Status of the three-phase line tension: a review

From a thermodynamic point of view, the concept of line tension is on solid ground; it is well recognized and defined. There is a notion in the literature that there is little consensus with regards to magnitude or sign of line tension. Partly, inappropriate comparisons of line tension values may have contributed to the current uncertainty, especially some between theoretical and experimental values. A wide range of reported line tension values may not necessarily be a result of conflicting findings, but it may reflect the diversity of systems studied. However, differences among similar approaches and systems observed are sometimes caused by one or several of the following factors: difficulties in sample preparation, poor experimental techniques, non-equilibrium, or conceptual and theoretical difficulties and oversimplifications. It is important to clearly establish the magnitude and sign of line tension as it may determine its relevance to technological applications such as microfluidic systems.

This review has examined a wide range of recent and past studies on the subject of line tension. The review classifies line tension studies into theoretical and experimental; it further groups the studies into liquid–liquid–vapor and solid–liquid–vapor systems for each category. The review provides a comparison between similar studies in an attempt to clarify the current status of line tension research. The majority of theoretical line tension studies (excluding the near wetting studies) have estimates for line tension of about 10⁻¹⁰ N; however, for special cases, values as high as 10⁻⁶ N are also reported. There is less consensus regarding the sign for the line tension as theoreticians often assert that line tension can have either a positive or a negative sign. In experimental studies of liquid–liquid–vapor systems the majority of studies reported positive values for line tension; for film studies, the magnitude of line tension reported was between 10⁻⁹ and 10⁻⁷ N, whereas for studies of liquid lens values as high as 10⁻⁶ N were also reported. The clear majority of studies for solid–liquid–vapor systems reported a positive sign for line tension. In studies involving particles, line tension values ranged from 10⁻⁹ to 10⁻⁶ N. Studies using drop size dependence of contact angles reported values mostly in the span of 10⁻⁹–10⁻⁶ N; however, a clear majority of the reported values fell in the higher end of the above range. The special topic of line tension near wetting was also studied. While the interface displacement model appears to have brought about consensus with respect to certain aspects of line tension behavior near wetting, this model maybe still too phenomenological to be satisfactory from a more fundamental standing. Experimentalists have just begun studying line tension for systems near wetting, this is encouraging, but more comprehensive studies are needed.     

Comparison of the capillary wave method and pressure tensor route for calculation of interfacial tension in molecular dynamics simulations

We have studied the calculation of surface and interfacial tension for a variety of liquid–vapor and liquid–liquid interfaces using molecular dynamics (MD) simulations. Because of the inherently small scale of MD systems, large pressure fluctuations can cause imprecise calculations of surface tension using the pressure tensor route. The capillary wave method exhibited improved precision and stability throughout all of the simulated systems in this study. In order to implement this method, the interface was defined by fitting an error function to the density profile. However, full mapping of the interface from coordinate files produced enhanced accuracy. Upon increasing the system size, both methods exhibited higher precision, although the capillary wave method was still more reliable. © 2013 Wiley Periodicals, Inc.     

Adsorption of hydrophobin proteins at hydrophobic and hydrophilic interfaces

The surface activity of two hydrophobin proteins, HFBII and SC3, at the solid–liquid, liquid–liquid and liquid–vapor interface has been investigated. Hydrophobins are fungal proteins that are known to adsorb and affect the physico-chemical properties of an interface. In this study, the surface activity was determined by measuring the interaction of hydrophobin molecules with various liquids, solid particles and films that are commonly used or produced in industrial processes. We found that a very low concentration of hydrophobin is required to facilitate the wet-in of hydrophobic solid particles, such as Teflon^®, into aqueous solutions. It is also capable of stabilizing aqueous dispersions of Kevlar^® nanopulp, reversing the wettability of hydrophobic films and stabilizing polyunsaturated fatty acid (PUFA) oil-in-water emulsions.     

Development of a simple sample preparation technique for gas chromatographic–mass spectrometric determination of nicotine in edible nightshades (Solanaceae)

The purpose of this study was to develop a rapid analytical method for the reliable determination of low concentrations of nicotine in foods for large numbers of samples. Food material was extracted using a simple liquid–liquid extraction method. For processed foods, further clean-up steps had to be employed to eliminate interfering compounds. The determination of nicotine was performed by gas chromatography–mass spectrometry. Quantitative analysis was accomplished using deuterium labeled nicotine as an internal standard. Recoveries of over 95% were obtained for a single step extraction, as well as for a multiple-stage extraction procedure, respectively. The method has been applied to the determination of nicotine in edible nightshades (i.e. tomatoes, potatoes and aubergines) and their processed products.     

Measurement of air-water interfacial area for multiple hysteretic drainage curves in an unsaturated fine sand

A new method has been developed to measure fluid-fluid interfacial area during multiple drainages, along with the measurement of hysteretic capillary pressure-saturation (P(c)-S) relationships in unsaturated porous media. The method makes use of an automated device which has been successfully used for rapid measurement of hysteretic P(c)-S relationships, in combination with a novel technique for interfacial area measurement. A pure anionic surfactant, sodium octylbenzene sulfonate (SOBS), is used as a surface-active tracer, and a flow-through UV spectrometer is used to monitor the real-time concentration change of SOBS solution due to adsorption to the fluid-fluid interface during drainage. The Gibbs and Langmuir adsorption equations are applied in combination with a continuous mole balance to calculate interfacial areas. Using this method, air-water interfacial area of a fine sand was measured as a function of capillary pressure and saturation during primary, secondary, and one scanning drainages to explore the influence of drying/wetting history on interfacial area. Results show that 8-20 and 12-22 cm(2)/g more air-water interface was generated in secondary and scanning drainages, respectively, than in primary drainage, with the magnitude of the difference varying as a function of saturation. An advantage of the method is that interfacial tension variations from the method itself are relatively small, typically on the order of 5 mN/m, so measured areas are not skewed by surface-tension-induced changes in interfacial area. In a measurement specifically designed to study the influence of surfactant-induced interfacial tension variations, approximately two times more interfacial area was observed for a 25 mN/m interfacial tension change, in comparison with a system with relatively constant interfacial tension. Implications of results of interfacial area measurements for hysteresis in the three-dimensional relationship between capillary pressure, saturation, and interfacial area are discussed.     

An improved sealed quartz‐tube method for the determination of hydrogen isotopes in water

Hydrogen stable isotope analysis has been a valuable tool in the fields of geochemistry and ecological research as well as many other research fields. The methods are mainly divided into the dual‐inlet method (off‐line method) and continuous flow method. The dual‐inlet method is complicated and inefficient, but it is still important because of its high precision and wide application range. Although the continuous flow method improves the experimental efficiency, the memory effect is noticeable and the accuracy is reduced. An improved sealed quartz‐tube method is proposed in this paper. The sample is sealed in a capillary tube and placed in a quartz tube containing chromium powder. It is then packaged, evacuated, reacted at a high temperature, and analyzed for hydrogen isotope ratio. Excellent data accuracy, good reproducibility (<1‰), and no memory effect occurred in the method. The process is relatively simple, and the experimental efficiency is greatly improved, which provides an effective method for the analysis of hydrogen isotopes in complex liquid samples.     

The liquid–gas interface of oxygen–nitrogen solutions: 1. Surface tension

The capillary method of surface tension measurement has been used to measure the surface tension of oxygen–nitrogen solutions in the temperature range from 80 to 132 K. At temperatures below the nitrogen critical temperature (T_c = 126.2 K) the capillary constant and the surface tension of solutions are smaller than their additive values and vary linearly with the temperature. Experimental data are compared with the results of calculating the surface tension by the theories of Pinnes and Rowlinson.     

Measurement of the surface tension of liquid marbles

The capillary rise and Wilhelmy plate methods have been used to study the "surface tension" of water marbles encapsulated with polytetrafluoroethylene (PTFE) powders of 1-, 35-, and 100-μm particle size. With the capillary rise technique, a glass capillary tube was inserted into a water marble to measure the capillary rise of the water. The Laplace pressure exerted by the water marble was directly measured by comparing the heights of the capillary rise from the marble and from a flat water surface in a beaker. An equation based on Marmur's model was proposed to calculate the water marble surface tension. This method does not require the water contact angle with the supporting solid surface to be considered; it is therefore a simple but efficient method for determining liquid marble surface tension. The Wilhelmy method was used to measure the surface tension of a flat water surface covered by PTFE powder. This method offers a new angle for investigating liquid marble shell properties. A discussion on the nature and the realistic magnitude of liquid marble surface tension is offered.     

Liquid—liquid—vapor phase equilibria behavior of certain binary ethane + n-alkylbenzene mixtures

The liquid—liquid—vapor loci for the binary mixtures ethane + n-nonylbenzene, ethane + n-decylbenzene and ethane + n-undecylbenzene were experimentally studied. The pressure, temperature, and compositions and molar volumes of the liquid phases are reported along the loci. n-Nonylbenzene was found to be the first member of the n-alkylbenzene homologous series to exhibit liquid—liquid—vapor immiscibility with ethane. For the three alkylbenzenes studied, the liquid—liquid—vapor loci have the same type of behavior: they extend from a lower critical end point (LCEP) to an upper critical end point (K-point).     

Determination of L-Dopa and L-Dopamine in Aqueous Solutions Using In-Loop SPME Coupled with LC

A new in-loop solid-phase microextraction (in-loop-SPME) technique, based on an aluminum capillary tube coupled to HPLC, is described for on-line isolation, concentration, and analysis of analytes from aqueous samples. L-Dopa and L-dopamine, in aqueous solutions, were selected as model compounds. The main conditions affecting extraction of the analytes from aqueous samples, desorption, injection, and chromatographic separation were investigated. The method is simple and reproducible. Using the proposed method, reliable determination of L-dopa and L-dopamine at parts-per-billion concentrations was achieved. The calibration plots were linear in the range of 2.5–1500 ng mL⁻¹ with correlation coefficients of 0.999 and 0.998 for L-dopa and L-dopamine, respectively. The detection limits were 0.5–1 ng mL⁻¹ with a relative standard deviation less than 4.1%. Concentration factors more than 100-fold were obtained for these compounds.     

Separation and determination of amitriptyline and nortriptyline by dispersive liquid-liquid microextraction combined with gas chromatography flame ionization detection

Dispersive liquid–liquid microextraction (DLLME) coupled with gas chromatography–flame ionization detection (GC–FID) was applied for the determination of two tricyclic antidepressant drugs (TCAs), amitriptyline and nortriptyline, from water samples. This method is a very simple and rapid method for the extraction and preconcentration of these drugs from environmental sample solutions. In this method, the appropriate mixture of extraction solvent (18 μL Carbon tetrachloride) and disperser solvent (1 mL methanol) are injected rapidly into the aqueous sample (5.0 mL) by syringe. Therefore, cloudy solution is formed. In fact, it is consisted of fine particles of extraction solvent which is dispersed entirely into aqueous phase. The mixture was centrifuged and the extraction solvent is sedimented on the bottom of the conical test tube. 2.0 μL of the sedimented phase is injected into the GC for separation and determination of TCAs. Some important parameters, such as kind of extraction and disperser solvent and volume of them, extraction time, pH and ionic strength of the aqueous feed solution were optimized. Under the optimal conditions, the enrichment factors and extraction recoveries were between 740.04–1000.25 and 54.76–74.02%, respectively. The linear range was (0.005–16 μg mL⁻¹) and limits of detection were between 0.005 and 0.01 μg mL⁻¹ for each of the analytes. The relative standard deviations (R.S.D.) for 4 μg mL⁻¹ of TCAs in water were in the range of 5.6–6.4 (n = 6). The performance of the proposed technique was evaluated for determination of TCAs in blood plasma.     

Simple interface for capillary electrophoresis-inductively coupled plasma atomic emission spectrometry

A simple interface has been developed to couple capillary electrophoresis (CE) to inductively coupled plasma atomic emission spectrometry (ICP-AES) for metal speciation. A concentric glass nebulizer with elongated tip is used as the CE-ICP interface. The CE capillary is the central tube of the nebulizer. A platinum wire is wrapped across the exit end of the CE capillary to provide electrical connection to the CE power supply. No sheath flow of buffer solution is needed. A simple cooling system has also been developed. A peristaltic pump circulates water through a plastic tube that encloses the section of the CE capillary between the CE instrument and the ICP spectrometer. Characteristics of the CE-ICP interface, e.g., elution time, nebulization and transport efficiency and peak broadening, versus carrier gas flow-rate have been studied. Comparisons to a previous design with the Pt electrode inserted into the end of the CE capillary are made where appropriate. The reproducibility (RSD) in ICP emission intensity of the system is <4%. Detection limits of Cr and Cu are approximately 5 ng/ml.     

Revision of the volumetric method for measurements of liquid–liquid equilibria in binary systems

A theoretical analysis of the accuracy of the volumetric method for the determination of liquid–liquid equilibrium was carried out. The results show that, under certain conditions, this method can be used to investigate systems showing relatively small mutual solubilities. Relations were derived to estimate standard deviations of the equilibrium compositions determined by the volumetric method.

In the experimental part of the work, an apparatus for measurements of mutual solubilities of liquids was constructed. A procedure that enabled us to determine precisely volumes of liquid phases was developed. This procedure and apparatus present the advantage that relatively small amounts of samples are required (approximately 2 × 20 ml). Theoretical conclusions concerning the applicability of the volumetric method were checked by measuring mutual solubilities at 303.15 K in systems methylcyclohexane + N,N-dimethylformamide, 1-butanol + water and dimethyl phthalate + water. Further, the method was used to measure systematically the liquid–liquid equilibrium in systems ethyl acetate + ethylene glycol and phenyl acetate + ethylene glycol at temperatures from 293 to 323 K. Data for these systems were acquired by means of other methods as well and a good agreement was observed on comparison.     

Study of adsorption of methylene blue and new methylene blue in liquid-solid interface by slab optical waveguide spectroscopy

A slab optical waveguide (SOWG) has been used for study of adsorption of both methylene blue (MB) and new methylene blue (NMB) in liquid–solid interface. Adsorption characteristics of MB and NMB on both bare SOWG and silanized SOWG by octadecyltrichlorosilane (ODS) were compared. Effect of pH on adsorption on MB and NMB was investigated. Binding rate constant analysis showed that both MB and NMB on bare SOWG demonstrates larger association constants than those on ODS-SOWG. Interactions of MB and NMB on bare SOWG and ODS-SOWG were analyzed by molecular mechanics calculation method. The binding energy change was in the following order: E_NMB–bare > E_MB–bare > E_NMB–ODS > E_MB–ODS.     

A differential approach to calculating osmotic equilibrium for multisolvent systems

A differential approach to the calculation of osmotic pressure of multisolvent systems within the Lewis–Randall framework is presented in this paper. Exact differential equations relating the osmotic pressure and the system composition along paths of constant solvent chemical potential are obtained and numerically solved. Although even for the simple case of an ideal solution no analytic expression for the osmotic pressure can be obtained, the system of differential equations does not pose numerical difficulties to be solved. Examples of the use of the proposed methodology are presented using the two-suffix Margules and Flory–Huggins equations, allowing an assessment of the influence of liquid-phase non-ideality on the performance of the method, and showing that it can be applied even for systems wherein liquid–liquid phase equilibrium occurs.     

Surface tension of water droplets upon homogeneous droplet nucleation in water vapor

A method has been proposed for determining interfacial free energy from the data of molecular dynamics simulation. The method is based on the thermodynamic integration procedure and is distinguished by applicability to both planar interfaces and those characterized by a high curvature. The workability of the method has been demonstrated by the example of determining the surface tension for critical nuclei of water droplets upon condensation of water vapor. The calculation has been performed at temperatures of 273–373 K and a pressure of 1 atm, thus making it possible to determine the temperature dependence of the surface tension for water droplets and compare the results obtained with experimental data and the simulation results for a “planar” vapor–liquid interface.     

Do polysaccharides such as dextran and their monomers really increase the surface tension of water?

It has been reported in the literature that sugars such as dextrose and sucrose increase the surface tension of water. The effect was interpreted as a depletion of the solute molecules from the water-air interface. This paper presents accurate measurements of the surface tension of different concentrations of dextrose solution as well as its polymer (i.e., dextran). An automated drop shape technique called axisymmetric drop shape analysis (ADSA) was used for the surface tension determination. The surface tension measurement is presented as a function of a shape parameter, P(s), which has been used to quantify the range of the applicability of ADSA. The results of the above study show that dextrose solutions decrease the surface tension of water in contradiction to the results obtained from the weight drop method in the literature. The surface tension decreases continuously with increasing concentration. A similar effect was observed for the dextran solutions. To verify that the setup and the methodology are capable of accurately measuring increases in surface tension, a similar experiment was conducted with a sodium chloride solution with a concentration of 1 M. It is well-known that electrolyte solutions, e.g., sodium chloride, increase the surface tension of water. The results obtained from ADSA verify that the sodium chloride increases the surface tension of water by 1.6 mJ/m(2). It is concluded that dextrose and dextran decrease the surface tension of water. Thus, there is no evidence of depletion. To identify the sources of discrepancy between the results of ADSA and those reported in the literature, the experiments were repeated for different concentrations and the rate of drop formation using the drop weight method. It was found that the rate of drop formation is most likely the source of error in the results reported in the literature.     

Determination of Oligosaccharides by Conventional High-Resolution Gas Chromatography

This paper describes a simple gas chromatographic method for the analysis of (higher) oligosaccharides without needing a high-temperature procedure. The newly developed method used an ordinary capillary column and a temperature program with 360 °C as the maximum temperature. Sample preparation consisted of the oxymation and silyation of the sugars. Oligosaccharides with degrees of polymerisation of up to 7 could be determined with good repeatability (variation coefficient < 7.5%), reproducibility (variation coefficient < 11.8%) and accuracy (mean recovery 103.9%). Analysis of oligosaccharide-enriched food products demonstrated that this cheap and easy procedure could be a valuable and reliable tool for the quantitative determination of oligosaccharides in foodstuffs.