Dynamic interfacial tension measurement

In this entry, the most used microfluidic devices are described and compared using criteria based on scaling relations, ease of operation, and productivity. In the future research section, the use of these devices as tools for dynamic interfacial tension measurement and emulsion stability is discussed among others. 

Emulsions and microfluidic structures have been used for many purposes, including fusion of reactants present in two droplets, preparation of gel beads, preparation of multiple emulsions, etc. for a comprehensive overview, please consult the review paper of Vladisavljevic and colleagues [1]. Besides this, the microfluidic systems discussed in this entry can be used as analytical tools in various ways. To illustrate this, the use of Y-shaped junctions for dynamic interfacial tension measurement [14] and the use of T-junctions in combination with a coalescence chamber for emulsion stability research [15] are discussed next. 

Steegmans MLJ, Warmerdam A, Schroen CGPH, Boom RM (2009) Dynamic interfacial tension measurements with microfluidic Y-junctions. Langmuir... 

During dynamic interfacial-tension measurement, the geometry of the drop, and thus the interfacial tension between the drop and the medium, change as a function of time as more and more amphiphilic molecules diffuse from the bulk to the interface. The process continues until equilibrium geometry and, hence, equilibrium interfacial tension are attained after a relatively long period of time. The equilibrium interfacial tension is reached when the concentration of amphiphilic molecule at the interface attains its equilibrium value. 

The oil-water dynamic interfacial tensions are measured by the pulsed drop (4) technique. The experimental equipment consists of a syringe pump to pump oil, with the demulsifier dissolved in it, through a capillary tip in a thermostated glass cell containing brine or water. The interfacial tension is calculated by measuring the pressure inside a small oil drop formed at the tip of the capillary. In this technique, the syringe pump is stopped at the maximum bubble pressure and the oil-water interface is allowed to expand rapidly till the oil comes out to form a small drop at the capillary tip. Because of the sudden expansion, the interface is initially at a nonequilibrium state. As it approaches equilibrium, the pressure, AP(t), inside the drop decays. The excess pressure is continuously measured by a sensitive pressure transducer. The dynamic tension at time t, is calculated from the Young-Laplace equation... 

While the quasistatic method is quite accurate, it requires a long time to determine a complete adsorption kinetics curve. This is because a new drop has to be formed at the tip of the capillary to determine one single measurement point. For example, if ten dynamic interfacial tension values are to be determined over a period of 30 min, -180 min will be required to conduct the entire measurement. On the other hand, the constant drop formation method is often limited because a large number of droplets have to be formed without interruption, which may rapidly empty the syringe. Furthermore, the critical volume required to cause a detachment of droplets depends on the density difference between the phases. If the density difference decreases, the critical volume will subsequently increase, which may exacerbate the problem of not having enough sample liquid for a complete run. 

An excellent comprehensive review of all theoretical and practical aspects of dynamic and static interfacial tension measurements written by the most prolific authors in the field of protein adsorption. Contains a wealth of additional references that the interested reader may consult to gain additional understanding of the field of research. 

Dynamic Interfacial Tension. Crude-oil-alkali systems are unusual in that they exhibit dynamic interfacial tension (Figure 11). A solution of 0.05 wt% sodium hydroxide in contact with David Lloydminster crude oil initially produces ultralow values of IFT. A minimum value is reached, after which IFT increases with time by nearly 3 orders of magnitude, measured in the spinning drop tensiometer. Taylor et al. (57) showed that dynamic inter-facial tension can also occur in crude-oil-alkali-surfactant systems. Figure 11 shows interfacial tension versus time for a solution containing 1 wt% sodium carbonate, and the same solution containing 0.02 wt% of Neodol 25-... 

Some interfacial tension measurement techniques are essentially static, i.e. they operate at low Deborah number (De 1) capillary rise, shapes of sessile and pendant drops. Others (drop weight and detachment techniques) require extension of the interface. Then the procedure is static or dynamic depending on the rate of extension relative to the rate of adsorption equilibration, i.e. on De. 

Here it may be appropriate to note that in the literature the term dynamic interfacial (or surface tension is used in two senses, viz. (i) as the tension obtained by a dynamic method such as scattering or (li) as that obtained under nonequilibrium conditions, l.e. tensions measured at non-zero De, for interfaces that relax during the observation. We shall reserve the term dynamic interfacial tension for the latter case, that is, for Interfaces that are not equilibrated (sec. 1.14). Interfacial tensions derived from scattering are essentially static, although... 

When the value of the interfacial tension is significantly less than 1 mN m 1, then we consider the measurement of ultra-low interfacial tension, which is common in liquid-liquid emulsification processes when effective surfactant solutions are used. The dynamic spinning drop tensiometer method is especially suitable for this purpose. Ultra-low interfacial tension measurement is important in the chemical industry because the cleaning of solid surfaces of dirt, grease, and oil the formulation of stable emulsions the recovery of petroleum, and other applications often rely on lowering the interfacial tension between immiscible liquids to ultra-low values by the use of surfactants. 

Fig. 5 Dynamic interfacial tension (y) measurements of a toluene-water interface during adsorption of 6-nm CdSe nanoparticles to a pendant water drop in toluene (CdSe concentration was 1.58 x 10-6 mol/L). The circles mark the time at which TEM samples shown in Fig. 6 were prepared. The inset depicts the data on a logarithmic time scale. Reprinted with permission from Physical Chemistry Chemical Physics [50], Copyright (2007) RSC Publishing...

Fig. 11 Dynamic interfacial tension (y) measurements of a hexane-water interface during adsorption of nanoparticles to a pendant water drop in hexane (for ail particle types, concentration was 1.2 x 10-4 mmol/L). The gold moieties were modified using dodecanethiol (DDT) or octade-canethiol (ODT). NP homogeneous nanoparticles, JP Janus particles. Reprinted with permission from Langmuir [68], Copyright (2006) American Chemical Society...

Dynamic properties of interfaces have attracted attention for many years because they help in understanding the behaviour of polymer, surfactant or mixed adsorption layers.6 In particular, interfacial rheology (dilational properties) is crucial for many technological processes (emulsions, flotation, foaming, etc).1 The present work deals with the adsorption of MeC at the air-water interface. Because of its amphiphilic character MeC is able to adsorb at the liquid interface thus lowering the surface tension. Our aim is to quantify how surface active this polymer is, and to determine the rheological properties of the layer. A qualitative and quantitative evaluation of the adsorption process and the dilata-tional surface properties have been realised by dynamic interface tension measurements using a drop tensiometer and an axisymmetric drop shape analysis. 

The most frequently used parameter to characterise the dynamic properties of liquid adsorption layers is the dynamic interfacial tension. Many techniques exist to measure dynamic tensions of liquid interfaces having different time windows from milliseconds to hours and days. 

Beside the very frequently used methods of dynamic surface and interfacial tension measurements, adsorption kinetics processes at liquid interfaces can also be studied by other methods, such as dynamic surface potentials, ellipsometry and other light scattering and reflection methods, X-ray techniques, neutron scattering, radiotracer techniques. These methods yield more or less relative information on the change of adsorption with time at different time resolutions. 

This chapter presents the state of the art of experimental equipment available for dynamic surface and interfacial tension measurements. The introductory section demonstrates the crucial importance of a special purity of surfactants and solvents, called surface-chemical purity, for interfacial studies. It turns out that studies of surfactant adsorption layers could be... 

DDRM is particularly useful for the binary polymer blends. The dynamic interfacial tension coefficient, Vj2, is determined from the time evolution of a distorted fluid drop toward its equilibrium form. Measurements of either low viscosity model systems or high viscosity industrial polymer mixtures led to a good agreement with values obtained from the widely used breaking thread method. DDRM enables to measure in polymeric blends of commercial interest — the high viscosity systems that frequently are impossible to characterize by other techniques. Furthermore, for the first time it is possible to follow the time dependence of Vj, thus unambiguously determine its dynamic and equilibrium values. 

Later in Chapter 6 a large variety of technologies based on adsorption effects will be described. It will also be shown that in general, these technologies work under dynamic conditions and an improvement of the surfactant s efficiency, made in the by past trial and error or thanks personal experience, is now more and more based on a systematic analysis of the entire technology and the particular impact of the surfactants used. The optimisation of surfactants and their mixtures requires specific knowledge of their dynamic adsorption behaviour [1]. The most frequently used parameter to characterise the dynamic properties of liquid adsorption layers is the dynamic interfacial tension. Many techniques exist to measure dynamic tensions of liquid interfaces having different time windows from milliseconds to hours and days. As direct measurements of the time process of adsorption of surfactants at liquid interfaces are rare... 

As mentioned above, the study of the dynamics of adsorption layers at liquid interfaces is mainly restricted to surface and interfacial tension measurements. Only for slow adsorption processes, methods such as radiotracer technique [163, 164], the significantly improved surface ellipsometry [165, 166], or the very recently developed technique of neutron reflectivity [167, 168, 169, 170] can be used to directly follow the change of surface concentration with time. Neutron reflectivity allows even distinguishing between different species adsorbed at a fluid interface [171, 172, 173]. These techniques are reviewed in more detail in the preceding chapter 3 as they yield data most of all for the equilibrium state of adsorption layers. 

Adsorption kinetics, mainly studied by dynamic surface tension measurements, shows many features very much different from that of typical surfactants (Miller et al. 2000). The interfacial tension isotherms for standard proteins such as BSA, HSA, (3-casein and (3-lactoglobulin were measured at the solution/air interface by many authors using various techniques. The state of the art of the thermodynamics of adsorption was discussed in Chapter 2 while isotherm data for selected proteins were given in the preceding Chapter 3. Here we want to give few examples of the dynamic surface pressure characteristics of protein adsorption layers. 

Typical experiments to study the adsorption of a surfactant at a water/oil interface consists in the measurement of the interfacial tension with time. An example is shown in Fig. 4.39. In this experiment, a drop of an aqueous surfactant solution is first formed in a cell filled with pure hexane. The volume ratio Q=10 of the volume of the water drop (containing the surfactant) to that of the hexane bulk. The dynamic interfacial tension y(t) is then monitored by the pendant drop technique. The time dependence of y for four initial concentrations of C13DMPO in aqueous solution is shown in Fig. 4.41. 

If we exchange the phases in the experiment completely different results are obtained. When we form a hexane drop inside a cell filled with an aqueous surfactant solution. The measured dynamic interfacial tensions for such an experiment are shown in Fig. 4.42 for three C13DMPO... 

In addition to the experimental data, the dependencies y(t) calculated from the theory of Eq. (4.94) are shown in Figs. 4.41 and 4.42. The results agree very well with the measured dynamic interfacial tension, in particular at the lower concentrations. Note that these calculations are not best fits to the experimental points but directly result from when the isotherm data for this surfactant are used (cf. Chapter 3). For larger concentrations, the deviation increases, which can possibly be caused by one of the assumptions, such a spherical symmetry for the drop and negligible deformation during the adsorption process. ... 

In this chapter an overview is given on the possibilities for a quantitative characterization of adsorption layers at liquid/liquid interfaces. After a general introduction to the fundamental thermodynamic relationships and particular ideas on surfactant and protein adsorption, the process of adsorption-layer formation is discussed on the basis of the most frequently used methodology, the measurement of dynamic interfacial tensions. This will also include measurements of extremely low interfacial tensions and the effect of inter-facial transfer between the two liquid phases. 

What distinguishes Y-junctions most from other shear-driven techniques is that the emulsion droplet size can solely be controlled by the continuous phase to predict the droplet size, a simple force balance can be used, linking viscous shear force and interfacial tension, indicating a one-step mechanism [10] (in the future directions, it is described how this feature of Y-junctions may be used to measure dynamic interfacial tensions). This suggests that emulsification in Y-junctions... 

As mentioned previously, at the Y-shaped junction droplet formation takes place in a one-step mechanism that is determined by the viscous shear force and the interfacial tension force [11]. Because of this special feature, it was possible to directly measure the effect of interfacial tension on the droplet size using various systems with different static interfacial tensions. Water/ ethanol mixtures were used as continuous phase, and hexadecane and silicon oils as to-be-dispersed phase. The size of the droplets was recorded and a calibration curve constructed, and based on that curve, the dynamic interfacial tension could be estimated in systems that contain surfactants. 

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