Dynamic surface tension measurement methods

Table 2.2 Dynamic Surface Tension Measurement Methods for Liquids ...

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. 

The Marangoni elasticity can be determined experimentally from dynamic surface tension measurements that involve known surface area changes. One such technique is the maximum bubble-pressure method (MBPM), which has been used to determine elasticities in this manner (24, 26). In the MBPM, the rates of bubble formation at submerged capillaries are varied. This amounts to changing A/A because approximately equal bubble areas are produced at the maximum bubble pressure condition at all rates. Although such measurements include some contribution from surface dilational viscosity (23, 27), the result will be referred to simply as surface elasticity in this work. 

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. 

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. 

When the diffusion time has comparable magnitude with the time of formation of the electric double layer, the quasiequilibrium model is not applicable. Lucassen et al. [Ill] and Joos et al. [112] established that mixtures of anionic and cationic surfactants diffuse as a electroneutral combination in the case of small periodic fluctuations of the surface area consequently, this process is ruled by the simple diffusion equation. The e/ec ro-diffusion problem was solved by Bonfillon et al. [113] for a similar case of small periodic surface corrugations related to the capillary-wave methods of dynamic surface-tension measurement. 

Surface tension methods measure either static or dynamic surface tension. Static methods measure surface tension at equilibrium, if sufficient time is allowed for the measurement, and characterize the system. Dynamic surface tension methods provide information on adsorption kinetics of surfactants at the air-liquid interface or at a liquid-liquid interface. Dynamic surface tension can be measured in a timescale ranging from a few milliseconds to several minutes [315]. However, a demarkation line between static and dynamic methods is not very sharp because surfactant adsorption kinetics can also affect the results obtained by static methods. It has been argued [316] that in many industrial processes, sufficient time is not available for the surfactant molecules to attain equilibrium. In such situations, dynamic surface tension, dependent on the rate of interface formation, is more meaningful than the equilibrium surface tension. For example, peaked alcohol ethoxylates, because they are more water soluble, do not lower surface tension under static conditions as much as the conventional alcohol ethoxylates. Under dynamic conditions, however, peaked ethoxylates are equally or more effective than conventional ethoxylates in lowering surface tension [317]. 

Dynamic surface tension measurements by Hirt et al. [316], based on the maximum-bubble-pressure method, revealed large differences between equilibrium and dynamic surface tension values of fluorinated surfactants (see Section 4.4). The surface tension transition from equilibrium values to dynamic diffusion-limited values depended on the surfactant type, concentration, and bubble generation rate. 

Dynamic surface tension has also been measured by quasielastic light scattering (QELS) from interfacial capillary waves [30]. It was shown that QELS gives the same result for the surface tension as the traditional Wilhelmy plate method down to the molecular area of 70 A. QELS has recently utilized in the study of adsorption dynamics of phospholipids on water-1,2-DCE, water-nitrobenzene and water-tetrachloromethane interfaces [31]. This technique is still in its infancy in liquid-liquid systems and its true power is to be shown in the near future. 

The Wilhelmy plate method provides an extremely simple approach that, unlike the ring detachment method, permits the measurement of continuously varying or dynamic surface tensions. If a thin plate (e.g., a microscope slide, a strip of platinum foil, or even a slip of filter paper) is attached to a microbalance and suspended so that its lower edge is just immersed in a liquid, the measured apparent weight Wj, is related to the actual weight of the plate Wp and the surface tension y by the following simple equation ... 

Two dynamic methods have been developed for measurement of the surface tensions, the method of ripples first employed by Rayleigh and the vibrating jet method developed by Bohr. 

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. 

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. 

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. ... 

The most suitable technique for studying adsorption kinetics and dynamic surface tension is the maximum bubble pressure method, which allows measurements to be obtained in the millisecond range, particularly if correction for the so-called dead time, t. The dead time is simply the time required to detach the bubble after it has reached its hemispherical shape. A schematic representation of the principle of maximum bubble pressure is shown in Figure 18.14, which describes the evolution of a bubble at the tip of a capillary. The figure also shows the variation of pressure p in the bubble with time. 

Equation 5.93 reflects the fact that in the diffusion regime the surface is always assumed to be equilibrated with the subsurface. In particular, if E, = 0, then we must have Cj = 0. In contrast, Equation 5.94 stems from the presence of barrier for time intervals shorter than the characteristic time of transfer, the removal of the surfactant from the interface (Tj = 0) cannot affect the subsurface layer (because of the barrier) and then Cij(O) = c. This purely theoretical consideration implies that the effect of barrier could show up at the short times of adsorption, whereas at the long times the adsorption will occur under diffusion control." The existence of barrier-affected adsorption regime at the short adsorption times could be confirmed or rejected by means of the fastest methods for measurement of dynamic surface tension. 

The maximum bubble pressure technique is a classical method in interfacial science. Due to the fast development of new technique and the great interest in experiments at very small adsorption times in recent years, commercial set-ups were built to make the method available for a large number of researchers. Rehbinder (1924, 1927) was apparently the first who applied the maximum bubble pressure method for measurement of dynamic surface tension of surfactant solutions. Further developments of this method were described by several authors (Sugden 1924, Adam Shute 1935, 1938, Kuffiier 1961, Austin et al. 1967, Bendure 1971,... 

One of the oldest experimental methods for the measurement of dynamic surface tensions of surfactant solutions is the oscillating jet (OJ) method. The idea is based on the analysis of a stationary jet issuing from a capillary pipe into the atmosphere which oscillates about its... 

Fig. 5.28 Dynamic surface tension of two TRITON X-100 solutions measured using the maximum bubble pressure (n- ) and inclined plate ( ) methods Cq= 0.2 ( ) 0.5 ( ) g/1 according to Fainerman et al. (1994a)...

---