Drop shape methods

In the present paper, interfacial tensions were measured for a number of heavy crude oils at temperatures up to 200°C using the spinning drop technique. However, reliable data cannot be obtained by this or any other drop shape method because of the small density difference between heavy crudes and water which, moreover, tends to decrease as the temperature increases. This problem was overcome by using aqueous D20 instead of H20 as has been previously described [5,8,211. The influence of surfactant type and concentration, mono- and divalent cation concentrations, and pH on the attainment of low interfacial tensions are reported and discussed. 

Drop shape method (can also be used for high pressure and temperature)... 

This unit will introduce two fundamental protocols—the Wilhelmy plate method (see Basic Protocol 1 and Alternate Protocol 1) and the du Noiiy ring method (see Alternate Protocol 2)—that can be used to determine static interfacial tension (Dukhin et al., 1995). Since the two methods use the same experimental setup, they will be discussed together. Two advanced protocols that have the capability to determine dynamic interfacial tension—the drop volume technique (see Basic Protocol 2) and the drop shape method (see Alternate Protocol 3)—will also be presented. The basic principles of each of these techniques will be briefly outlined in the Background Information. Critical Parameters as well as Time Considerations for the different tests will be discussed. References and Internet Resources are listed to provide a more in-depth understanding of each of these techniques and allow the reader to contact commercial vendors to obtain information about costs and availability of surface science instrumentation. 

Many different techniques have been developed for the measurement of contact angles 17.8). Of these, the three most useful methods are the Wilhelnty technique, the technique of capillary rise at a vertical plate, and the drop shape methods. These techniques require the solid surface to be flat and smooth. Direct measurement of contact angles on fibers (of uniform thickness) can also be performed using the Wilhelmy technique. For nonflat surfaces or particles, indirect methods such as capillary penetration into columns of powders, sedi-... 

In the following subsections, the three most commonly used contact angle techniques, the Wilhelmy technique, capillary rise at a vertical plate, and drop shape methods, are described in detail. 

Drop Volume and Drop Shape Methods, Video-image Digitization Techniques... 

Figure 6.2 Liquid surface tension determination by the drop shape method a. A pendant drop is formed by suspending the liquid from the tip of a thin tube. b. A sessile air (or vapor) bubble is formed in a liquid by injecting the gas from the tip of a needle connected to a syringe.

Drop shape method and video-image digitization techniques... 

Future developments will also focus on the combination of different techniques, such as drop pressure and drop shape methods. A more efficient approach would be to combine macroscopic with microscopic or molecular methods, for example drop shape or pressure experiments with ellipsometric or spectroscopic techniques. Another useful possibility involves linking, for example, the inclinded plate or overflowing cylinder technique with scattering experiments, which would allow studies of structure formation under dynamic conditions and at freshly formed surfaces (Howe et al. 1993). 

From rm experimental point of view, studies with protein solutions are very difficult due to their extremely high surface activity. In many experimental techniques the adsorption at the interface, or sometimes even at the surface of the container, tubes and connectors of the dosing system, can lead to a depletion of protein in the bulk. Estimations have shown that the protein mass in the bulk of a drop and in the adsorption layer at the drop surface, are comparable for drops of a radius of 1.5 mm and a bulk concentration of c < 20 mg/1 [234]. The use of the drop shape method however may be considerably extended to small surface pressures, usually n < 2 mN/m and hence to low bulk concentrations when taking into account the protein mass... 

Drop-weight method (can be also used for high pressure and temperature) Drop shape method (can be also used for high pressure and temperature)... 

This equation was firstly used by Bashforth and Adams [13] and represents the starting point for most of the drop shape methods developed so far. By definition, p is positive for captive... 

In the last 20 years, large improvements in numerical techniques were obtained by the availability of fast computers. Hence, drop shape methods became more popular due to the direct estimation of the surface tension by profile fitting techniques. Moreover, the utilisation of automatic imaging techniques also provided an easy way to perform measurements in almost real-time conditions. Several set-ups and minimisation procedures have been proposed [21 - 22, 23, 24, 25, 26, 27, 28, 29], in order to increase the measurement accuracy and the sampling rate. Typically, the output of these modem procedures are the best fit values of P and b. 

Today, thanks to the fast development of computer enhanced imaging techniques and numerical fitting procedures, the accuracy and the sampling rate of drop shape methods are substantially increased. Thus, this technique is an important tool for the investigation of adsorption dynamics, and it is particularly suitable for studying processes with characteristic times from a few seconds up to hours and even longer. In fact, there is a large number of experimental studies in which the drop shape technique is used to evaluate the adsorption equilibrium properties, like adsorption isotherms and the dynamic surface tension behaviour. The method is also extensively utilised in the study of surfactants and proteins both in liquid/liquid and liquid/air systems. 

Fig. 12 Dynamic surface tension during the adsorption of C10E5 at water/air interface. From top, the bulk concentrations are 6 10", and lO mol/cm the empty symbols refer to data acquired by the dynamic maximum bubble pressure method, while the filled ones to data acquired by the drop shape method the solid lines are the theoretical prediction by the diffusion controlled adsorption with the two-state isotherm...

The drop shape method is possibly the most useful one for the investigation of the adsorptive transfer, i.e. the adsorption kinetics at the interface between two liquid phases containing the surfactant from the partition equilibrium. This phenomenon is particularly significant when situations far from the partition equilibrium are considered, in systems characterised by a high solubility of the surfactant in the recipient phase or by a large solubility of the surfactant in both phases. The latter case represents a typical situation for many types of ionic surfactants in water-oil and water-alkane systems, as demonstrated by the partition coefficients measured for various solvents [52, 53, 54, 55, 56]. 

An example of such behaviour, studied by the drop shape method described here, is shown in Fig. 1 lb, where the dynamic surface tension during the adsorptive transfer of CioEOg at a fresh water/hexane interface is shown. The diffusion controlled approach can be applied to model the... 

The problem of the adsorptive transfer in finite systems can be successfully studied with the drop shape method by arranging large volume ratios between the droplet and the surrounding liquid. Moreover, in these studies the contact between the two liquids must be avoided until the interface has formed. This can be conveniently arranged by leaving an air bridge between the two liquids into the capillary, that is blown out and eliminated by the buoyancy, when forming the drop. 

Woodward, R.P., 2010. Surface tension measurement using the drop shape method. Firsttenangstroms. com. First Ten Angstroms. Web. http //www.firsttenangstroms.com/pdfdocs/STPaper.pdf. 

Figure 12.10. Dynamic surface tensions obtained during three subsequent square pulse perturbations of a 5 x 10 mol/cm human serum albumin (HSA) solutions, as measured by the drop shape method (PATl, SINTECH-Berlin, Germany)...

Axisymmetric Drop Shape Analysis-Diameter As discussed earlier, a drop shape method, such as ADSA-P, utilizes the meridian profile of a sessile drop to calculate the contact angle and the interfacial tension. Despite its versatility, this technique does suffer from some limitations. In cases of very low contact angles (less than 20°), it may be difficult to measure contact angles accurately by using profile techniques, e.g. ADSA-P. In order to avoid this difficulty, an alternate method called ADSA-(C)D (contact diameter) has been developed by utilizing the drop contact diameter (see Figure 14.6(b)... 

Interfacial tension (7,2) between immiscible liquids can, in principle, be measured by the same methods with which liquid/vapor surface tensions are mea-smed (Adamson, 1990). However, espedally for the measurement of fairly small 7,2 values, a few of these methods are better adapted for this particular purpose, e.g., the pendant drop-shape method (see above), the simpler, but less accurate dropnweight method (Adamson, 1990), and a few other methods using deformed interfaces, sudi as the rotating drop method (Vonnegut,... 

Woodward RP contact angle measurements using the drop shape method. http //firsttenang-stroms.com/pdfdocs/CAPaper.pdf... 

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