旋转滴方法研究界面扩张流变性质

采用旋转滴方法, 对2-丙基-4,5-二庚烷基苯磺酸钠(DHPBS)在癸烷-水界面上的扩张流变性质进行了研究, 较为详细地介绍了SVT20N视频旋转滴张力仪的装置和实验方法, 考察了油滴注入体积、基础转速及振荡振幅等实验条件对扩张模量的影响. 研究结果表明, 旋转滴方法是一种研究扩张流变性质的新型手段, 在涉及低界面张力现象的领域具有良好的应用前景.     

Oscillating drop/bubble tensiometry: effect of viscous forces on the measurement of interfacial tension

The oscillating drop/bubble technique is increasingly popular for measuring the interfacial dilatational properties of surfactant/polymer-laden fluid/fluid interfaces. A caveat of this technique, however, is that viscous forces are important at higher oscillation frequencies or fluid viscosities; these can affect determination of the interfacial tension. Here, we experimentally quantify the effect of viscous forces on the interfacial-tension measurement by oscillating 100 and 200 cSt poly(dimethylsiloxane) (PDMS) droplets in water at small amplitudes and frequencies ranging between 0.01 and 1 Hz. Due to viscous forces, the measured interfacial tension oscillates sinusoidally with the same frequency as the oscillation of the drop volume. The tension oscillation precedes that of the drop volume, and the amplitude varies linearly with Capillary number, Ca=DeltamuomegaDeltaV/gammaa(2), where Deltamu=mu(D)-mu is the difference between the bulk Newtonian viscosities of the drop and surrounding continuous fluid, omega is the oscillation frequency of the drop, DeltaV is the amplitude of volume oscillation, gamma is the equilibrium interfacial tension between the PDMS drop and water, and a is the radius of the capillary. A simplified model of a freely suspended spherical oscillating-drop well explains these observations. Viscous forces distort the drop shape at Ca>0.002, although this criterion is apparatus dependent.     

Effect of pH on the adsorption kinetics of egg yolk at the triacylglycerol–water interface and viscoelastic properties of interfacial egg yolk films: a dynamic drop tensiometry study

Interfacial properties of normal egg yolk (EY), as well as stabilized, i.e. enzymatically modified with phospholipase A2, egg yolk (SEY) at the triacylglyceride (TAG) oil–water interface have been investigated with the use of the dynamic drop tensiometry (DDT) technique in the wide interval of pH values of aqueous EY solutions. We found that for both EY and SEY pH values of their aqueous solutions affect absolute values of interfacial tension at the TAG oil–water interface. In the presence of EY this effect was more pronounced, with minimum of interfacial tension values at pH nearly equal to the isoelectric point of EY proteins. For SEY solutions no clear trend was noticed, although a reduction of interfacial tension near pH 6 was also observed. Moreover, the pH-dependence of nearly steady values of interfacial tension in the presence of EY was substantially less pronounced than it has been reported previously. It was also found that there is a difference in the interfacial viscoelasticity of SEY and EY films formed at the TAG oil–water interface. Although the dependence of dilational modulus, ∣ε∣, versus surface pressure for SEY solutions goes through a maximum, absolute values of ∣ε∣ increase for EY solutions in a wide pH range. At the same time, no visible effect of pH on the viscoelasticity of EY and SEY interfacial films was noticed. It became clear from the dilational modulus versus surface pressure curves for both EY and SEY that adsorption of their surface-active components at the TAG oil–water interface occurs in a step-wise manner. We found also that the phase angle values for the adsorbed EY layers were lower that those observed in the presence of SEY, indicating an increasing viscous contribution to the dilational modulus in the SEY-containing system.     

Interfacial dilational properties of tri-substituted alkyl benzene sulfonates at air/water and decane/water interfaces

The dilational rheological properties of absorbed film of three pairs of structural isomers, tri-substituted alkyl benzene sulfonates, at the air-water and decane-water interfaces have been investigated by drop shape analysis method. The influences of bulk concentration on dilational elasticity and viscosity were expounded. Interfacial tension relaxation method was employed to obtain dilational parameters in a reasonably broad frequency range. The experimental results showed that the meta-alkyl to sulfonate group plays a crucial role in the interfacial dilational properties: the longer meta-alkyl will lead to higher dilational parameters for air-water interface and lower ones for decane-water interface when the total alkyl carbon numbers are equal. For alkyl benzene sulfonates with shorter meta-alkyl, the surface dilational properties are similar to interfacial dilational properties, whereas the surface dilational parameters are obviously higher than the interfacial dilational parameters for alkyl benzene sulfonates with longer meta-alkyl in general. The possible mechanism has been proposed and ensured by Cole-Cole plots.     

Dilational rheology of protein films adsorbed at fluid interfaces

This report aims at (i) presenting a quantitative interpretation of interfacial dilational moduli (|E|) for four proteins at three different interfaces and (ii) identifying the main parameters responsible. The proteins were adsorbed from aqueous solution against air, n-tetradecane and sunflower seed oil, as a function of protein concentration and adsorption time.Experimentally, a dynamic drop tensiometer is a convenient instrument to generate the required sinusoidal oscillations for compression/expansion of interfaces (Benjamins et al., 1996 [1]).Theoretically, a simple two-dimensional solution model with a constant molecular area of the protein described the data only at fairly low pressures. Much better agreement over the entire elastic range was found with a recent extension of the model. This extension accounted for adsorbed proteins adopting smaller molecular areas with increasing surface pressure.Three factors dominated the values of the dilational modulus: (i) rigidity of protein molecules, (ii) degree of interfacial non-ideality and (iii) tension of the clean interface (Benjamins et al., 2006 [2]). The last factor is clearly of great relevance to food emulsions.For each protein at different interfaces, the elasticity increased with the enthalpy parameter (Η^S) of the equation of state. Elasticity and Η^S both increased with the clean-interface tension, γ⁰, i.e., with decreasing polarity of the interface (Benjamins et al., 2006 [2]; Fainerman et al., 2003 [3]). The elasticity of the different proteins also increased with increasing rigidity of the molecules, indicating a lower compressibility of the molecular area at the interface.Pure viscosities were never observed in our experience. However, viscoelastic behaviour was found at high pressures, i.e., in densely packed surfaces. The measured viscous phase angles strongly decreased at still higher pressures, indicating that the active relaxation mechanism slowed down with increasing molecular packing density. Specific kinetic models are yet to be developed for such mechanisms.     

Interfacial dilational properties of anionic gemini surfactants with polymethylene spacers

The dilational properties of anionic gemini surfactants alkanediyl-α,ω-bis(m-octylphenoxy sulfonate) (C₈C_mC₈) with polymethylene spacers at the water–air and water–decane interfaces were investigated by oscillating barriers and interfacial tension relaxation methods. The influences of oscillating frequency and bulk concentration on the dilational properties were explored. The experimental results show that the linking spacer plays an important role in the interfacial dilational properties. The moduli pass through one maximum for all three gemini surfactants at both water–air and water–decane interfaces. However, the values of moduli at the water–air interface are obviously higher than those at the water–decane interface because the sublayer formed by spacer chains will be destroyed by the insertion of oil molecules. Moreover, with the increase of spacer length, the surface adsorption film becomes more viscous at high concentration, which can be attributed to the process involving the formation of the sublayer. On the other hand, the spacers of the adsorbed C₈C₆C₈ molecules will extend into the oil phase when the interface is compressed. As a result, the interfacial film becomes more elastic with the increase of spacer length at high concentration. The experimental results obtained by the interfacial tension relaxation measurements are in accord with those obtained by the oscillating barriers method.     

Effect of Molecular Weight on the Interfacial Dilational Viscoelasticity of Anionic Polyelectrolyte/Surfactant Systems

In this article, the effect of molecular weight on the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water-octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. PSS/cationic surfactant CTAB systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be well explained by electrostatic interaction. Molecular weight of PSS plays a crucial role in the nature of adsorption film. The complex formed by CTAB and higher molecular weight PSS, which has larger dimension and stronger interaction, results in higher dilational modulus at lower surfactant bulk concentration. In the case of PSS/anionic surfactant SDS systems, the co-adsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational modulus at lower surfactant bulk concentration. For PSS/nonionic surfactant T × 100 systems, PSS may form a sublayer contiguous to the aqueous phase, which has little effect on interfacial tension but slightly decreases dilational modulus.     

Dilational viscoelasticity of anionic polyelectrolyte/surfactant adsorption films at the water–octane interface

In this article, the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water–octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. Polystyrene sulfonate sodium (PSS)/cationic surfactant hexadecanetrimethyl–ammonium bromide systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be explained well by electrostatic interaction. In the case of PSS/anionic surfactant sodium dodecyl sulfate (SDS) systems, the coadsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational elasticity. For PSS/nonionic surfactant TX100 systems, PSS may form a sub-layer contiguous to the aqueous phase with partly hydrophobic polyoxyethylene chain of TX100, which has little effect on the TX100 adsorption film and interfacial tension.     

A Study of Dynamic Interfacial Properties as Related to Foaming Properties of Sodium 2,5-dialkyl Benzene Sulfonates

In this article, foaming properties and dynamic interfacial properties of a series of sodium 2,5-dialkyl benzene sulfonates in aqueous solutions were carried out to elucidate the relationship between foaming properties and dynamic interfacial properties. The properties of foams generated from bubbling air through different surfactant solutions were measured using a modified Bikerman device. The dynamic surface tension and surface dilational elasticity were obtained from an image analysis technique based on the oscillating bubble method. The surfactants molecular adsorption at the air/water interface was introduced with Rosen empirical equation and the rate of adsorption was determined from measurements of the dynamic surface tension. The surfactant with the longest alkyl chain shows the lowest dynamic surface activity, which lead to the lowest foam volume. The short ortho straight alkyl chain has little effect on the arrangement of molecules at the interface and the foam stability changes a little with the changing of the ortho alkyl chain length. The foam stability is correlated with both the higher surface dilational elasticity and the larger surface monolayer strength.     

Foams and surface rheological properties of β-casein, gliadin and glycinin

Interfacial rheological properties and their suitability for foam production and stability of two vegetable proteins were studied and compared to β-casein. Proteins used ranged from flexible to rigid/globular in the order of β-casein, gliadin and soy glycinin. Experiments were performed at pH 6.7. Network forming properties were characterised by the surface dilational modulus (determined with the ring trough) and the critical falling film length (L_still) at which a stagnant protein film will break. Gliadin had the highest dilational modulus, followed by glycinin and β-casein, whereas glycinin formed the strongest film against fracture in the overflowing cylinder. The rate of decrease in the surface tension was studied at the air–water (Wilhelmy plate method) and the oil–water interface (bursting membrane) and the dynamic surface tension during compression and expansion in the caterpillar. Gliadin had the lowest equilibrium interfacial tensions and β-casein the lowest dynamic surface tension during expansion. Hardly any foam could be formed at a concentration of 0.1 g/l by shaking. At a concentration of 1.4 g/l most foam was formed by β-casein, followed by gliadin and glycinin. It seems that in the first place the rate of adsorption is important for foam formation. For the vegetable proteins, adsorption was slow. This resulted in lower foamability, especially for glycinin.     

A study of interfacial dilational properties of two different structure demulsifiers at oil-water interfaces

The interfacial dilational viscoelastic properties of two demulsifiers with straight chain (SP-169) and branched chain (AE-121) at the oil-water interfaces were investigated by means of the longitudinal waves method and the interfacial tension relaxation method, respectively. The results obtained by the longitudinal waves method showed that the dilational viscous component for AE-121 and SP-169 also passed through a maximum value with increasing concentration. It was found that the maximum value appeared at different demulsifier concentrations during our experiment frequency; and the higher is the dilational frequency, the lower is the concentration. The influences of AE-121 and SP-169 on the dilational viscoelastic properties of the oil-water interface containing surface-active fraction from Iranian crude oil have been measured. The results clearly stated that both demulsifiers could obviously decrease the dilational elasticity of oil-water interface containing surface-active fraction. At low concentration, because of stronger adsorption ability, SP-169 has stronger ability to decreasing the dilational modulus than AE-121. We also found that the dilational modulus of the interface contained surface-active fraction passed through a minimum value with increasing demulsifier concentration for both demulsifiers. This result indicated the dosage of demulsifier had an optimum value. The results obtained by means of interfacial tension relaxation method showed that the slow relaxation processes involve mainly rearrangement in the conformation of the molecules appeared with increasing demulsifier concentration.     

Evaluation of the surface tension measurement of axisymmetric drop shape analysis (ADSA) using a shape parameter

A quantitative criterion called “shape parameter” to evaluate the quality of surface tension measurement of Axisymmetric Drop Shape Analysis (ADSA) is presented. ADSA is a powerful technique for the measurement of interfacial tensions and contact angles of pendant drops, sessile drops, and bubbles. Despite the general success of ADSA, deficient results may be obtained for drops close to spherical shape. Therefore, the “shape parameter” was used to determine the range of drop shapes in which ADSA succeeds or fails. The “shape parameter” is a dimensionless parameter that expresses quantitatively the difference in shape between a given experimental profile and an inscribed circle. The surface tension measurements of ADSA were evaluated for both pendant drop and constrained sessile drop configurations using the shape parameter. Different shapes of the pendant drop were studied using different sizes and materials of holders. For each drop configuration, a “critical shape parameter” was defined based on the minimum value of the shape parameter that guarantees an error of less than ±0.1 mJ/m². Furthermore, the effects of the type of liquid and constellation on the “critical shape parameter” were studied.     

Effect of solid walls on spontaneous wave formation at water/oil interfaces

The regulation of spontaneous waves at water/oil interfaces was investigated, focusing on effects of materials and sizes of containers. Trimethylstearylammonium chloride was dissolved in an aqueous phase. Nitrobenzene with potassium iodide and iodine was used as an organic phase. Rotation of interfacial waves with almost triangular shape was observed only in containers made of glass. The nature of interfacial waves is sensitive to container size. There was no interfacial wave in PFA (Teflon) containers. However, when a glass plate was soaked vertically to the interface, oscillation of contact angles of water/oil interfaces to glass plates was observed. The oscillation generated wave propagation along the plate. Dynamic interfacial tension was measured by Wilhelmy method and the pendant drop technique. Results with the Wilhelmy method in small glass containers exhibited spontaneous oscillation. However, oscillations in dynamic interfacial tension were not observed for other cases, i.e., the Wilhelmy method for large glass containers, for PFA containers, and for the pendant drop technique. It was concluded that all nonlinear behavior such as wave generation and apparent tension oscillation could be attributed to the effect of the sidewalls of container on the adsorption/desorption kinetics of the surfactant. We propose a possible scenario which can explain all of the qualitative features of the present experimental findings.     

Compression/expansion rheology of oil/water interfaces with adsorbed proteins. Comparison with the air/water surface

Dynamic interfacial tensions and surface dilational moduli were measured for four proteins at three fluid interfaces, as a function of time and concentration. The proteins-beta-casein, beta-lactoglobulin, bovine serum albumin, and ovalbumin-were adsorbed from aqueous solution against air, n-tetradecane, and a triacylglycerol oil. The sinusoidal interfacial compression/expansion, at frequencies ranging from 0.005 to 0.5 Hz, was effected in a dynamic drop tensiometer suited to viscous oil phases. Generally, at interfacial pressures up to 15 mN/m, dilational moduli were purely elastic at frequencies from 0.1 Hz. In this elastic range, in-surface relaxation either was essentially completed or had not yet started within a time on the order of 10 s. Within this time span, protein exchange with the bulk solution was negligible. In cases where in-surface relaxation was completed in the imposed time, the moduli depended only on the equilibrium Pi(Gamma) relationship. We interpret these results in terms of a simple two-dimensional solution model, based on a Gibbs dividing surface, accounting for nonideal mixing to the first order with respect to both entropy and enthalpy. Interfacial mixing enthalpy is shown to have a major effect on the elasticity, with both quantities increasing in the sequence triacylglycerol < tetradecane < air. We also suggest a strong correlation between enthalpy and clean-interface tension that increases in the same order. At each interface, the enthalpy increases with increasing molecular rigidity: beta-casein < beta-lactoglobulin < bovine serum albumin < ovalbumin. Best agreement with the experimental data was obtained with a recently extended version of the model accounting for proteins adopting smaller molecular areas with increasing surface pressure. For interfacial pressures above 15 mN/m, the moduli were generally no longer purely elastic, with viscous loss angles ranging up to 36 degrees. In this range of high pressures, the moduli depended on relaxation mechanisms for which specific kinetic models must be developed.     

Protein-lipid interactions at the oil-water interface

The distribution of proteins and lipids in food emulsions and foams is determined by competitive and cooperative adsorption between the two types of emulsifiers at the fluid-fluid interfaces, and by the nature of protein-lipid interactions, both at the interface and in the bulk phase. The existence of protein-lipid interactions can have a pronounced impact on the surface rheological properties of these systems. Therefore, these results are of practical importance for food emulsion formulation, texture, and stability. In this study, the existence of protein-lipid interactions at the interface was determined by surface dynamic properties (interfacial tension and surface dilational modulus). Systematic experimental data on surface dynamic properties, as a function of time and at long-term adsorption, for protein (whey protein isolate (WPI)), lipids (monoglycerides), and protein-lipid mixed films at the oil-water interface were measured in an automated drop tensiometer. The dynamic behaviour of protein+lipid mixed films depends on the adsorption time, the lipid and the protein/lipid ratio in a rather complicated manner. The protein determined the interfacial characteristics of the mixed film as the protein at WPI>/=10(-2)% wt/wt saturated the film, no matter what the concentration of the lipid. However, there exists a competitive or cooperative adsorption of the emulsifier (WPI and monoglycerides), as the concentration of protein in the bulk phase is far lower than that for interfacial saturation.     

The array and interfacial activity of sodium dodecyl benzene sulfonate and sodium oleate at the oil/water interface

There is a close correlation between the interfacial activity and the adsorption of the surfactant at the interface, but the detailed molecular standard information was scarce. The interfacial activity of two traditional anionic surfactants sodium dodecyl benzene sulfonate (SDBS) and sodium oleate (OAS) were studied by experimental and computer simulation methods. With the spinning drop method and the suspension drop method, the interfacial tension of oil/aqueous surfactant systems was measured, and the influence of surfactant concentration and salinity on the interfacial tension was investigated. The dissipative particle dynamics (DPD) method was used to simulate the adsorption of SDBS and OAS at the oil/water interface. It was shown that it is beneficial to decrease interfacial tension if the hydrophobic chains of the surfactant and the oil have similar structure. The accession of inorganic salts causes surfactant molecules to form more compact and ordered arrangements and helps to decrease the interfacial tension. There is an osculation relation between interfacial density and interfacial activity. The interfacial density calculated by molecular simulation is an effective parameter to exhibit the interfacial activity.     

The effect of temperature on food emulsifiers at fluid-fluid interfaces

Heat-induced interfacial aggregation of a whey protein isolate (WPI) with a high content of beta-lactoglobulin (>92%), previously adsorbed at the oil-water interface, was studied by means of interfacial dynamic characteristics performed in an automatic drop tensiometer. Protein concentration in aqueous bulk phase ranging between 1x10(-1) and 1x10(-5) % wt/wt was studied as a variable. The experiments were carried out at temperatures ranging from 20-80 degrees C with different thermal regimes. During the heating period, competition exists between the effect of temperature on the film fluidity and the increase in mechanical properties associated with the interfacial gelation process. Interfacial crystallisation of food polar lipids (monopalmitin, monoolein, and monolaurin) previously adsorbed at the oil-water interface, was studied by interfacial dynamic characteristics (interfacial tension and surface dilational properties). The temperature, ranging between 40 and 2 degrees C, and the lipid concentration in aqueous oil phase, ranging between 1x10(-2) and 1x10(-4) % wt/wt, were studied as variables. Significant changes in interfacial dynamic characteristics associated with interfacial lipid crystallisation were observed as a function of lipid concentration in the bulk phase. Interfacial crystallisation of food polar lipids (monopalmitin, monoolein, and monolaurin) at the air-water interface, was studied by pi-A isotherms performed in a Langmuir trough coupled with Brewster angle microscopy (BAM). A condensation in monoglyceride monolayers towards lower molecular area was observed as the temperature decreased. This effect was attributed to lipid crystallisation at lower temperatures. BAM images corroborated the effect of temperature on the monolayer structure, as a function of the monoglyceride type.     

Partition and water/oil adsorption of some surfactants

Adsorption isotherms have been determined at the water/oil interface for five biphasic systems involving surfactants (non-ionic and ionic) present in both phases at partition equilibrium. The systems studied were polyoxyethylene(23)lauryl ether (Brij35) in water/hexane and four ionic surfactants, hexadecyltrimethylammonium bromide (CTAB), and a series of three tetraalkylammonium dodecylsulfate (TEADS, TPADS, and TBADS) in water/CH 2Cl 2. Interfacial tension measurements performed at the water/air and water/oil interfaces provided all the necessary information for the determination of the adsorption parameters by taking partition into account. These measurements also allowed the comparison of the adsorption properties at both interfaces which showed an increase of the adsorption equilibrium constant and a decrease of the maximum surface concentration at the water/oil interface compared to water/air. The values of the critical aggregation concentration showed, in all cases, that only the surfactant dissolved in the aqueous phase contribute to the decrease of the water/oil interfacial tension. In the case of the four ionic surfactants, the critical aggregation concentration obtained in biphasic conditions were lowered because of the formation of mixed surfactant-CH 2Cl 2 aggregates.     

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 measurements at the interface between two highly immiscible electrolyte solutions: the trapped air bubble method

A dynamic method for the measurement of interfacial tension at a liquid–liquid interface under controlled interfacial potential difference is described. The interface was formed as a drop suspended at the tip of a liquid filled glass micro-syringe into which a trapped air bubble of known volume had been introduced. Changes in volume of the air bubble allow changes in the surface tension to be calculated. Application to measurement of the variation in interfacial tension with cyclic variation in interfacial potential difference (‘cyclic volttensiometry’) is demonstrated.