Structure of nonpolarizable water/nitrobenzene interface: potential distribution, ion adsorption, and interfacial tension

Adsorption of hydrophobic and hydrophilic ions at the nonpolarizable interface between two immiscible electrolyte solutions was investigated. The results were analyzed in three different models: (i) Gouy-Chapman model, (ii) ions as hard spheres, and (iii) ion pair formation at the interface. In the Gouy-Chapman model, an analytical expression for the interfacial tension was obtained. It predicts that interfacial tension should be proportional to the square root of the electrolyte concentration, which does not agree with experimental data. Modeling ions as hard spheres only slightly improves the agreement. The third model of interfacial ion pairing as the main origin of adsorption was analyzed using the amphiphilic isotherm (Markin-Volkov isotherm). A good agreement between ion-pairing theory and experimental values was achieved. The MV isotherm takes into account the limited number of adsorption sites, final size of molecules, complex formation at the interface, and interaction between adsorbed particles. The analysis revealed repulsion between adsorbed tetraalkylammonium ions at the nitrobenzene/water interface and demonstrated linear dependence between adsorption site area and the size of a molecule.     

Determination of Water-Oil Interfacial Area during 3-Phase Gravity Drainage in Porous Media

Water-oil interfacial area in porous media was determined in laboratory experiments using sand columns consisting of either 2 (water and oil) or 3 (water, oil, and air) fluid phases. Surfactant sorption at the water-oil interface was directly measured for a wide range of water, oil, and air saturations undergoing gravity drainage. Differing values of the water-oil interfacial tension were also examined. The Gibbs adsorption equation was then used to obtain values for the water-oil interfacial area. Both 2- and 3-phase water-oil experiments showed a linear increase in interfacial area with decreasing water saturation. Results also showed that interfacial areas were not affected by changes in interfacial tension. The interfacial areas in the 3-phase experiments were less than half the calculated values of the corresponding 2-phase experiments, which contradicts predictions from a conventional pore level analysis of 3-phase flow. Copyright 2000 Academic Press.     

Interfacial tension in phase-separated aqueous cationic/anionic surfactant mixtures

Interfacial tensions in two aqueous phase-separated cationic/anionic surfactant mixtures, CTAB/AS and 12-3-12/AS, without and with NaBr added were determined by the spinning drop method at 318.15 K. CTAB, 12-3-12 and AS are the abbreviations for cetyltrimethylammonium bromide, 1,3-propanediyl-bis(dodecyldimethylammonium bromide) and sodium dodecyl sulfonate, respectively. The interfacial tension sigma was found to be in the range of 0.06-21 microNm(-1). Toward a better understanding of the influence of the concentration difference between the separated phases in aqueous two-phase systems (ATPS) to interfacial tension, compositions of equilibrium phases were determined by elemental analysis coupled with material balance and electroneutrality. The investigation indicates that the concentration differences of surfactant ions between the separated phases and the adsorption of surfactant ions at the interface are the decisive factors determining the magnitude of interfacial tension.     

Dendritic PtCo alloy nanoparticles as high performance oxygen reduction catalysts

We studied the physical properties and the concentration profile of benzene+water+caprolactam mixtures near the fluid-fluid interface using self-consistent field (SCF) theory. This yields the interfacial tension which plays an important role in describing the stability of transient liquid droplets of one phase in the other. The studies were performed at a fixed temperature of 313K. Flory-Huggins binary interaction parameters and the compound lattice segment numbers are input parameters for the applied SCF theory. These parameters were derived from activity coefficient relations, which are used to describe experimental liquid-liquid and vapor-liquid phase equilibrium measurements. Using first principles, the benzene-water interface was studied and the resulting interfacial tension was found to be in agreement with experimental values. This study illustrates that caprolactam accumulates at the benzene-water interface, acting as a weak surfactant. The interfacial tension is also demonstrated to be affected by the caprolactam concentration and the SCF results are in fair agreement with the experimental observations.     

Demulsification and Interfacial Properties of Crosslinking Phenol-Amine Resin Block Polyether Demulsifiers

Six novel crosslinking phenol-amine resin block polyether demulsifiers were synthesized for demulsification of surfactant-polymer flooding emulsion. The demulsification performances of these demulsifiers were investigated by conventional graduated bottle test. Their interfacial behaviors at water-oil interface were explored by dynamic interfacial tension and interfacial dilational viscoelasticity measurements. The results show that the demulsification efficiency is dependant on the hydrophilic-hydrophobic balance (HLB) value of these demulsifiers. It was also correlated to the interfacial activity and the dilational elasticity at the water-oil interface. The higher the HLB value of demulsifiers, the better the demulsification efficiency is.     

Molecular features of the air/carbonate solution interface

The nature of the air/carbonate solution interface is considered with respect to water structure by sum-frequency vibrational spectroscopy (SFVS) and molecular dynamics simulations (MDS). Results from this study provide further understating regarding previous observations that the surface tensions of structure making sodium carbonate solutions have been shown to be significantly greater than the surface tensions of structure breaking bicarbonate solutions at equivalent concentrations. This difference in surface tension and its variation with salt concentration is related to the organization of water and ions at the air/solution interface. Spectral results from SFVS show at equivalent concentrations that, for the carbonate solution, the strong water structure signal of 3200 cm(-1) at the air/carbonate solution interface is increased by a factor of 4 when compared to the same signal for the air/bicarbonate solution interface, which spectrum is weaker than the spectrum for the air/water interface in the absence of salt. These results from SFVS are explained by the results from MDS which show that in the case of carbonate solutions the structure making carbonate ions are excluded from the interfacial water region which region is extended in depth. On the other hand, in the case of bicarbonate solutions, the bicarbonate ions are accommodated in the interfacial water region and there is no evidence of an increase in the extent of water structure. These SFVS experimental and MD simulation results provide further information to understand interfacial phenomena of soluble salts at the molecular level.     

Surface tension of quantum fluids from molecular simulations

We present the first molecular simulations of the vapor-liquid surface tension of quantum liquids. The path integral formalism of Feynman was used to account for the quantum mechanical behavior of both the liquid and the vapor. A replica-data parallel algorithm was implemented to achieve good parallel performance of the simulation code on at least 32 processors. We have computed the surface tension and the vapor-liquid phase diagram of pure hydrogen over the temperature range 18-30 K and pure deuterium from 19 to 34 K. The simulation results for surface tension and vapor-liquid orthobaric densities are in very good agreement with experimental data. We have computed the interfacial properties of hydrogen-deuterium mixtures over the entire concentration range at 20.4 and 24 K. The calculated equilibrium compositions of the mixtures are in excellent agreement with experimental data. The computed mixture surface tension shows negative deviations from ideal solution behavior, in agreement with experimental data and predictions from Prigogine's theory. The magnitude of the deviations at 20.4 K are substantially larger from simulations and from theory than from experiments. We conclude that the experimentally measured mixture surface tension values are systematically too high. Analysis of the concentration profiles in the interfacial region shows that the nonideal behavior can be described entirely by segregation of H(2) to the interface, indicating that H(2) acts as a surfactant in H(2)-D(2) mixtures.     

Molecular dynamics simulations of the surface tension of ionic liquids

We report molecular dynamics computer simulations of the surface tension and interfacial thickness of ionic liquid-vapor interfaces modeled with a soft core primitive model potential. We find that the surface tension shows an anomalous oscillatory behavior with interfacial area. This observation is discussed in terms of finite size effects introduced by the periodic boundary conditions employed in computer simulations. Otherwise we show that the thickness of the liquid-vapor interface increases with surface area as predicted by the capillary wave theory. Data on the surface tension of size-asymmetric ionic liquids are reported and compared with experimental data of molten salts. Our data suggest that the surface tensions of size-asymmetric ionic liquids do not follow a corresponding states law.     

Molecular dynamics simulations of the solution-air interface of aqueous sodium nitrate

Molecular dynamics simulations have been used to investigate the behavior of aqueous sodium nitrate in interfacial environments. Polarizable potentials for the water molecules and the nitrate ion in solution were employed. Calculated surface tension data at several concentrations are in good agreement with measured surface tension data. The surface potential of NaNO3 solutions at two concentrations also compare favorably with experimental measurements. Density profiles suggest that NO3- resides primarily below the surface of the solutions over a wide range of concentrations. When the nitrate anions approach the surface of the solution, they are significantly undercoordinated compared to in the bulk, and this may be important for reactions where solvent cage effects play a role such as photochemical processes. Surface water orientation is perturbed by the presence of nitrate ions, and this has implications for experimental studies that probe interfacial water orientation. Nitrate ions near the surface also have a preferred orientation that places the oxygen atoms in the plane of the interface.     

Interfacial tension and interfacial profiles: an equation-of-state approach

A quasi-thermodynamic approach of inhomogeneous systems is used for modeling the fluid-fluid interface. It is based on the recently introduced QCHB (quasi-chemical hydrogen bonding) equation-of-state model of fluids and their mixtures, which is used for the estimation of the Helmholtz free energy density difference, Deltapsi(0), between the system with interface and another system of the same constitution but without interface. Consistent expressions for the interfacial tension and interfacial profiles for various properties are presented. The interfacial tension is proportional to the integral of Deltapsi(0) along the full height of the system, the proportionality constant being equal to 1, when no density gradient contributions are taken into consideration, 2, when the Cahn-Hilliard approximation is adopted, and 4, when the full density gradient contributions are taken into consideration. A satisfactory agreement is obtained between experimental and calculated surface tensions. Extension of the approach to mixtures is examined along with the associated problems for the numerical calculations of the interfacial profiles. A new equation is derived for the chemical potentials in the interfacial region, which facilitates very much the calculation of the composition profiles across the interface.     

Experimental observation and prediction of interfacial tension and viscoelastic emulsion model behavior in novel phosphate glass-polymer hybrids

The interfacial tension of hybrids composed of a tin-based phosphate glass (Pglass) and thermoplastic polymers, low-density polyethylene (LDPE), polystyrene (PS), and polypropylene (PP) was investigated using pendant drop and droplet deformation methods. High surface tension values were determined for the pure Pglass and subsequently used to obtain interfacial tension values that were found to be greater than that of most polymer blends reported in the literature. Small amplitude oscillatory shear data were fitted to the Choi-Schowalter and Palierne emulsion models in order to estimate the interfacial tension and to validate the accuracy (or lack thereof) of using a polymer emulsion model on the special Pglass-polymer systems. Although some of the hybrids showed satisfactory agreement with the emulsion models, wide ranges of interfacial tensions were obtained, suggesting that a more complicated theory that explicitly takes the Pglass-polymer interactions, shape factor, and size distributions of the dispersed Pglass phase into account may be necessary for more accurate modeling of these special hybrid systems with enhanced benefits.     

Orientation‐Induced Adsorption of Hydrated Protons at the Air–Water Interface

The surface tension of the air—water interface increases upon addition of inorganic salts, implying a negative surface excess of ionic species. Most acids, however, induce a decrease in surface tension, indicating a positive surface excess of hydrated protons. In combination with the apparent negative charge at pure air–water interfaces derived from electrokinetic experiments, this experimental observation has been a source of intense debate since the mid‐19th century. Herein, we calculate surface tensions and ionic surface propensities at air–water interfaces from classical, thermodynamically consistent molecular dynamics simulations. The surface tensions of NaOH, HCl, and NaCl solutions show outstanding quantitative agreement with experiment. Of the studied ions, only H₃O⁺ adsorbs to the air–water interface. The adsorption is explained by the deep potential well caused by the orientation of the H₃O⁺ dipole in the interfacial electric field, which is confirmed by ab initio simulations.     

The QCHB model of fluids and their mixtures

The essentials of the QCHB (quasi-chemical hydrogen-bonding) equation-of-state model are presented along with some applications for calculations of phase equilibria and interfacial properties of fluids and their mixtures. This is a model applicable to non-polar systems as well as to highly non-ideal systems with strong specific interactions, to systems of small molecules as well as to macromolecules, including polydisperse polymers, glasses, and gels, to liquids as well as to vapours including supercritical systems, to homogeneous as well as to inhomogeneous systems. A quasi-thermodynamic approach of inhomogeneous systems is used for modeling the fluid–fluid interface. Consistent expressions for the interfacial tension and interfacial profiles for various properties are presented. A satisfactory agreement is obtained between experimental and calculated surface tensions. Extension of the approach to mixtures is examined along with the associated problems for the numerical calculations of the interfacial profiles. A new equation is derived for the chemical potentials in the interfacial region, which facilitates very much the calculation of the composition profiles across the interface. The relation of the model with the COSMO-RS approach is also discussed.     

Simultaneous prediction of interfacial tension and phase equilibria in binary mixtures: An approach based on cubic equations of state with improved mixing rules

Vapor–liquid interfacial tensions of miscible mixtures have been predicted by applying the gradient theory to an improved Peng–Robinson equation of state. The modified Huron–Vidal mixing rule model has been considered for fitting vapor–liquid equilibrium data of miscible polar and non-polar mixtures and, then, for predicting the interfacial tension of these mixtures. According to results, an accurate and globally stable fitting of the vapor–liquid equilibrium data results on a physically coherent prediction of interfacial tensions in the full concentration range. In addition, we present a criteria based on the geometry of the grand potential function along the interface for assessing the predictive value of the GT. Calculations for subcritical binary mixtures are presented and compared to experimental data and the Parachor method for demonstrating the potential of the unified approach suggested in this work.     

A simple operational relationship between drop-time and interfacial tension at a charged mercury/aqueous solution interface

This paper describes a simple operational relationship between the drop-time of a dropping mercury electrode and the interfacial tension at a charged mercury/aqueous solution interface. An apparatus and technique for measuring drop-times is reported, and examples are given of the fit of experimental drop-times to literature values of interfacial tensions. The operational relationship is independent of temperature in the range 293-313 K.     

Estimation of line tension and contact angle from heterogeneous nucleation experimental data

Using the classical nucleation theory corrected with line tension and experimental data of heterogeneous nucleation of n-nonane, n-propanol, and their mixture on silver particles of three different sizes, the authors were able to estimate the line tensions and the microscopic contact angles for the above mentioned systems. To do this they applied generalized Young's equation for the line tension and calculated the interfacial tensions using Li and Neumann's equation [Adv. Colloid Interface Sci. 39, 299 (1992)]. It has been found that, for both unary and binary systems, the line tension is negative and the resulting microscopic contact angle derived from experimental nucleation data is most of the time larger than the macroscopic one. This is in contrast to earlier studies where the influence of line tension has not been accounted for. The values of the three phase contact line tension obtained in this way are of the same order of magnitude as the estimations for other systems reported in literature. The line tension effect also decreases considerably the nucleation barrier.     

Adsorption kinetics of asphaltenes at the oil-water interface and nanoaggregation in the bulk

Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop-drop coalescence. As a result, asphaltenes unfavorably impact water-oil separation processes and consequently oil production rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water-oil interfaces. This study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barrier-controlled regime rather than to a conformational relaxation regime.     

The solid surface free energy calculation. I. In defense of the multicomponent approach

The acid-base approach to the calculation of solid surface free energy and liquid-liquid interfacial tensions is a practical example of application of correlation analysis, and thus it is an approximate approach. In these limits, and provided that wide and well-obtained sets of contact angles or interfacial tension data are used for their computation, surface tension components can be considered as material properties. Although their numerical value depends on the characteristics of the chosen reference material, their chemical meaning is independent on the selected scale. Contact angles contain accessible information about intermolecular forces; using surface tension component (STC) acid-base theory, one can extract this information only making very careful use of the mathematical apparatus of correlation analysis. The specific mathematical methods used to obtain these results are illustrated by using as an example a base of data obtained by the supporters of the equation-of-state theory (EQS). The achievements are appreciably good and the agreement between STC and EQS is discussed.     

非离子表面活性剂在气液界面的分子动力学模拟

采用分子动力学方法研究了十二烷基醇聚氧乙烯醚系列非离子表面活性剂单分子层在气液界面的微观结构,并通过表面张力的计算考察了表面活性剂分子结构与性能的关系.研究结果表明,随着表面活性剂分子乙氧基基团个数的增加,模拟所得的表面张力的变化趋势与实验一致,所有分子的计算误差在5 mN·m-1以内.同时,随着乙氧基基团数目的增加,...