Dynamic Interfacial Tension of Surfactant Mixtures at Liquid-Liquid Interfaces

Dynamic interfacial tensions for surfactant mixtures at liquid-liquid interfaces were obtained with a drop volume tensiometer. The surfactants tested were Triton X-100, palmitic acid, and Span 80 at both the water-hexadecane and water-mineral oil interfaces. Two-surfactant mixtures were examined with the surfactants initially dissolved in different phases to minimize bulk-phase interactions. For concentrations below the CMC, it was found that the adsorption kinetics of palmitic acid and Triton X-100 mixtures were dominated by the latter surfactant. Apparent diffusion coefficients were obtained for Triton X-100 both in the absence and in the presence of palmitic acid. These values were largely insensitive to the presence of palmitic acid. For mixtures of Span 80 and Triton X-100, the adsorption kinetics were found to be influenced significantly by both surfactants. In this case, relative changes in surfactant concentrations affected the dynamic interfacial tension of the mixed system. A previously proposed multicomponent adsorption model described the dynamic interfacial tension adequately at low concentrations of Triton X-100, when desorption could be neglected. At higher concentrations, modifications were needed to account for solubilization into the oil phase. These corrections allowed the model to describe the long time adsorption quite well. However, predicted values of short time interfacial tensions were overestimated, likely due to a synergistic interaction of the two surfactants. Copyright 1999 Academic Press.     

A Growing Drop Technique for Measuring Dynamic Interfacial Tension

A novel, growing drop technique is described for measuring dynamic interfacial tension due to sorption of surface-active solutes. The proposed method relates the instantaneous pressure and size of expanding liquid drops to the interfacial tension and is useful for measuring both liquid/gas and liquid/liquid tensions over a wide range of time scales, currently from 10 ms to several hours. Growing drop measurements on surfactant-free water/ air and water/octanol interfaces yield constant tensions equal to their known literature values. For surfactant-laden, liquid drops, the growing drop technique captures the actual transient tension evolution of a single interface, rather than interval times as with the classic maximum-drop-pressure and drop-volume tension measurements. Dynamic tensions measured for 0.25 mM aqueous 1-decanol solution/air and 0.02 kg/m³ aqueous Triton X-100 solution/dodecane interfaces show nonmonotonic behavior, indicating slow surfactant transport relative to the imposed rates of interfacial dilatation. The dynamic tension of a purified and fresh 6 mM aqueous sodium dodecyl sulfate (SDS) solution/air interface shows only a monotonic decrease, indicating rapid surfactant transport relative to the imposed rates of dilatation. Conversely, an aged SDS solution, naturally containing trace dodecanol impurities, exhibits dynamic tensions which reflect a superposition of the rapidly equilibrating SDS and the slowly adsorbing dodecanol.     

Experimental study on dynamic interfacial tension with mixture of SDS-PEG as surfactants in a coflowing microfluidic device

In this work, a coflowing microfluidic device was used to determine the influence of different mixed sodium dodecyl sulfate (SDS)-poly(ethylene glycol) (PEG) compound systems on dynamic interfacial tension and, by extension, corresponding emulsion droplet sizes. The aqueous solutions were used as the continuous phase in the microfluidic device, while octane was used as the organic dispersed phase. Combined SDS-PEG systems lower the interfacial tension more than either component can alone up to the critical aggregation concentration (CAC) of SDS. Octane droplet sizes produced in the microfluidic device using combined SDS-PEG systems were smaller than those produced using SDS alone, and a reduction in dynamic interfacial tension as determined by drop size followed a pattern similar to that observed in the static case (PEG4000 > PEG600 > PEG400 > PEG200 > PEG8000) with the exception of PEG8000. Finally, a previously formulated model relating interfacial tension to droplet size was used to estimate the dynamic interfacial tensions in the microfluidic device.     

The dynamic effects of surfactants on droplet formation in coaxial microfluidic devices

Droplet emulsification in microfluidic devices involves the constant formation of fresh interfaces between two immiscible fluids. When the multiphase system contains surfactant, dynamic mass transfer of the surfactant onto the interface results in a dynamic interfacial tension different from the static interfacial tension measured in an equilibrium state. In this work, we have systematically investigated the effects of surfactant concentration and type on the dynamic interfacial tension of two different liquid-liquid two phase systems [N-hexane/water-sodium dodecyl sulfate (SDS) and N-hexane/water-cetyltrimethylammonium bromide (CTAB)] rapidly producing relatively small droplets in coaxial microfluidic devices. Dynamic interfacial tension experiments using the pendent drop method and a tensiometer were conducted, and a semiempirical equation was developed to put into context the effects of surfactants and the experimental conditions on droplet formation and dynamic interfacial tension in dynamic microchannel flows. The results presented in this work provide a more in-depth understanding of the dynamic effects of surfactants on droplet formation and the precise controllable preparation of monodispersed droplets in microfluidic devices.     

Interfacial tension between polystyrene and a liquid crystal polymer

In this work contact angles formed by drops of polystyrene (PS) on a surface of liquid crystalline polymer (LCP) Vectra A910 were measured as a function of temperature for temperatures ranging from 180 to 230°C. The values were used together with the surface tensions of both polymers to evaluate the interfacial tension between PS and the LCP. In order to validate the method used to evaluate this interfacial tension, the interfacial tension between polypropylene (PP) and PS was evaluated using values of contact angles formed by a drop of PP on PS and the values of surface tension of both polymers in the molten state. The values of interfacial tension between PP and PS corroborated well the values obtained using the pendant drop method. The values of interfacial tension between PS and the LCP were shown to decrease linearly with temperature.     

Incomplete phase inversion W/O/W emulsion and formation mechanism from an interfacial perspective

Water-in-oil-in-water (W/O/W) double emulsion can be prepared by incomplete phase inversion method using both medium chain triglycerides (MCT) and isopropyl myristate (IPM) as oil phase, Span 85-Tween 80 (HLB values of 2.5-3.0) as mixed emulsifiers. The preparation method was simple, and the final double emulsions were proved of good microstructure and particle size distribution. Owning to the addition of Tween 80 to Span 85, interfacial tension, interfacial viscosity and modulus decreased, which contributed to the phase inversion. Furthermore, formation of reverse micelles under high-speed dispersion may be a hypothesis to explain the incomplete phase inversion phenomenon.     

Dynamic and Rheological Properties of Span 80 at Liquid–Liquid Interfaces

The adsorption characteristics of Span 80 at liquid/liquid interfaces were investigated. The equilibrium interfacial tension values were successfully fitted with a Langmuir isotherm resulting in the determination of a mean molecular area from 25 to 35 Å²/mol. The measured interfacial tension values and deduced adsorption parameters depend on the experimental technique used to obtain them, either Du Noüy ring or profile analysis tensiometry. Two possible explanations to such phenomenon are provided. Adsorption kinetics of Span 80 at liquid/liquid interfaces were studied, and it was concluded that the diffusion of Span 80 molecules from the bulk is the rate determining step of the adsorption. Finally the interfacial rheology properties were investigated and compared to the Lucassen–van den Tempel model. A good match was obtained when the isotherm parameters determined by profile analysis tensiometry were used.      

表面活性剂双水相界面性质的研究

表面活性剂双水相是指正、负离子表面活性剂混合水溶液在一定浓度及混合比 范围内，自发分离形成的两个互不相溶的水相。前文报道了将其作为一种新型萃取 体系，用于生物活性物质的分离。目前有关其相行为、化学物质和生物大分子的分 配方面已有较多研究，但未见两相之间界面化学性质研究的报道。表面活性剂双水 相的形成是一种奇特的相分离现象，两个稀水溶液（含水量可高达99%以上）互不 相溶、平衡共存，其界面结构和界面张力必有其特殊性。     

Interfacial tension in aqueous biopolymer-surfactant mixtures

The current study offers a first insight into the interfacial properties of pullulan-sodium dodecyl sulphate (SDS) aqueous two-phase systems (ATPS) in the presence of sodium chloride (NaCl). The effect of composition on the interfacial tension (sigma) in these ATPS was investigated over a wide range of pullulan, SDS and NaCl concentrations. An increase in the interfacial tension was observed with increasing pullulan and SDS concentrations and a small increase was also observed as the NaCl concentration was increased. In both cases the interfacial tension increases were closely related to the phase behaviour of these systems; as a consequence of increasing the pullulan, SDS and/or NaCl concentrations, the system moves further away from the critical point. In all systems interfacial tensions (of the order of muN/m) were comparable with those reported for polymer-polymer ATPS. Interfacial tensions sigma can be well correlated with the difference in pullulan and SDS concentrations between the phases (DeltaC pul and DeltaC SDS) and also the tie-line length (TLL); all yield straight lines on a log-log scale.     

Effect of stabilizer on formation of "onionlike" multilayered polystyrene-block-poly(methyl methacrylate) particles

The effect of the kind of stabilizers on the formation of "onionlike" multilayered polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) particles was studied. The release of toluene from PS-b-PMMA/toluene droplets dispersed in aqueous medium resulted in the formation of onionlike multilayered structures in the particles for all stabilizers used. However, the surface composition of the particles was strongly affected by the kind of stabilizer. When sodium dodecyl sulfate (SDS) and poly(vinyl alcohol) (PVA) were used as stabilizers, the surface of the particles was occupied by PMMA phase. On the other hand, in the cases of Emulgen 911 (polyoxyethylene nonylphenyl ether) and Tween 80 (polyoxyethylene sorbitan monooleate) as stabilizers, the PS phase occupied the surfaces. These results for SDS, PVA, and Emulgen 911 are consistent with the surface layer of the PS-b-PMMA particles being occupied by the polymer phase, which gives a lower interfacial tension than that of another phase. However, in the case of Tween 80, interfacial tensions between water and toluene solutions of the polymer showed almost the same values making it impossible to predict the surface polymer phase.     

Determination of Low Interfacial Tension with a Laser Light Scattering Technique and a Comparative Analysis with Drop Shape Methods

A comparative analysis of laser light scattering and drop shape techniques for measuring low interfacial tension is provided with particular reference to the gas condensate systems in petroleum reservoirs. Measurements of interfacial tensions for two real reservoir gas condensate samples were undertaken at elevated pressure and temperature reservoir conditions with the laser light scattering technique. The purposes of this study are (a) to clarify some existing confusion on the usefulness of three major interfacial tension measurement methods, pendant drop, spinning drop, and laser light scattering; and (b) to show the applicability of the laser light scattering technique to real reservoir gas condensate interfaces at low interfacial tensions. The laser light scattering technique has appeared to be the most suitable approach for the determination of low gas condensate interfacial tensions at reservoir conditions in terms of precision and operation. Copyright 2001 Academic Press.     

Novel calibration of a dynamic surface tension detector: flow injection analysis of kinetically-hindered surface active analytes

First, a novel technique for calibration of a dynamic surface tension detector (DSTD) is described. The DSTD measures the differential pressure as a function of time across the liquid-air interface of growing drops that repeatedly form and detach at the end of a capillary tip. The calibration technique utilizes the ratio of pressure signals acquired from the drop growth of two separate solutions, i.e. a standard solution and a corresponding mobile phase, such as water, both of which have a known surface tension. Once calibrated, the dynamic surface tension of an analyte is obtained from the ratio of the pressure signals from the analyte solution to that of the mobile phase solution. Thus, this calibration technique eliminates the need to optically image the radius of the expanding drop of liquid. Accurate dynamic surface tension determinations were achieved for aqueous sodium dodecyl sulfate (SDS) solutions over a concentration range of 0.5-5.4 mM. The measured surface tensions for these SDS solutions range from 70.3 to 46.8 dyne/cm and were in excellent agreement with the literature. A precision of 0.2 dyne/cm (1 S.D.) was routinely obtained. Second, the DSTD with this calibration technique was combined with flow injection analysis (FIA) for the study of model protein solutions and polymer solutions. The kinetic surface tension behavior of aqueous bovine serum albumin (BSA) solutions as a function of concentration and flow rate is presented. Evaluation of the dynamic surface tension data illustrates that a protein such as BSA initially exhibits kinetically-hindered surface tension lowering, i.e. a time dependence, as BSA interacts with the liquid-air interface of an expanding drop. FIA/DSTD is then shown to be an effective tool for the rapid study of kinetically-hindered surfactant mixtures. It was found that mixtures of SDS and the polymeric surfactant Brij(R)-35 (lauryl polyoxyethylene ether with an average molecular weight of 1200 g/mol) result in essentially an additive lowering of the surface tension. Mixtures of polyethylene glycol (PEG), with an average molecular weight of 1470 g/mol, and Brij(R)-35, however, result in a competitive (non-additive) surface tension with the Brij(R)-35 dominating the response.     

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.     

Dynamic adsorption and characterization of phospholipid and mixed phospholipid/protein layers at liquid/liquid interfaces

Drop profile analysis tensiometry is applied to study the adsorption dynamics of phospholipids, proteins and phospholipid/protein mixtures at liquid/liquid interfaces. Measurements of the dynamic interfacial tension of phospholipid layers give information on the adsorption mechanism and the structure of the adsorption layer. The equilibrium and dynamic adsorption of pure protein solutions, i.e. human serum album (HSA), beta-lactoglobulin (beta-LG), beta-casein (beta-CA), can be explained well by the thermodynamic model of Frumkin and the diffusion-controlled adsorption theory. The adsorption behavior from mixed phospholipid/protein solutions was also investigated in terms of dynamic interfacial tensions. Interestingly, a "skin-like" folded film of pure protein or phospholipid/protein complex layers can be observed at curved surfaces at the water/oil interfaces. The addition of phospholipids accelerates the formation of the folded structure at the drop surface through co-adsorption of proteins.     

Preparation of Water-in-Diesel Fuel Nanoemulsions Using High-Energy Emulsification Method and a Study of Some of Their Surface Active Properties

In this work, formations of water-in-diesel fuel nanoemulsions using water/mixed nonionic surfactant/diesel fuel system has been studied. The high-energy emulsification method was used to form three emulsions using different water contents: 5, 10, and 14% (v/v) namely; E1, E2, and E3, respectively. These nanoemulsions were stabilized with emulsifiers having different hydrophilic lipophilic balance (HLB), namely, Span 80 (HLB = 4.3), Emarol 85 (HLB = 11), and their mixture (SE) with HLB = 10. The effect of water on the droplet size formation has been investigated. The interfacial tension and thermodynamic properties of the individual and emulsifiers blends have been studied. The interfacial tension (γ) measurements at 30°C were used to determine the critical micelle concentration (CMC) and surface active properties of these emulsifiers. The water droplet sizes were measured by dynamic light scattering (DLS). From the obtained data, it was found that mean sizes between 19.3 and 39 nm were obtained depending on the water content and concentration of blend emulsifiers (SE). Also, the results show that the interfacial tension (γ) gives minimum value (10.85 mN/m) for SE comparing with individual emulsifier (17.13 and 12.77 mN/m) for Span 80 and Emarol 85, respectively. The visual inspection by transmission electron microscopy showed that the obtained results support the data obtained by dynamic light scattering.     

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.     

Study on the Interfacial Tensions Between Oils and Alkylbenzene Sulfonate Gemini Surfactant Solutions

Interfacial tensions (IFT) of five alkylbenzene sulfonate Gemini surfactants Ia, Ib, Ic, Id, and Ie at different oils/water systems were measured by a spinning drop interfacial tensiometer. And critical micelle concentration (CMC), the interfacial tension at CMC (γ_CMC), maximum interfacial excess concentration (Γ _max) and the surface area per molecule (A_min) were calculated. The results indicated that the CMC values determined with interfacial tension method were lower than those determined with surface tension method. And γ_CMC for Ie is larger than that for Ia, Ib, Ic, and Id. In addition, the effects of temperature and hydrophobic chains on dynamic IFT were also studied. With the increment of temperature, dynamic IFT is easier to reach a stable value. However, with the increment of hydrophobic chains, dynamic IFT is more difficult to reach a stable value. Each Gemini surfactant produces a minimum IFT when measured against a different n-alkane.     

Revisiting the morphology development of solvent-swollen composite polymer particles at thermodynamic equilibrium

Morphology of polystyrene (PS)/poly(methyl methacrylate) (PMMA)/toluene droplets, in which phase separation proceeds, dispersed in SDS aqueous solution was examined. It changed from ex-centered PS-core/PMMA-shell to hemisphere with increasing SDS concentration. At low polymer weight fraction (wp), PS and PMMA phases contained non-negligible amount of PMMA and PS, respectively. The small amount of PS and PMMA in PMMA and PS phases, respectively, affected significantly the interfacial tension between polymer/toluene and aqueous solutions. Interfacial tension between PS and PMMA phases at low wp was measured by the spinning drop method, showing a quite low value ( approximately 10-2 mN/m). Predicted morphology obtained from calculation of minimum total interfacial free energy of the droplets using the interfacial tensions agreed well with the experimental observation.     

Influence of dynamic interfacial tension on droplet formation during membrane emulsification

Membrane emulsification is a promising and relatively new technique for producing emulsions. The purpose of this study was to better understand the influence of interfacial tension on droplet formation during membrane emulsification. Droplet formation experiments were carried out with a microengineered membrane; the droplet diameter and droplet formation time were studied as a function of the surfactant concentration in the continuous phase. These experiments confirm that the interfacial tension influences the process of droplet formation; higher surfactant concentrations lead to smaller droplets and shorter droplet formation times (until 10 ms). From drop volume tensiometer experiments we can predict the interfacial tension during droplet formation. However, the strong influence of the rate of flow of the to-be-dispersed phase on the droplet size cannot be explained by the predicted values. This large influence of the oil rate of flow is clarified by the hypothesis that snap-off is rather slow in the studied regime of very fast droplet formation.     

正负离子混合表面活性剂双水相界面张力的研究

用旋转滴法测定了正负离子混合表面活性剂形成的双水相界面张力, 研究了双水相界面张力与表面活性剂的分子结构、正负离子表面活性剂的摩尔比、总浓度、外加无机盐及温度的关系. 结果表明, 双水相界面张力在一定正、负离子表面活性剂的摩尔比时属于超低界面张力范围. 观察到三种界面张力曲线类型, 第一类为摩尔比1:1 的两边的两条曲线, 界面张力随过剩表面活性剂组分的比例增加而降低; 第二类为一条跨过摩尔比1:1的马鞍型曲线; 第三类为位于摩尔比1:1的一边的一条马鞍型曲线. 界面张力曲线的类型主要取决于表面活性剂的分子结构, 包括亲水基类型、疏水链长度及对称性.