Van der Waals Forces: Theory and Experiment

Enhancing Zn (II) recovery efficiency: Bi-divalent nickel-cobalt ferrite spinel NiXCo1-xFe2O4 as a Game-changing Adsorbent-an experimental and computational study

This study presents a comprehensive investigation into NiXCo1-xFe2O4 (x = 0.5) spinel nanoparticles synthesized through a one-pot hydrothermal method using Co(NO3)2.6H2O and Ni(NO3)2.6H2O salts. XRD, FTIR, FESEM, and VSM analyses confirmed a cubic structure of NiXCo1-xFe2O4 (x = 0.5) nanoparticles without impurities. These nanoparticles exhibit efficient Zn (II) adsorption characteristics, following Langmuir isotherm and pseudo-second-order kinetics. The maximum adsorption capacity was measured to be 666.67 mg g-1 at pH = 7, with mechanisms involving both electrostatic attraction and cation exchange. Desorption studies indicate more than 75% Zn (II) recovery in an acidic environment (pH = 2) after three cycles. Computational analysis was used to validate the experimental results through Molecular Dynamics simulations, initially focusing on NiXCo1-xFe2O4 (x = 0.5). Further exploration involved variations in x at 0.25 and 0.75 to identify the optimal Ni and Co ratio in this bivalent cation spinel ferrite. Computational analyses reveal the superior performance of NiXCo1-xFe2O4 (x = 0.75) in Zn (II) removal, supported by radial distribution analysis, VdW energy, Coulombic energy, mean square displacement (MSD), root mean square displacement (RMSD), and interaction energy. This comprehensive study provides valuable insights into the adsorption behavior and structural stability of NiXCo1-xFe2O4 nanoparticles, showcasing potential applications in Zn (II) removal.

Transcending Lifshitz Theory: Reliable Prediction of Adhesion Forces between Hydrocarbon Surfaces in Condensed Phases Using Molecular Contact Thermodynamics

Lifshitz theory is widely used to calculate interfacial interaction energies and underpins established approaches to the interpretation of measurement data from experimental methods including the surface forces apparatus and the atomic force microscope. However, a significant limitation of Lifshitz theory is that it uses the bulk dielectric properties of the medium to predict the work of adhesion. Here, we demonstrate that a different approach, in which the interactions between molecules at surfaces and in the medium are described by a set of surface site interaction points (SSIPs), yields interaction free energies that are correlated better with experimentally determined values. The work of adhesion W(Lifshitz) between hydrocarbon surfaces was calculated in 260 liquids using Lifshitz theory and compared with interaction free energies ΔΔG calculated using the SSIP model. The predictions of these models diverge in significant ways. In particular, ΔΔG values for hydrocarbon surfaces are typically small and vary little, but in contrast, W(Lifshitz) values span 4 orders of magnitude. Moreover, the SSIP model yields significantly different ΔΔG values in some liquids for which Lifshitz theory predicts similar values of W(Lifshitz). These divergent predictions were tested using atomic force microscopy. Experimentally determined works of adhesion were closer to the values predicted using the SSIP model than Lifshitz theory. In mixtures of methanol and benzyl alcohol, even greater differences were found in the interaction energies calculated using the two models: the value of ΔΔG calculated using the SSIP model declines smoothly as the benzyl alcohol concentration increases, and values are well correlated with experimental data; however, W(Lifshitz) decreases to a minimum and then increases, reaching a larger value for benzyl alcohol than for methanol. We conclude that the SSIP model provides more reliable estimates of the work of adhesion than Lifshitz theory.

Activated carbons-preparation, characterization and their application in CO2 capture: A review

In this paper, we provide a comprehensive review of the latest research trends in terms of the preparation, and characteristics of activated carbons regarding CO2 adsorption applications, with a special focus on future investigation paths. The reported current research trends are primarily closely related to the synthesis conditions (carbonization and physical or chemical activation process), to develop the microporosity and surface area, which are the most important factors affecting the effectiveness of adsorption. Furthermore, we emphasized the importance of regeneration techniques as a factor determining the actual technological and economic suitability of a given material for CO2 capture application. Consequently, this work provides a summary and potential directions for the development of activated carbons (AC). We attempt to create a thorough theoretical foundation for activated carbons while also focusing on identifying and specific statements of the most relevant ongoing research scope that might be advantageous to progress and pursue in the coming years.

Friction, Surface Science and Tribology—A Personal View

This paper is a rather personalized account of research carried out over many years by a physicist exploring the basic mechanisms of friction and the general field of tribology. It indicates how side-tracks have often led to other rich pastures and reflects the sometimes fruitful unpredictability of scientific research. This paper records several such felicitous surprises emerging from studies of friction. These include the direct measurement of surface forces, the adhesion of elastomers in terms of surface energy and viscoelastic properties, the skid-resistant properties of tyres, diffusion in polymers and measurement at the molecular level of the shear properties of oriented layers of boundary lubricants.

In all this work the author is conscious of the original initiatives of the late Professor Bowden and recognizes the major contributions made by research students and colleagues. He is also conscious of many elements of serendipity.

Membrane autopsy based bio-fouling investigation of distillery spent wash RO treatment plant

In this paper, a thorough investigation has been done to evaluate the effects of different pollutants on membrane performance by autopsy. Autopsy was performed on fresh and fouled reverse osmosis (RO) membrane elements from a distillery spent wash treatment plant by different analyses, such as field emission scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy and X-ray diffraction, to identify the cause of poor performance of an RO plant. Results obtained from the analysis of membranes indicate that a mixture of colloids and organic matters (polysaccharides and amines) along with the presence of multivalent ions (Ca, Mg, Fe and SO4) causes membrane fouling, which in turn affects membrane performance. Possible measures to improve treated effluent quality and mitigate fouling have been suggested for this particular case study.

The electrochemical surface forces apparatus: the effect of surface roughness, electrostatic surface potentials, and anodic oxide growth on interaction forces, and friction between dissimilar surfaces in aqueous solutions

We present a newly designed electrochemical surface forces apparatus (EC-SFA) that allows control and measurement of surface potentials and interfacial electrochemical reactions with simultaneous measurement of normal interaction forces (with nN resolution), friction forces (with μN resolution), and distances (with Å resolution) between apposing surfaces. We describe three applications of the developed EC-SFA and discuss the wide-range of potential other applications. In particular, we describe measurements of (1) force-distance profiles between smooth and rough gold surfaces and apposing self-assembled monolayer-covered smooth mica surfaces; (2) the effective changing thickness of anodically growing oxide layers with Å-accuracy on rough and smooth surfaces; and (3) friction forces evolving at a metal-ceramic contact, all as a function of the applied electrochemical potential. Interaction forces between atomically smooth surfaces are well-described using DLVO theory and the Hogg-Healy-Fuerstenau approximation for electric double layer interactions between dissimilar surfaces, which unintuitively predicts the possibility of attractive double layer forces between dissimilar surfaces whose surface potentials have similar sign, and repulsive forces between surfaces whose surface potentials have opposite sign. Surface roughness of the gold electrodes leads to an additional exponentially repulsive force in the force-distance profiles that is qualitatively well described by an extended DLVO model that includes repulsive hydration and steric forces. Comparing the measured thickness of the anodic gold oxide layer and the charge consumed for generating this layer allowed the identification of its chemical structure as a hydrated Au(OH)(3) phase formed at the gold surface at high positive potentials. The EC-SFA allows, for the first time, one to look at complex long-term transient effects of dynamic processes (e.g., relaxation times), which are also reflected in friction forces while tuning electrochemical surface potentials.

Forces between surfaces in liquids

Recent developments in the direct measurements of forces between surfaces in liquids at the ångstrom resolution level are reviewed. The results reveal a rich variety of interactions and interaction potentials that depend on the nature of the surfaces and intervening liquids. These results also shed new insights into liquid structure adjacent to surfaces and the interactions occurrig in complex systems, with implications in many different areas of chemical physics, biology, and technology. The origin of some important fundamental interactions, such as repulsive "hydration" forces and attractive "hydrophobic" forces, are still not understood and offer a challenge for experimental and theoretical work in this area.

Numerical methods for fluctuation-driven interactions between dielectrics

We develop a discretized theory of thermal Casimir interactions to numerically calculate the interactions between fluctuating dielectrics. From a constrained partition function we derive a surface free energy, while handling divergences that depend on system size and discretization. We derive analytic results for parallel plate geometry in order to check the convergence of the numerical methods. We use the method to calculate vertical and lateral Casimir forces for a set of grooves.

Gravity as a factor of aggregative stability and coagulation

Gravity is a potential factor of aggregative stability and/or coagulation for any heterogeneous system having a density contrast between the dispersed phase and its dispersion medium. However, gravity becomes comparable to other stability factors only when the particle size becomes large enough. Since the particle size may grow in time due to various other instabilities, even nano-systems may eventually become susceptible to gravity. There have been many attempts in the last century to incorporate gravity in the overall theory of aggregative stability, but the relevant papers are scattered over a wide variety of journals, some of which are very obscure. Reviews on this subject in modern handbooks are scarce and inadequate. No review describes the role of gravity at all three levels introduced by DLVO theory for characterizing aggregative stability, namely: particle pair interaction, collision frequency and population balance equation. Furthermore, the modern tendency towards numerical solutions overshadows existing analytical solutions. We present a consistent review at each DLVO level. First we describe the role of gravity in particle pair interactions, including both available analytical solutions as well as numerical stability diagrams. Next we discuss a number of works on collision frequency, including works for both charged and non-charged particles. Finally, we present analytical solutions of the population balance equation that takes gravity into account and then compare these analytical solutions with numerical solutions. In addition to the traditional aggregate model we also discuss work on a fractal model and its relevance to gravity controlled stability. Finally, we discuss many experimental works and their relationship to particular theoretical predictions.

Effects of confinement on freezing and melting

We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting. We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials). The most commonly used molecular simulation, theoretical and experimental methods are first presented. We also provide a brief description of the most widely used porous materials. The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surface-driven and confinement-driven phases are then discussed. We also address how confinement affects the glass transition.

Some properties of clay-water dispersions

The bulk properties of clay-water dispersions, particularly with montmorillonites, depend on the very large surface area of the materials and their plate-like form. Owing to isomorphous substitution in the clay lattice the plates are charged and hence interact electrostatically with each other. The form of the interaction was tested using macroscopic clay surfaces in the form of cleaved mica and the results compared with those obtained by compression of concentrated aqueous dispersions of various montmorillonites in a homoionic form. The results obtained with mica and lithium montmorillonite were comparable suggesting the latter disperses as single plates. When sodium, potassium and caesium were used as the counter-ions for montmorillonite, differences were observed which were attributed to face-face association. The consequences of isomorphous substitution in either the tetrahedral or the octahedral layer were also examined.

Van der Waals forces in oil–water systems from the study of thin lipid films - I. Measurement of the contact angle and the estimation of the van der Waals free energy of thinning of a film

Free energies of formation of ‘black’ lipid films have been determined from measurements of their contact angles. The contact angles were calculated from the interference fringes formed in monochromatic light reflected from either the Plateau–Gibbs border or from lenses of bulk lipid solution trapped in the films. It is concluded that the electrostatic repulsion between the two surfaces of a film is negligibly small and that the ‘ steric’ interaction between the adsorbed monolayers of lipid molecules is of such short range that the free energy change during film formation originates almost entirely from work done by the van der Waals forces. The free energies determined for a range of different films all agree to within a factor of three with the free energy calculated from Lifshitz theory for water phases interacting across an isotropic layer of liquid hydrocarbon. Nevertheless, a systematic trend in the experimental data suggests that this picture of the lipid film is too simple and that either the polar groups of the lipid or the structure of the hydrocarbon region (or both) of the film have a significant influence on the results.

The stability of superspheres

The stability of superspheres containing a few tens of particles, formed by the aggregation of weakly charged spherical colloidal crystalites, can be explained by the existence of two opposing forces of different range within the superspheres: long-range electrostatic repulsion and shortrange attractive dispersion. A condition of stability is found that is in quantitative agreement with observation. The superspheres are a manifestation of frustration, analogous to frustration in spin glasses. The generality of the phenomenon is discussed.

Adhesion: Molecules and Mechanics

There is a difference between adhesion at the molecular level and adhesion in engineering. There is no doubt that molecules of solid materials stick together and can be separated mechanically. The problem is explaining the connection between molecular attractions and mechanical measurements. False ideas such as keying and gluing require critical assessment because they confuse molecules and mechanics. Mechanisms such as adhesive hysteresis, stringing, and clustering deserve evaluation. A rational theory of these phenomena should be based on the theoretical concept of reversible work of adhesion and on the measured quantity of adhesive energy, which includes the extra energy required to restructure the interface as surfaces move.

Forces between proteins and model polypeptides adsorbed on mica surfaces

The forces of interaction between proteins adsorbed onto mica have been measured as a function of the distance of separation between the two mica surfaces in aqueous solutions. The results for three proteins, myelin basic protein, concanavalin A and cytochrome c, are presented together with the results for a model basic protein, poly(L-lysine). With the exception of cytochrome c at large separations, the forces of interaction are due to charges on the protein surfaces and may be fitted closely to theoretical predictions. For cytochrome c, however, no long-range electrical repulsion is observed, indicating that the negatively charged mica surface has been neutralised by the adsorption of the positively charged protein. At short surface separations, an attraction between the protein surfaces was noted. For concanavalin A, a weak attraction was observed in the presence of calcium and manganese ions only. For poly(L-lysine) and cytochrome c the attraction can be explained simply in terms of van der Waals interactions between the proteins. However, for myelin basic protein the observed attraction was an order of magnitude larger than that predicted by van der Waals theory. We believe that this additional attraction may be due to hydrophobic interactions between the adsorbed myelin basic protein molecules.

On mechanical aspects of vesicular transport

From the chain of events leading to secretion we have identified and isolated stages in which mechanical and physical mechanics may play important roles. These include the vesicle motion towards the cell wall, drainage of the cytoplasmic fluid from the gap between the membranes, reorganization of the membrane constituents, failure of the membrane structure and coalescence into a new configuration. We suggest a unified mechanism, relevant to the neural, secretory and vascular systems, based on physical factors as flow, pressure and stress distributions, and membranes properties. The simulation of several stages of secretion is coupled with experimental observations. By use of the proposed hypothesis it is possible to explain some observed phenomena, such as spontaneous and induced secretion, membrane failure, protein lateral dislocation and the omega-shapes in electron microscopic exposures of fusion sites.