact based on Lennard–Jones potential law

The present study explores the effect of surface tension on adhesive contact behavior where the adhesion is interpreted by long-range intermolecular forces. The adhesive contact is analyzed using the equivalent system of a rigid sphere and an elastic half space covered by a membrane with surface tension. The long-range intermolecular forces are modeled with the Lennard‒Jones (L‒J) potential law. The current adhesive contact issue can be represented by a nonlinear integral equation, which can be solved by Newton‒Raphson method. In contrast to previous studies which consider intermolecular forces as short-range, the present study reveals more details of the features of adhesive contact with surface tension, in terms of jump instabilities, pull-off forces, pressure distribution within the contact area, etc. The transition of the pull-off force is not only consistent with previous studies, but also presents some new interesting characteristics in the current situation.

The effect of surfaces on molecular ordering in thin liquid-crystal systems.

A theoretical method for analyzing the interplay between pair long-range intermolecular forces and nonlocal, relatively short-range, surface interactions in liquid crystals, confined between plates of thin planar cells, is developed. It is shown that this method, as involving the concept of local orientational and translational order parameters, enables detailed investigations of the emergence of smectic A, nematic, and isotopic phases, as well as yields an insight into phase transitions between them, in cases of systems strongly affected by surfaces. The evidence of various surface effects, including the coexistence of different phases and the inward propagation of surface melting under the increase of temperature, is also given. The underlying numerical procedure, based on the algorithm of self-consistent calculations of local order parameters, is found to be very effective, allowing one to consider model systems of rather large thicknesses, corresponding to thicknesses of real sample cells.

Fluid structure in the immediate vicinity of an equilibrium three-phase contact line and assessment of disjoining pressure models using density functional theory

We examine the nanoscale behavior of an equilibrium three-phase contact line in the presence of long-ranged intermolecular forces by employing a statistical mechanics of fluids approach, namely, density functional theory (DFT) together with fundamental measure theory (FMT). This enables us to evaluate the predictive quality of effective Hamiltonian models in the vicinity of the contact line. In particular, we compare the results for mean field effective Hamiltonians with disjoining pressures defined through (i) the adsorption isotherm for a planar liquid film, and (ii) the normal force balance at the contact line. We find that the height profile obtained using (i) shows good agreement with the adsorption film thickness of the DFT-FMT equilibrium density profile in terms of maximal curvature and the behavior at large film heights. In contrast, we observe that while the height profile obtained by using (ii) satisfies basic sum rules, it shows little agreement with the adsorption film thickness of the DFT results. The results are verified for contact angles of 20°, 40°, and 60°.

Short and long time drop dynamics on lubricated substrates

Liquid infiltrated solids have been proposed as functional solvent-phobic surfaces for handling single and multiphase flows. Implementation of such surfaces alters the interfacial transport phenomenon as compared to a dry substrate. To better understand the interface characteristics in such systems we study experimentally the dynamics of a pendant water drop in air that contacts a substrate coated by thin oil films. At short times the water drop is deformed by the oil that spreads onto the water-air interface, and the dynamics are characterized by inertial and viscous regimes. At late times, the the oil film under the drop relaxes either to a stable thin film or ruptures. In the thin film rupture regime, we measure the waiting time for the rupture as a function of the drop equilibrium contact angle on a dry substrate and the initial film height. The waiting time is rationalized by lubrication theory, which indicates that long-range intermolecular forces destabilize the oil-water interface and is the primary mechanism for the film drainage.

Combined influences of electrostatic component of disjoining pressure and interfacial slip on thin film evaporation in nanopores

Transport characteristics of evaporating thin liquid films in nanopores play a critical role in designing effective thermal management of high performance electronic devices and systems. In such devices and systems, the thin film evaporating region is formed as a combined consequence of the capillary and long-range intermolecular forces. Traditionally, dispersion forces are considered as the dominant element of long-range intermolecular forces and are represented by disjoining pressures using Hamaker’s constant. In the present study, we investigate the implications of electrostatic component in disjoining pressure on interfacial transport of evaporating liquid confined in cylindrical nanopores in presence of interfacial slip. The most significant finding from the present study is a nonlinear alteration in the thin film profiles, as a combined confluence of the electrostatic component in disjoining pressure and interfacial slip. We reveal that the transition region of the evaporating meniscus is extended when we consider the electrostatic component in disjoining pressure. Interestingly and non-trivially, this extension turns out not only to be a function of the parameters dictating the electrostatic component in disjoining pressure, but also on the extent of interfacial slip and other relevant parameters, namely, the pore size, the wall temperature and the vapor-pressure ratio. We further reveal that the evaporative mass flux is always more with the consideration of electrostatic component in disjoining pressure than that without the consideration of electrostatic component in disjoining pressure and the mass flux increases with an increase in the level of interfacial slip.

Geometry-induced phase transition in fluids: Capillary prewetting

We report a new first-order phase transition preceding capillary condensation and corresponding to the discontinuous formation of a curved liquid meniscus. Using a mean-field microscopic approach based on the density functional theory we compute the complete phase diagram of a prototypical two-dimensional system exhibiting capillary condensation, namely that of a fluid with long-ranged dispersion intermolecular forces which is spatially confined by a substrate forming a semi-infinite rectangular pore exerting long-ranged dispersion forces on the fluid. In the T-μ plane the phase line of the new transition is tangential to the capillary condensation line at the capillary wetting temperature T(cw). The surface phase behavior of the system maps to planar wetting with the phase line of the new transition, termed capillary prewetting, mapping to the planar prewetting line. If capillary condensation is approached isothermally with T>T(cw), the meniscus forms at the capping wall and unbinds continuously, making capillary condensation a second-order phenomenon. We compute the corresponding critical exponent for the divergence of adsorption.

From Boltzmann’s Equation to the Incompressible Navier–Stokes–Fourier System with Long-Range Interactions

We establish a rigorous demonstration of the hydrodynamic convergence of the Boltzmann equation towards a Navier–Stokes–Fourier system under the presence of long-range interactions. This convergence is obtained by letting the Knudsen number tend to zero and has been known to hold, at least formally, for decades. It is only more recently that a fully rigorous mathematical derivation of this hydrodynamic limit was discovered. However, these results failed to encompass almost all physically relevant collision kernels due to a cutoff assumption, which requires that the cross sections be integrable. Indeed, as soon as long-range intermolecular forces are present, non-integrable collision kernels have to be considered because of the enormous number of grazing collisions in the gas. In this long-range setting, the Boltzmann operator becomes a singular integral operator and the known rigorous proofs of hydrodynamic convergence simply do not carry over to that case. In fact, the DiPerna–Lions renormalized solutions do not even make sense in this situation and the relevant global solutions to the Boltzmann equation are the so-called renormalized solutions with a defect measure developed by Alexandre and Villani. Our work overcomes the new mathematical difficulties coming from the consideration of long-range interactions by proving the hydrodynamic convergence of the Alexandre–Villani solutions towards the Leray solutions.

Role of Long-Range Intermolecular Forces in the Formation of Inorganic Nanoparticle Clusters

An understanding of the role played by intermolecular forces in terms of the electron density distribution is fundamental to the understanding of the self-assembly of molecules in the formation of a molecular crystal. Using ab initio methods capable of describing both short-range intramolecular interactions and long-range London dispersion interactions arising from electron correlation, analyses of inorganic dimers of As(4)S(4) and As(4)O(6) molecules cut from the structures of realgar and arsenolite, respectively, reveal that the molecules adopt a configuration that closely matches that observed for the crystal. Decomposition of the interaction energies using symmetry-adapted perturbation theory reveals that both model dimers feature significant stabilization from electrostatic forces as anticipated by a Lewis acid/Lewis base picture of the interaction. London dispersion forces also contribute significantly to the interaction, although they play a greater role in the realgar structure near equilibrium than in arsenolite.

Crossover from layering to island formation in Langmuir-Blodgett growth: Role of long-range intermolecular forces

Combined studies by atomic force microscopy, x-ray reflectivity, and Fourier transform infrared spectroscopy on transition-metal stearate (M-St, M = Mn, Co, Zn, and Cd) Langmuir-Blodgett films clearly indicate association of bidentate coordination of the metal-carboxylate head group to layer-by-layer growth as observed in MnSt and CoSt and partially in ZnSt. Crossover to islandlike growth, as observed in CdSt and ZnSt, is associated with the presence of unidentate coordination in the head group. Morphological evolutions as obtained from one, three, and nine monolayers (MLs) of M-St films are consistent with Frank van der Merwe, Stranski-Krastanov, and Volmer Weber growth modes for M=Mn/Co, Zn, and Cd, respectively, as previously assigned, and are found to vary with number (n) of metal atoms per head group, viz. n=1 (Mn/Co), n=0.75 (Zn), and n=0.5 (Cd). The parameter n is found to decide head-group coordination such that n=1.0 corresponds to bidentate and n=0.5 corresponds to unidentate coordination; the intermediate value in Zn corresponds to a mixture of both. The dependence of the growth mode on head-group structure is explained by the fact that in bidentate head groups, with the in-plane dipole moment being zero, intermolecular forces between adjacent molecules are absent and hence growth proceeds via layering. On the other hand, in unidentate head groups, the existence of a nonzero in-plane dipole moment results in the development of weak in-plane intermolecular forces between adjacent molecules causing in-plane clustering leading to islandlike growth.