Drop volume technique

Figure D3.5.6 Adsorption kinetics of a small molecule surfactant. Surface tension of polyoxyethylene (10) lauryl ether (Brij) at the air-water interface decreases as time of adsorption increases. Brij concentration is 0.1 g/liter, as measured by the drop volume technique and the maximum bubble pressure method (UNITD3.6).

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. 

DYNAMIC INTERFACIAL TENSION DETERMINATION BY THE DROP VOLUME TECHNIQUE... 

Figure D3.6.3 Schematic diagram of a drop volume technique (DVT) tensiometer.

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). 

Figure D3.6.8 Drop growth at the tip of a capillary and subsequent drop detachment during adsorption kinetics measurements using the drop volume technique (DVT).

The time required to conduct an interfacial tension experiment depends largely on the properties of the surfactants and less on the chosen measurement method. A notable exception is the drop volume technique, which, due to the measurement principle, requires substantial ly more time than the drop shape analysis method. Regardless of the method used, 1 day or more may be required to accurately determine, e.g., the adsorption isotherm (unit D3.s) of a protein. This is because, at low protein concentrations, it can take several hours to reach full equilibrium between proteins in the bulk phase and those at the surface due to structural rearrangement processes. This is especially important for static interfacial tension measurements (see Basic Protocol 1 and Alternate Protocols 1 and 2). If the interfacial tension is measured before the exchange of molecules... 

Fainerman, V.B. and Miller, R. 1995. Hydrodynamic effects in measurements with the drop volume technique at small drop times. 2. Drop time and drop volume bifurcations. Colloids Surf., A 97 255-262. 

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. 

The interfacial tension decay of the three food proteins at the air-water interface at 25°C has been monitored with an apparatus based on the drop volume technique. A full description of the interfacial tension apparatus is given elsewhere (2). The following procedure was used (for details cf. ref. 2)- A drop of a certain volume, corresponding to a certain interfacial tension (y) value, is expelled rapidly, and the time necessary for the interfacial tension to fall to such a value that the drop becomes detached is measured. This procedure is repeated for differing drop sizes, ue. for different values of the interfacial tension. 

Tomberg, E. The application of the drop volume technique to measurements of the adsorption of proteins at interfaces, /. Colloid Interface Sci., 154, 316, 1978. 

The determination of the effective surface age is the key for comparison of results obtained by different experimental techniques. If for example the drop volume technique is used in its "classical" version, which is based on continuously growing drops, dynamic surface tensions are obtained as a function of drop formation time. It was shown in the previous chapter, that the process of adsorption at the surface of a growing drop is overlapped by a radial flow inside the drop, which changes the diffusion profile. In addition, the drop area increases and... 

In the drop volume technique the volume of a drop formed at the tip of a given capillary has to be determined accurately which is typically realised by means of a precise dosing system. In Fig. 4.13 the principle of the drop volume apparatus TVT2 from Lauda is shown as an example. 

When the drop volume technique is applied with fast drop formation, i.e. less than 10 seconds per drop, additional hydrodynamic effects appear which simulate slightly higher interfacial tension values. The procedure to correct for the hydrodynamic effect is quite cumbersome however, as a good approximation the apparent value of y can be multiplied then by the factor... 

Recently, the competitive adsorption dynamics of phospholipid/protein mixed system at the chloroform/water interface was investigated by using the drop volume technique. The three proteins P-Lactoglobulin, P-Casein, and Human Serum Albumin were used in this study. To investigate the influence of the phospholipid structure at concentrations close to the CAC (critic aggregation concentration) the four lipids dipalmitoyl phosphatidyl choline (DPPC), dimyristoyl phosphatidyl choline (DMPC), dimyristoyl phosphatidyl ethanolamine (DMPE)... 

The interfacial tension decay of food proteins adsorbing from a subphase has, in this study, been monitored with an apparatus based on the drop volume technique (1. 2. 3). The following procedure was used (for details cf. ref. 2). 

A relatively simple way to measure interfacial tensions is the drop weight or drop volume technique. The size of a pendant drop (Figure 1.7) is slowly increased imtil the drop can no longer be prevented from falling and breaks off. The total weight or volume of a known number of drops is measured. 

The drop volume technique is limited in its application. Under conditions of fast drop formation and larger tip radii, drop formation shows irregular behaviour. 

The drop volume technique, as is implemented in the TVT2 instrument from LAUDA, also allows... 

The time dependent surface tension decay was measured according to the drop-volume technique as outlined by Tornberg [18,19]. The automatization procedure according to Arnebrant and Nylander [20] was employed. In this method surface tension reduction by macromolecules during adsorption at the air-water interface is measured by formation of drops of certain volumes. Time for detachment of the droplets is recorded. Surface tension calculated [19, 20] was plotted against detachment time and the value attained after 2000 seconds was set as the equilibrium value. The surface tension of the solutions is still decreasing after this period of time, but the rate of decrease is small, less than 0.05 mNm" per 100 seconds. The maximum error in surface tension values is 1.5%. 

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