Showing papers on "Surface tension published in 2001"

Practical implications of some recent studies in electrospray ionization fundamentals.

Abstract: In accomplishing successful electrospray ionization analyses, it is imperative to have an understanding of the effects of variables such as analyte structure, instrumental parameters, and solution composition. Here, we review some fundamental studies of the ESI process that are relevant to these issues. We discuss how analyte chargeability and surface activity are related to ESI response, and how accessible parameters such as nonpolar surface area and reversed phase HPLC retention time can be used to predict relative ESI response. Also presented is a description of how derivitizing agents can be used to maximize or enable ESI response by improving the chargeability or hydrophobicity of ESI analytes. Limiting factors in the ESI calibration curve are discussed. At high concentrations, these factors include droplet surface area and excess charge concentration, whereas at low concentrations ion transmission becomes an issue, and chemical interference can also be limiting. Stable and reproducible non-pneumatic ESI operation depends on the ability to balance a number of parameters, including applied voltage and solution surface tension, flow rate, and conductivity. We discuss how changing these parameters can shift the mode of ESI operation from stable to unstable, and how current-voltage curves can be used to characterize the mode of ESI operation. Finally, the characteristics of the ideal ESI solvent, including surface tension and conductivity requirements, are discussed. Analysis in the positive ion mode can be accomplished with acidified methanol/water solutions, but negative ion mode analysis necessitates special constituents that suppress corona discharge and facilitate the production of stable negative ions.

Molecular Structure of Salt Solutions: A New View of the Interface with Implications for Heterogeneous Atmospheric Chemistry

Abstract: Most salts raise the surface tension of water. Interpretation of this phenomenon via the Gibbs adsorption equation has led to the commonly held belief that the ions are repelled from the air/solution interface. Here, we report results from molecular dynamics simulations of a series of sodium halide solution/air interfaces. The simulations reproduce the experimentally measured increases in surface tension relative to pure water. Analysis of the structure reveals that the small, nonpolarizable fluoride anion is excluded from the interface, in accord with the traditional picture. However, all of the larger, polarizable halide anions are present at the interface, and bromide and iodide actually have higher concentrations in the interfacial region than in the bulk. On the basis of the simulations we develop a molecular picture of hydrogen bonding in the interfacial region that might be tested by surface sensitive spectroscopic experiments. The novel, microscopic view of the interfacial structure of aqueous sal...

Interfacial tension of alkane + water systems

Abstract: Interfacial tension was measured for hexane + water, heptane + water, octane + water, nonane + water, decane + water, undecane + water, and dodecane + water, using the emergent drop experimental technique with a numerical method based on a fourth degree spline interpolation of the drop profile. The experimental equipment used to generate the drop consists of a cell with a stainless steel body and two Pyrex windows. The inner cell was previously filled with water. A surgical needle (at the bottom of the cell) was used to introduce the organic phase into the cell (forming the emergent drop). Water was used to keep the temperature constant inside the cell (between 10 °C and 60 °C). The cell was illuminated from the back using a fiber optic lamp and a diffuser. A video camera (with a 60 mm microlens and an extension ring) was located at the front window. The emergent drop image was captured and sent to the video recording system. The cell and the optical components were placed on an optical table with vibration isolation legs. A new correlation was found to predict interfacial tension (γ) as a function of temperature (t) and the number of carbon atoms (n) with a deviation of less than 0.05% from experimental values.

Surface Tension Measurements of N-Alkylimidazolium Ionic Liquids

Abstract: We have measured the surface tensions, with use of a ring tensiometer, of a series of closely related ionic liquids [Cnmim][X] where [Cnmim] represents the 1-CnH2n+1-3-methylimidazolium cation (n = 4 [bmim], 8 [omim], and 12 [C12mim]) and X = PF6-, BF4-, Cl-, or Br-. The values of surface tension span an unusually wide range for compounds of such similar structure, from 45 mJ/m2 ([bmim][PF6]) to 24 mJ/m2 ([C12mim][PF6]) at 336 K. They all show a linear variation of surface tension with temperature allowing a separation of the surface excess entropy and energy components. The surface excess quantities are alkyl chain length-dependent; both the surface excess entropy and energy decrease as the alkyl chain in the 1-position of the cation is lengthened for a particular anion. For a common cation, a reduction in the surface excess entropy occurs with decreasing anion size. A similar effect occurs in the surface excess energy when the anion decreases in size for the shorter alkyl chain compounds. The wide range...

Determination of critical micelle concentration (CMC) of nonionic surfactants by donor-acceptor interaction with lodine and correlation of CMC with hydrophile-lipophile balance and other parameters of the surfactants

Abstract: The nonionic surfactants form donor-acceptor complexes with iodine in aqueous medium. The spectral absorption and the shift in the λmax of l2 upon complexation have been exploited to determine the critical micelle concentration (CMC) of Tweens, Brijs, and Triton X-100. The CMC values obtained closely agree with those determined by other methods, including measurements of static surface tension, differential refractive index, and iodine solubilization. The spectral characteristics of the complex salt Kl3 can be utilized as well to derive similar information. The CMC and the spectral shift can be correlated with the weight fraction of the polyoxyethylene groups and the hydrophile-lipophile balance (HLB) in various ways, with the parameters in these relationships depending on the series to which the surfactant belong. Because both CMC and HLB depend on temperature, the results and the relations obtained are temperature-dependent; those presented are with reference to 298 K.

The dynamics of strong turbulence at free surfaces. Part 1. Description

Abstract: A free surface may be deformed by fluid motions; such deformation may lead to surface roughness, breakup, or disintegration. This paper describes the wide range of free-surface deformations that occur when there is turbulence at the surface, and focuses on turbulence in the denser, liquid, medium. This turbulence may be generated at the surface as in breaking water waves, or may reach the surface from other sources such as bed boundary layers or submerged jets. The discussion is structured by consideration of the stabilizing influences of gravity and surface tension against the disrupting effect of the turbulent kinetic energy. This leads to a two-parameter description of the surface behaviour which gives a framework for further experimental and theoretical studies. Much of the discussion is necessarily heuristic, and is often limited by a lack of appropriate experimental observations. It is intended that such experiments be stimulated, to test the value or otherwise of our two-parameter description.

A two‐phase model for compaction and damage: 1. General Theory

Abstract: A theoretical model for the dynamics of a simple two-phase mixture is presented. A classical averaging approach combined with symmetry arguments is used to derive the mass, momentum, and energy equations for the mixture. The theory accounts for surficial energy at the interface and employs a nonequilibrium equation to relate the rate of work done by surface tension to the rates of both pressure work and viscous deformational work. The resulting equations provide a basic model for compaction with and without surface tension. Moreover, use of the full nonequilibrium surface energy relation allows for isotropic damage, i.e., creation of surface energy through void generation and growth (e.g., microcracking), and thus a continuum description of weakening and shear localization. Applications to compaction, damage, and shear localization are investigated in two companion papers.

Surface Tension of Electrolytes: Specific Ion Effects Explained by Dispersion Forces

Abstract: The surface tension of ionic solutions shows marked specific ion effects not explained by existing theories. The effects are here accounted for by invoking a dispersion force between ions and the air−water interface. The measurements can be explained with reasonable values for the dispersion potential.

Effect of Undulations on Surface Tension in Simulated Bilayers

Abstract: To understand the effect of the finite size of simulation cells on the equilibrium properties of bilayers, an extensive series of glycerolmonoolein bilayer molecular dynamics simulations in which the surface area and system size were systematically changed have been conducted. Systems ranging from 200 to 1800 lipids were simulated, covering length scales up to 20 nm. The dependence of the surface tension on the area per lipid is shown, although long simulation times were needed (up to 40 ns) to obtain reliable estimates. As the size of simulated patches increases, long wavelength undulatory modes appear with a concomitant increase in the area compressibility due to coupling of undulation modes to area fluctuations. Both the undulatory intensities and the peristaltic intensities of the bilayer fluctuations can be fitted in the long wavelength limit to continuum model predictions. The effect of system size on surface tension appears to depend on the stress conditions.

Scaling of Hydrophobic Solvation Free Energies

Abstract: We have calculated the free energy of solvation for hard sphere solutes, as large as 20 A in diameter, in two simple-point-charge models of water. These results were obtained using umbrella sampling of ensembles with fixed, ambient temperature and pressure. For the same water models, we have also calculated the surface tension of a liquid−vapor interface at room temperature. Analogous calculations were carried out for three thermodynamic states of the Lennard-Jones (LJ) fluid near liquid−vapor coexistence. For both water and the LJ fluid at the conditions we have simulated, extrapolation of our results suggests that the planar interface between coexisting liquid and vapor phases has the same surface tension as the planar limit of hard sphere solvation. We expect this correspondence to be a general result for fluids at thermodynamic states close to phase coexistence, as measured by the difference in chemical potential between bulk liquid and vapor phases, and far from the critical point. The solvation free...

Tip streaming from a drop in the presence of surfactants.

Abstract: Drop breakup in a linear extensional flow is simulated numerically using a nonlinear model for the surface tension that accounts for maximum packing at the interface. Surface convection sweeps surfactant to the drop poles, where it accumulates and drives the surface tension to near zero. The drop assumes a transient shape with highly pointed tips. From these tips, thin liquid threads are pulled. Subsequently, small, surfactant-rich droplets are emitted from the termini of these threads. The scale of the shed drops depends on the initial surfactant coverage. Dilute initial coverage leads to tip streaming, while high initial coverage leads to the tip dropping breakup mode.

Uses and applications of microemulsions

Abstract: Since the discovery of microemulsion s, they have attained increasing significance both in basic r esearch and in industry. Due to their unique properties, namely, ultralow interfacial tension, large interfacial area, thermodynamic stability and the ability to solubilize otherwise immiscible liquids, uses and applications of microemulsion s have been numerous. The objective of this review is to present briefly the possible applications of the novel co mpartmentallized systems of microemulsions. IT is well established that large amounts of two immi scible liquids (e.g. water and oil) can be brought into a single phase (macroscopically homogeneous but microscopically heterogeneous) by addition of an appropriate surfactant or a surfactant mi xture. This unique class of optically clear, thermodynamically stable and usually low viscous solutions, called ‘microemulsions’ 1 , have been the subject of extensive research over the last two decades primarily because of their scientific and tech nological importance. Microemulsions can have characteristic properties such as ultralow interfacial tension, large interfacial area and c apacity to solubilize both aqueous and oil-soluble compounds. For detailed information one can consult several books and review articles 2–5 . The essential distinction between normal emulsion and microemulsion is their particle size and stability; the former is ‘kinetically stable’ whereas the latter is ‘thermodynamically stable’. The stability of the m icroemulsion can be influenced by addition of salt, other additives, temperature or pressure. Normal emulsions age by co alescence of droplets and Ostwald ripening (transfer of material from small droplets to larger ones), since these processes lead to a decrease in the free e nergy of dispersion (the system is inherently thermod ynamically unstable). Thermodynamic stability of the microemulsions has been proposed by Ruckenstein and Chi 6 who considered that the free energy of formation comprises interfacial free energy, interaction e nergy between droplets and entropy of dispersion. The interaction energy between droplets has been shown to be negligible and the free energy of formation can be zero or even negative if the interfacial tension is of the

Development of a eulerian model for the “atomization” of a liquid jet

Abstract: In this article, a Eulerian model for the atomization of a liquid jet is proposed. Atomization is considered as turbulent mixing in a flow with variable density in the limit of large Reynolds and Weber numbers. An assumption similar to the Kolmogorov hypothesis has been invoked: large-scale features of the flow are supposed to be independent of viscosity and surface tension at high Reynolds and Weber numbers; small-scale features do depend on viscosity and surface tension. Dispersion of the liquid in the gas phase is computed by a classical equation for the turbulent diffusion flux of the liquid. The mean size of the liquid fragments is obtained with a new equation for the mean surface area of the liquid–gas interface per unit of volume. Discussions concerning this new equation are presented. Several comparisons with experiments are presented, сoncerning the liquid dispersion as well as the mean size of fragments produced.

Structure and dynamics of amorphous silica surfaces

Abstract: We use molecular dynamics computer simulations to study the equilibrium properties of the surface of amorphous silica. Two types of geometries are investigated: (i) clusters with different diameters (13.5, 19, and 26.5 A) and (ii) a thin film with thickness 29 A. We find that the shape of the clusters is independent of temperature and that it becomes more spherical with increasing size. The surface energy is in qualitative agreement with the experimental value for the surface tension. The density distribution function shows a small peak just below the surface, the origin of which is traced back to a local chemical ordering at the surface. Close to the surface the partial radial distribution functions as well as the distributions of the bond–bond angles show features which are not observed in the interior of the systems. By calculating the distribution of the length of the Si–O rings we can show that these additional features are related to the presence of two-membered rings at the surface. The surface den...

Coalescence of liquid drops by surface tension.

Abstract: The merging of two mercury drops at very low kinetic energy is observed using fast, digital, and analog imaging techniques. Sequences showing the time evolution of the overall-surface shape as well as an amplified view of the contact region are shown. Qualitative and quantitative comparisons with computations of the Navier-Stokes equation with a free surface are made. In the model, the surface is tracked by a marker-chain method.

Integrated multilayer flow system on a microchip.

Abstract: We utilized microchip technology and found that the multilayer flow of liquids can be formed in microchannels. Liquid/liquid interfaces were formed parallel to the side wall of the microchannels, because the surface tension and friction force are stronger than the force of gravity. A water/ethylacetate/water interface was formed in a 70-µm-wide and 30-µm-deep channel. The interface was observed to be quite stable and to be maintained for a distance of more than 18 cm. As an example of a multilayer flow application, we demonstrated the liquid/liquid extraction of Co-dimethylaminophenol complex in a microchannel. The solvent-extraction process of the complex into m-xylene in the multilayer flow was found to reach equilibrium in 4 s, while it took 60 s in a simple two-phase extraction.

Alveolar surface forces and lung architecture.

Abstract: The entire alveolar surface is lined by a thin fluid continuum. As a consequence, surface forces at the air-liquid interface are operative, which in part are transmitted to the delicate lung tissue. Morphologic and morphometric analyses of lungs show that the alveolar surface forces exert a moulding effect on alveolar tissue elements. In particular, in lungs at low degrees of inflation, equivalent to the volume range of normal breathing, there is a derecruitment of alveolar surface area with increasing surface tensions which reflects equilibrium configurations of peripheral air spaces where the sum of tissue energy and surface energy is minimum. Thus, changes in surface tension alter the recoil pressure of the lung directly and indirectly by deforming lung tissue and hence changing tissue tensions. However, the interplay between tissue and surface forces is rather complex, and there is a marked volume dependence of the shaping influence of surface forces. With increasing lung volumes the tissue forces transmitted by the fiber scaffold of the lung become the predominant factor of alveolar micromechanics: at lung volumes of 80% total lung capacity or more, the alveolar surface area-volume relation is largely independent of surface tension. Most important, within the range of normal breathing, the surface tension, its variations and the associated variations in surface area are small. The moulding power of surface forces also affects the configuration of capillaries, and hence the microcirculation, of free cellular elements such as the alveolar macrophages beneath the surface lining layer, and of the surfaces of the peripheral airways. Still enigmatic is the coupling mechanism between the fluid continua of alveoli and airways which might also be of importance for alveolar clearance. As to the surface active lining layer of peripheral air spaces, which determines alveolar surface tension, its structure and structure-function relationship are still ill-defined owing to persisting problems of film preservation and fixation. Electron micrographs of alveolar tissue, of lining layers of captive bubbles, and scanning force micrographs of surfactant films transferred on mica plates reveal a complex structural pattern which precludes so far the formulation of an unequivocal hypothesis.

The atomistic nature of crystal growth

Abstract: 1. Introduction.- 2. Thermodynamics.- 3. Statistical Thermodynamics.- 4. Equilibrium Between Large Phases The Vapor Pressure of Solids.- 5. The Surface Tension of Crystals.- 6. Equilibrium Between Large Three- and Two-Dimensional Phases: Adsorption Phenomena.- 7. Thin Films, Surface Roughening, and Surface Alloys.- 8. Equilibrium Between a Small and a Large Phase.- 9. Equilibrium Shapes of Crystals.- 10. Homogeneous Nucleation the Phase Approach.- 11. Homogeneous Nucleation the Chemical Approach.- 12. Nucleation on a Foreign Substrate.- 13. Some Specific Cases of Nucleation.- 14. Time-Dependent Nucleation Kinetics.- 15. Elementary Processes on the Surface of a Crystal.- 16. Growth of a "Perfect" K Face.- 17. Growth of an F Face of a Perfect Crystal.- 18. Growth of an F Face of an Imperfect Crystal.- 19. Conclusion.- Appendices.- A. Legendre Transformations.- B. Method of Lagrange Multipliers.- C. Euler's Theorem.- D. Stirling's Approximation.- E. Maximum Term Approximation.- References.

Deformation Mechanisms during Latex Film Formation: Experimental Evidence

Abstract: During the deformation stage of latex film formation, a close-packed array of particles is consolidated to form a structure with volume fraction unity There are a number of different possible driving forces to achieve this Proposed mechanisms range from surface tension between polymer particles and either water or air to capillary forces at the water−air interface We review the different driving forces and the literature supporting them A recent model we have proposed predicts the conditions under which each mechanism operates Experiments in the literature correlate well with our model, although the need for experiments with well-defined values for the critical parameters is highlighted

Computer simulation study of the interface width of the liquid/liquid interface.

Abstract: Four molecular dynamics computer simulations have been performed to study the intrinsic width and the width due to thermal fluctuations of the water/carbon tetrachloride interface. We observed that thermal fluctuations have a capillary wave character. The surface tension calculated by using capillary wave formalism shows a very good agreement with the value obtained from the components of the pressure tensor.

Reply to Comment on "Stripe glasses: self generated randomness in a uniformly frustrated system"

Abstract: Using our previous results for the configurational entropy of a stripe glass as well as a variational result for the bare surface tension of entropic droplets we show that there is no disagreement between the numerical simulations of Grousson et al. and our theory. The claim that our theory disagrees with numerical simulations is based on the assumption that the surface tension is independent of the frustration parameter Q of the model. However, we show in this Reply that it varies strongly with Q and that the resulting Q-dependence of the kinetic fragility agrees with the one obtained by Grousson et al. We believe that this answers the questions raised in the Comment by Grousson et al.