Liquid Phase Surface Research Articles

Lateral Dynamics of Surfactants at the Free Water Surface: A Computer Simulation Study

Molecular dynamics simulations of the adsorption layer of five different surfactant molecules, i.e., pentyl alcohol, octyl alcohol, dodecyl alcohol, sodium dodecyl sulfate, and dodecyl trimethyl ammonium chloride are performed at the free surface of their aqueous solution at two surface densities, namely 1 and 4 μmol/m(2) at 298 K. The results are analyzed in terms of the two-dimensional single molecule dynamics, in particlular, lateral diffusion of the surfactants at the liquid surface, in order to distinguish between two possible adsorption scenarios, namely the assumptions of localized and mobile surfactants. The obtained results, in accordance with the dynamical nature of the liquid phase and liquid surface, clearly support the latter scenario, as the time scale of lateral diffusion of the surfactant molecules is found to be comparable with that of the three-dimensional diffusion of water in the bulk liquid phase. The mechanism of this lateral diffusion is also investigated in detail by calculating binding energy distribution of the water molecules in the first hydration shell of the surfactant headgroups and that of the nonfirst shell surface waters, and by calculating the mean residence time of the water molecules in the first hydration shell of the surfactant headgroups. This time is found to be at least an order of magnitude smaller than the characteristic time of the lateral diffusion of the surfactants, revealing that surfactant molecules move without their first shell hydration water neighbors at the surface.

Liquid phase surface alloying of CP-titanium with aluminum in an atmosphere of argon and nitrogen

In the present work, surface alloying of CP-titanium with different mixtures of titanium and aluminum powders in a gas mixture of 20% argon and 80% nitrogen was carried out using tungsten inert gas (TIG) process. The microstructures of the alloyed layers were investigated by optical microscope (OM), scanning electron microscope (SEM), and the phases formed were studied by X-ray diffraction analysis. The hardness of these layers was also evaluated using a microhardness machine. The results showed that the surface hardness was significantly enhanced from 175HV0.1 for the untreated substrate to more than 1000HV0.1 for the alloyed layers, due to the formation of Ti3Al2N2 as well as Ti3Al and TiN in the modified layers. It was also noticed that the alloyed layers exhibited better wear resistance as compared with the untreated substrate.

Liquid phase surface alloying of AZ91D magnesium alloy with Al and Ni powders

In this paper, liquid phase surface alloying of AZ91D magnesium alloy was carried out by pre-placing of Al and Ni powder mixture and subsequent tungsten inert gas (TIG) melting process. The effects of TIG processing parameters on both microstructures and resulting hardness were investigated. Microstructures of alloyed layers were studied by optical microscope, and scanning electron microscope equipped with energy dispersive X-ray spectroscopy (EDS) analyzer, and the phases were identified by X-ray diffraction analysis. The microhardness of the surface alloyed layer was also measured. The surface hardness was increased from 80 HV0.1 for AZ91D magnesium alloy to as high as 162 HV0.1 for alloyed sample due to the formation of Mg17Al12 and AlNi3 intermetallic compounds in the alloyed region and structural refinement. Hardness improvement reduced the wear rate of the surface alloyed layer to almost half of that of the untreated substrate.

Strengthening adsorptive amelioration: Isotherm modeling in liquid phase surface complexation of Pb (II) and Cd (II) ions

The mitigation of recalcitrant heavy metals in an economic fashion is still a headline issue even though various methodologies and techniques have been explored. In this present study, inexpensive and abundant Eucalyptus ( Eucalyptus globulus) bark has been utilized for adsorbent development; AdsEbc for mitigation of Pb (II) and Cd (II) ions from aqueous phase successfully. AdsEbc showed promising adsorption capacity of 26.12 and 35.65 mg g − 1 for Pb (II) and Cd (II) ions at pH 5 and 8, respectively, calculated by best fitted isotherm modeling. Thermodynamics results revealed the feasibility and endothermic nature of adsorption process. Langmuir, Freundlich, Langmuir–Freundlich (Sips), Redlich–Peterson (R–P) and Dubinin–Radushkevich (D–R) models were used for the mathematical description of the relationship between adsorbed and aqueous concentration of adsorbate on adsorbent surfaces at equilibrium. Spectroscopic examinations of adsorbent were carried out with scanning electron microscopy and Fourier transform infrared spectroscopy.

Friction and wear behaviour of electron beam surface treated aluminium alloys AlSi10Mg(Cu) and AlSi35

Abstract Because of their low density, Al based materials are increasingly used for state-of-the-art lightweight construction solutions. With regard to systems subjected to wear, Al alloys cannot be used for highly stressed components, unless they have been made resistant against wear by additional measures such as surface treatment. Electron beam (EB) surface treatment offers a possibility for producing hard wear-resistant layers on components made of Al alloys. Concerning Al materials, distinct improvements of layer properties can be achieved exclusively by the use of liquid phase surface processes (remelting, alloying, dispersing and cladding). The paper deals with current results of investigations in the field of EB surface remelting and alloying technologies of cast alloy AlSi10Mg(Cu) and spray-formed alloy AlSi35. The EB surface treatment of these alloys causes significant changes of properties in surface layers with depths of 0.5–4.0 mm basing on a local modification of microstructure. The hardness and scratch energy density of surface layers are 2–7 times higher than those of the base materials. Moreover, studies on friction and wear behavior under oil lubricated conditions at 80 °C impressively demonstrate the upgraded surface layer properties. Depending on the type of EB-alloying additives (Co, Cu, Ni powder) the specific wear rate decreases by a factor of 10–50 by effectuating a lower coefficient of friction simultaneously. The technologies discussed give a picture of the potential of EB technologies and offer new possibilities for improving service life and reliability of engine components, among others.

Surface tension and vapour–liquid phase coexistence of variable-range hard-core attractive Yukawa fluids

The vapour–liquid phase coexistence and surface tension of hard-core Yukawa fluids with short attraction range, λ = 8.0, 9.0 and 10.0, are reported using grand-canonical transition-matrix Monte Carlo (GC-TMMC) with the histogram reweighting method. Surface tension is calculated using finite-size scaling approach of Binder. We also compare GC-TMMC results with the available literature data for Yukawa fluids with λ = 1.8, 3.0 and 4.0. Critical properties obtained from rectilinear diameter approach and least square-technique are also reported. GC-TMMC results are found to be more precise than the previous reported values. We also present the corresponding state of surface tension for extremely short-range attractive Yukawa fluids.

Vapor−Liquid Phase Coexistence, Critical Properties, and Surface Tension of Confined Alkanes

Configurational-bias grand-canonical transition-matrix Monte Carlo simulations are conducted to investigate various thermophysical properties, such as phase coexistence, critical properties, density and orientation profiles of liquid and vapor phases, and vapor−liquid surface tension of methane, ethane, propane, n-butane, and n-octane in bulk and slit pores of graphite and mica surfaces. An exponential-6 (exp-6) model is used for the investigation of normal alkanes with a cutoff radius, 15 A. It is found that the surface tension of the bulk n-alkane, C1−C4, based on a truncated exp-6 model, agrees reasonably well with the experimental data. Critical properties are reported by means of the rectilinear diameter approach and least-squares technique. The shift in the critical temperature under confinements follows more than two linear regimes with an inverse in the slit width, as the slit width approaches the two-dimensional limit. This is contrary to what has been previously reported in the literature. The b...

Constrained Inverse Fluidization of Light Particles in a Draft Tube Airlift Reactor

The objective of this work was to investigate the main fluidodynamic behavior of a constrained inverse fluidized bed in a draft tube airlift reactor (DT-ALR), with liquid in batch and air in continuous mode. The inverse fluidized bed was formed of relatively large particles (4.3-5.4 mm) that were composed of light polyethylene (PE), in the first case, and of particles (3.7-4.7 mm) that were composed of extremely light polystyrene (PS), in the second case. The influence of physical properties of liquid phase (surface tension and viscosity) on basic fluidodynamic characteristics was examined using the coalescence-inhibiting medium (1 wt % aqueous solution of ethanol) and viscous medium (46 wt % aqueous solution of sucrose). It is shown that the overall gas holdup, induced liquid velocity, minimum fluidization velocity, and bed expansion are dependent on the superficial gas velocity and liquid and solid properties.

A comparison of liquid and gas phase surface preparation of III–V compound semiconductors for atomic layer deposition

Native oxide removal and surface termination of InAs(1 0 0) and InSb(1 0 0) using liquid and gas phase HF chemistries were studied using X-ray photoelectron spectroscopy. Aqueous HF etching removed the native oxides on InAs and produced elemental As, which reoxidized when exposed to air. On InSb the native oxides were not completely removed due to F-termination, which passivated the surface. Gas phase HF etching of InSb native oxide completely removed Sb 2O 5 producing a stoichiometric semiconductor surface terminated by F atoms on primarily In surface sites. On InAs gas phase HF completely removed As 2O 3 producing two surface stoichiometries. For the majority of HF to water molar ratios studied, a stoichiometric bulk metal and an As-rich overlayer were produced. For a lean HF composition, an As-rich bulk metal and In-rich overlayer were produced. Deposition of Al 2O 3 by atomic layer deposition (ALD) at 170 °C directly onto F-terminated InSb produced a chemically sharp Al 2O 3/InSb interface. ALD of Al 2O 3 on an In-rich overlayer on InAs resulted in an interfacial layer containing As-oxide.

Numerical studies of bubble necking in viscous liquids

The pinch-off of a gas bubble from a tiny nozzle immersed vertically in another quiescent viscous fluid due to buoyancy is numerically investigated. The dynamics of bubble growth and pinch-off are described by the full Navier-Stokes equations for both gas and liquid phases. The equations are solved with a finite-volume method based on the SIMPLE scheme, coupled with a front tracking method to locate the interface between the two phases. The effects of liquid viscosity, surface tension, and gas density on the bubble pinch-off dynamics, which are always coupled in experiments, are investigated separately through simulations. The numerical results are compared with experimental observations on the bubble pinch-off for validation purposes. The simulation results show that the radius of the necking region decreases in a power law mode with time as r approximately tau;{alpha} , where tau is the time to pinch-off and the exponent alpha varies in the range 0.5-1.0 depending strongly upon the liquid properties such as viscosity and surface tension. In addition, the surface tension can significantly affect the bubble pinch-off exponent alpha when the surface tension coefficient is smaller than 0.030 N/m with a Bond number higher than 0.72. It is also found that both higher viscosity of the liquid phase and higher surface tension may result in a delayed pinch-off process and a larger bubble. The effect of gas phase density on the pinch-off is also investigated. As reported in the literature, the gas density variation has minimal effect on the necking process because the density ratio of the gas phase to the liquid phase is small.

An algorithm for predicting the hydrodynamic and mass transfer parameters in bubble column and slurry bubble column reactors

A large number of experimental data points obtained in our laboratory as well as from the literature, covering wide ranges of reactor geometry (column diameter, gas distributor type/open area), physicochemical properties (liquid and gas densities and molecular weights, liquid viscosity and surface tension, gas diffusivity, solid particles size/density), and operating variables (superficial gas velocity, temperature and pressure, solid loading, impurities concentration, mixtures) were used to develop empirical as well as Back-Propagation Neural Network (BPNN) correlations in order to predict the hydrodynamic and mass transfer parameters in bubble column reactors (BCRs) and slurry bubble column reactors (SBCRs). The empirical and BPNN correlations developed were incorporated in an algorithm for predicting gas holdups (εG, εG-Small, εG-Large); volumetric liquid-side mass transfer coefficients (kLa, kLa-Small, kLa-Large); Sauter mean bubble diameters (dS, dS-Small, dS-Large); gas–liquid interfacial areas (a, aSmall, aLarge); and liquid-side mass transfer coefficients (kL, kL-Large, kL-Small) for total, small and large gas bubbles in BCRs and SBCRs.The developed algorithm was used to predict the effects of reactor diameter and solid (alumina) loading on the hydrodynamic and mass transfer parameters in the Fisher–Tropsch (F–T) synthesis for the hydrogenation of carbon monoxide in a SBCR, and to predict the effects of presence of organic impurities (which decrease the liquid surface tension) and air superficial mass velocity in the Loprox process for the wet air oxidation of organic pollutants in a BCR. In the F–T process, the predictions showed that increasing the reactor diameter from 0.1 to 7.0 m and/or increasing the alumina loading from 25 to 50 wt.% significantly decreased εG, kLaH2 and kLaCO and increased dS. The decrease of the total gas holdup was found to be controlled by the holdup of small gas bubbles. The increase of the Sauter mean bubble diameter increased both kLH2 and kLCO, however, the decrease of the total gas holdup coupled with the increase of dS resulted in a dramatic decrease of the gas–liquid interfacial area, a, and subsequently kLaH2 and kLaCO. Thus, in the churn-turbulent flow regime, the hydrodynamic and mass transfer behaviors of the F–T SBCR were controlled by the holdup and the gas–liquid interfacial area of small bubbles. In the Loprox process, the predictions showed that increasing the liquid surface tension (removal of organic impurities from water) significantly increased dS and decreased both εG and kLaO2. The decrease of the total gas holdup with increasing liquid-phase surface tension was due mainly to the decrease of the liquid-phase foamability which led to the decrease of the holdup of small gas bubbles. The increase of the Sauter mean bubble diameter and the decrease of the total gas holdup resulted in a strong decrease of the gas–liquid interfacial area, and subsequently kLaO2. Increasing the air superficial mass velocity increased εG, dS, a, kL-O2 and kLaO2. Within the conditions used in the Loprox BCR, the hydrodynamics and mass transfer parameter behaviors of the process appeared also to be controlled by the gas holdup of small gas bubbles; and the gas–liquid interfacial area.

Hydrodynamics and mass transfer in bubble column: Influence of liquid phase surface tension

According to literature, few experiments are performed in organic solvents which are mostly used in commercial gas–liquid reactors. However, it is commonly accepted that data obtained in aqueous solution allow to predict the surface tension effects, and to model the behaviour of organic solvents. In this work, we examine the validity of this approximation. In this objective, the flows observed in two pure media having similar viscosity but different surface tension—respectively, water (reference) and cyclohexane (solvent)—are successively compared at two scales: in a bubble column and in bubble plumes. In bubble plumes, as expected, the mean bubble size is smaller in the medium having the smallest surface tension (cyclohexane), but for this medium the destabilisation of flow is observed to occur at smaller gas velocity, due to break-up and coalescence phenomena. In bubble column, these phenomena induce the bubbling transition regime at lower gas velocity, whatever the operating conditions for liquid phase: batch or continuous. Consequently, when the two media are used at similar gas superficial velocity, but in different hydrodynamic regimes, greater gas hold-up and smaller bubble diameter can be observed in water; the interfacial area is then not always higher in cyclohexane. This result differs from the behaviour observed in the literature for aqueous solutions. The analysis of bubble plumes in aqueous solutions of butanol shows that this difference is due to a fundamental difference in coalescent behaviour between pure solvents and aqueous mixtures: the surface tension effect is less important in pure liquid than in aqueous solutions, because of the specific behaviour of surfactants. It is then still difficult to predict a priori the bubbling regime or the flow characteristics for a given medium, and all the more to choose an appropriate liquid as a model for industrial solvents.

High efficiency photoluminescence from silicon nanocrystals prepared by plasma synthesis and organic surface passivation

AbstractAn efficient synthesis route for highly luminescent silicon quantum dots is presented. Silicon nanocrystals were synthesized in the gas phase using an argon‐silane non‐thermal plasma in a continuous flow reactor. Liquid phase surface passivation was used to improve the optical properties and avoid surface oxidation of the silicon quantum dots. Various alkenes, such as octadecene, dodecene and styrene, were covalently bonded to the plasma‐produced particles, while strictly avoiding exposure of the raw powder to oxidizing agents. Stable colloidal solutions of silicon quantum dots with ensemble quantum yields between 60% and 70% were achieved. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasma synthesis and liquid-phase surface passivation of brightly luminescent Si nanocrystals

While silicon's optical properties are improved at the nanoscale, they also become highly sensitive to the properties of the surfaces and interfaces of silicon nanostructures. For instance, while reported quantum yields for photoluminescence of silicon quantum dots covered by a native oxide are often in the few percent range, quantum yields as high as 30% have been found in quantum dots whose surfaces were passivated by covalently bonded organic molecules. In this paper, we describe an approach that is based on the gas phase synthesis of silicon quantum dots in a nonthermal plasma, and the subsequent organic surface passivation in the liquid phase. Nanocrystals are formed within a few milliseconds with a high mass yield in a nonthermal plasma. Various organic ligands such as octadecene, dodecence, and styrene are grafted onto the nanocrystal surfaces in a reaction known as hydrosilylation. Materials are characterized through transmission electron microscopy, atomic force microscopy, and fluorescence measurements. The particle size distributions are found to be relatively monodisperse and are well controllable through the plasma process parameters. Photoluminescence quantum yields as high as 60–70% have been achieved for particles luminescing in the red range of the visible spectrum.

Prediction of liquid–vapour surface tension in multi-component systems

It is shown, using the lattice theory and regular solution assumption, that the activity coefficient of each component at a liquid–vapour interface is equal to the second root of that component's activity coefficient in the bulk liquid phase. Making use of this important finding, an equation for prediction of liquid–vapour surface tension in multi-component systems has been developed. The new equation needs the temperature, composition of the liquid phase, activity coefficient and partial molar surface area of each compound as input data. The equation does not include any empirical parameter and has no limitation with the number of components. Comparing the predicted results with the experimental data of various systems revealed the reliability of the new model.

Surface behaviour and undercooling in the liquid Ga-Bi binary system detected by optical second harmonic generation

We employ an optical second harmonic generation(SHG) technique to investigate the surface behaviours at the liquid(solid)/vapour interface of the Ga-Bi binary metallic system. In a heating and cooling cycle between 280°C and room temperature, there is no change of the SH-intensity in the heating process, whereas there exists an abrupt and abnormal change of the SH-intensity in the cooling process. It is interesting to find that a macroscopic Bi-rich solid layer is floating on the surface of the Ga-rich liquid phase just below the monotectic temperature (222°C±2°C) in the cooling process, in spite of the Bi-rich phase being heavier than the Ga-rich phase. On the other hand, different undercooling behaviours are observed at the surface and in the bulk. The behaviours of surface solidification and surface melting are different from those in the bulk.

Phase equilibria and surface tension of pure fluids using a molecular layer structure theory (MLST) model

This paper presents a new model based on thermodynamic and molecular interaction between molecules to describe the vapour–liquid phase equilibria and surface tension of pure component. The model assumes that the bulk fluid can be characterised as set of parallel layers. Because of this molecular structure, we coin the model as the molecular layer structure theory (MLST). Each layer has two energetic components. One is the interaction energy of one molecule of that layer with all surrounding layers. The other component is the intra-layer Helmholtz free energy, which accounts for the internal energy and the entropy of that layer. The equilibrium between two separating phases is derived from the minimum of the grand potential, and the surface tension is calculated as the excess of the Helmholtz energy of the system. We test this model with a number of components, argon, krypton, ethane, n-butane, iso-butane, ethylene and sulphur hexafluoride, and the results are very satisfactory.

Modeling and simulation of grain growth in Si3N4. III. Tip shape evolution

Simulations based on the anisotropic Ostwald ripening model have been performed for the tip shape evolution of the β-Si3N4 crystal in the liquid phase. It was found that tip shape is a function of the liquid concentration and the relationship between the interfacial reaction and diffusion constants. Depending on the concentration of the liquid phase, a convex, almost flat, or concave surface appeared at the tip surface. The following two factors are crucial for understanding the development of concavity at the tip surface of the β-Si3N4 crystal: (1) strong growth anisotropy of the β-Si3N4 crystal that is the interfacial reaction and diffusion controlled kinetics of the (100) and (001) surfaces, respectively, and (2) supersaturation of the liquid phase.

Relation between interfacial shear strength and compressive strength of carbon fiber/epoxy resin composite strands

The mechanism with which the fiber-matrix interfacial strength exerts its influence on the compressive strength of fiber reinforced composites has been studied by measuring the axial compressive strength of carbon fiber/epoxy resin unidirectional composite strands having different levels of interfacial shear strength. The composite strands are used for experiments in order to investigate the compressive strength which is not affected by the delamination taking place at a weak interlayer of the laminated composites. The interfacial strength is varied by applying various degrees of liquid-phase surface treatment to the fibers. The efficiency of the compressive strength of the fibers utilized in the strength of the composite strands is estimated by measuring the compressive strength of the single carbon filaments with a micro-compression test. The compressive strength of the composite strands does not increase monotonically with increasing interfacial shear strength but showes lower values at higher interfacial shear strengths. With increasing interfacial shear strength, the suppression of the interfacial failure in the misaligned fiber region increases the compressive strength, while at higher interfacial shear strengths, the enhancement of the crack sensitivity decreases the compressive strength.

Effect of liquid-phase surface tension on hydrodynamics of a three-phase airlift reactor with an enlarged degassing zone

The effect of the addition of ethanol (10 g/l) to the liquid-phase on gas and solids holdup, circulation and mixing times and interstitial liquid velocity in a three- phase airlift reactor was investigated. The airlift reactor (60 l) is of the concentric draught-tube type with an en- larged degassing zone. Ca-alginate beads were used as solid-phase and airflow rate (from 1.9 to 90.2 l/min) and solids loading (0-30% (v/v)) were manipulated. Riser and downcomer gas holdup were found to increase with the addition of ethanol, leading to a decrease on the relative solids holdup. The presence of ethanol seems to have no influence on the circulation time. On the other hand, mixing time variation depends on the solids loading and airflow rate. Riser and downcomer interstitial liquid ve- locity are lower for ethanol solution than for water. List of symbols Ad downcomer cross-section area (m) Ar riser cross-section area (m) d vertical distance between two points of the riser and of the downcomer (m) H1)H2 pressure difference between two points of the riser and of the downcomer (cmH2O) tc circulation time (s) tm mixing time (s) uld downcomer interstitial liquid velocity (m/s) uli interstitial liquid velocity in section i (m/s) ulr riser interstitial liquid velocity (m/s) vgr riser superficial gas velocity (m/s) Vs solids volume measured in each sample (l) VT sample total volume (l) Dt time required by the tracer to travel between the two acquisition points in the riser and in the downcomer (s) egd downcomer gas holdup