Drop methods dynamic surface tension

Joos, P. and Rillaerts, E. 1981. Theory on the determination of dynamic surface tension with the drop volume and maximum bubble pressure method. J. Colloid Interface Sci. 79 96-100. 

Kloubek, J., Friml, K., and Krejci, F. 1976. Determination of dynamic surface tension with various stalagmometers by drop weighing method. Collect. Czech. Chem. Commun. 41 1845-1852. 

A number of methods are available for the measurement of surface and interfacial tension of liquid systems. Surface tension of liquids is determined by static and dynamic surface tension methods. Static surface tension characterises the surface tension of the liquid in equilibrium and the commonly used measurement methods are Du Notiy ring, Wilhelmy plate, spinning drop and pendant drop. Dynamic surface tension determines the surface tension as a function of time and the bubble pressure method is the most common method used for its determination. 

Various experimental methods for dynamic surface tension measurements are available. Their operational timescales cover different time intervals. - Methods with a shorter characteristic operational time are the oscillating jet method, the oscillating bubble method, the fast-formed drop technique,the surface wave techniques, and the maximum bubble pressure method. Methods of longer characteristic operational time are the inclined plate method, the drop-weight/volume techniques, the funnel and overflowing cylinder methods, and the axisym-metric drop shape analysis (ADSA) " see References 54, 55, and 85 for a more detailed review. 

Fig. 5.30 Dynamic surface tension of a 0.025 mol/l pt-BPh-EOlO solution measured using the maximum bubble pressure ( ) and drop volume ( ) methods original data ( - ), corrected data ( ) according to Miller et al. (1994d)...

Dynamic surface tensions of an aqueous l.5510" mol/cm Triton X-100 solution measured with the dynamic capillary (0), inclined plate (A,A), drop volume ( ), strip ( ) and Wilhelmy plate ( ) methods according to Rillaerts Joos (I9S2)... 

It has been already indicated (Fig. 7) that micelles can lead to an essential acceleration of the adsorption process. Therefore, special experimental techniques are necessary for its investigation, allowing measurements of the dynamic surface tension in a time interval of milliseconds. The maximum bubble pressure method [78, 81, 83, 89,90,93] and the oscillating jet method [77, 82, 86, 87, 88, 90, 92, 93, 156] are most frequently used for these purposes. The inclined plate method [83, 89, 90, 93], the method of constant surface dilation [85] and the drop volume method [84] have been used also for slow adsorbing surfactants. 

Application of tensiometry and surface rheology in medical research was shown by Kazakov et al. (2000). For example, selected dynamic surface tension values of serum or urine correlate with the health state of patients suffering from various diseases. In the course of a medical treatment these values then change from a pathological level back to the normal values determined as standard for a certain group of people (age and sex). Fig. 36 shows the surface tension response after a step-type area change of a pendant drop area by about 10% for 6 serum samples from the same patient at different stages of his acute kidney insufficiency. It appears to be efficient to use such easy to handle methods for therapy control in the medical practice. 

After having gained some experimental experience, Heyrovsky simplified KuCera s method by measuring the drop-time of several drops under constant mercury pressure instead of collecting and weighing each time 80 drops of 2-second duration. However, even after 3 years of tedious work, he could not reconcile the results of the dynamic surface tension measurements with those of the static method. 

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 dynamic surface tension of [3-casein solutions at three concentrations 5 10, 10 and 10 mol/1 are shown in Fig. 14. As one can see the results from the two methods differ significantly. For the bubble the surface tension decrease starts much earlier. The surface tensions at long times, and hence the equilibrium surface tension from the bubble experiment are lower than those from the drop. However, the establishment of a quasi-equilibrium for the drop method is more rapid at low (3-casein concentrations while at higher P-casein concentrations this process is more rapid for the bubble method. This essential difference between solutions of proteins and surfactants was discussed in detail elsewhere [50]. In brief, it is caused by simultaneous effects of differences in the concentration loss, and the adsorption rate, which both lead to a strong difference in the conformational changes of the adsorbed protein molecules. 

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... 

Figure 12.7. Dynamic surface tension data of two diethylde-cylphosphine oxide solutions, as measured by the drop volume method (TVT2, LAUDA, Germany) in the dynamic mode , Co = 10 mol/cm O, Co = 10 mol/cm ...

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)...

Another experimental example of the use of the drop and bubble shape technique is given in Figure 12.11. The latter shows the dynamic surface tensions of blood serum as measured by the maximum bubble pressure (filled symbols) and drop shape techniques (open symbols). It becomes evident that both methods complement each other perfectly when the data from the MPT2 studies are plotted as a function of /efi- The results impressively demonstrate that during the radiotherapy process the dynamic surface tension parameters return to the normal values (2), i.e. approach those characteristic of healthy females from the respective control group. 

Figure 12.11. Dynamic surface tensions of blood serum samples as measured by the drop shape technique (PATl, SIN-TECH-Berlin, Germany, (0, )) and maximum bubble pressure method (MPT2, LAUDA, Germany, ( , )), samples taken from a 43 years old woman suffering from cervical carcinoma , , before radiotherapy o, at the end of treatment...

In many experimental techniques used for dynamic surface-tension measurements (such as the MBP method and the drop-volume method [14,76,82]), the surface expands gradually with time. In such a case, the convective terms in Eqs. (24) and (25) carmot be neglected. Nevertheless, it can be demonstrated that with the help of the new independent variables. 

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

As we shall have occasion to note in dealing with solutions, the composition of the surface phase is very different from that of the bulk liquid. When a liquid interface is newly formed the system is unstable until the surface phase has acquired its correct excess or deficit of solute by diffusion from or into the bulk of the solution. This process of diffusion is by no means instantaneous and, as has been observed in discussing the drop weight method, several minutes may elapse before equilibrium is established. In the ripple method the surfece is not renewed instantaneously but may be regarded as undergoing a series of expansions and contractions, thus we should anticipate that the value of the surface tension of a solution determined by this method would lie between those determined by the static and an ideal dynamic method respectively. 

Equation (46), one form of the Gibbs equation, is an important result because it supplies the connection between the surface excess of solute and the surface tension of an interface. For systems in which y can be determined, this measurement provides a method for evaluating the surface excess. It might be noted that the finite time required to establish equilibrium adsorption is why dynamic methods (e.g., drop detachment) are not favored for the determination of 7 for solutions. At solid interfaces, 7 is not directly measurable however, if the amount of adsorbed material can be determined, this may be related to the reduction of surface free energy through Equation (46). To understand and apply this equation, therefore, it is imperative that the significance of r2 be appreciated. 

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. 

Modem drop volume tensiometers are connected to a computer with sophisticated software that can be used to automatically record the surface tension as a function of the true interfacial age. Adsorption kinetics experiments with the drop volume technique can be conducted using either the constant drop formation method or the quasistatic method (for details, see Commentary). The choice of the dynamic measurement method depends primarily on the time range over which the adsorption kinetics needs to be measured. 

The theoretical foundation of the drop volume technique (DVT) was developed by Lohnstein (1908, 1913). Originally, this method was only intended to determine static interfacial tension values. Over the past 20 years, the technique has received increasing attention because of its extended ability to determine dynamic interfacial tension. DVT is suitable for both liquid/liquid and liquid/gas systems. Adsorption kinetics of surface-active substances at liquid/liquid or liquid/gas interfaces can be determined between 0.1 sec and several hours (see Fig. D3.6.5). 

Provides measuring techniques of contact angle, surface tension, interfacial tension, and bubble pressure. Suitable methods for both static and dynamic inteifacial tension of liquids include du Nous ring, Wilhelmy plate, spinning drop, pendant drop, bubble pressure, and drop volume techniques. Methods for solids include sessile drop, dynamic Wilhelmy, single fiber, and powder contact angle techniques. 

Surface Tension Measurement. The surface tension of the surfactant solution was determined by means of the Dynamic Contact Angle Tester FIBRO DAT 1100 (FIBRO Systems, Sweden) using the pendant drop method. It was also an output of the ADSA captive bubble contact angle measurements with surfactant solutions. 

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

The dynamic methods depend on the fact that certain vibrations of a liquid cause periodic extensions and contractions of its surface, which are resisted or assisted by the surface tension. Surface tension therefore forms an important part, or the whole, of the restoring force which is concerned in these vibrations, and may be calculated from observations of their periodicity. Dynamic methods include determination of the wave-length of ripples, of the oscillations of jets issuing from non-circular orifices, and of the oscillations of hanging drops. Dynamic methods may measure a different quantity from the static methods, in the case of solutions, as the surface is constantly being renewed in some of these methods, and may not be old enough for adsorption to have reached equilibrium. In the formation of ripples there is so little interchange of material between the surface and interior, and so little renewal of the surface, that the surface tension measured is the static tension ( 12. ... 

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