Tensiometer, pendant drop

Recently, surfactant adsorption and y have been measured at C02-water and C02-organic interfaces with a tandem variable-volume tensiometer (Harrison, 1996). A pendant drop of an aqueous or organic phase, saturated with C02, may be suspended in C02 or a C02-surfactant mixture and equilibrated. From the digitized droplet shape and density difference between the phases, y may be calculated from the Laplace equation. In Figure 8.1, y of the binary C02-water (da Rocha et al., 1999), -polyethylene... 

Interfacial Tensions. Initially interfacial tensions were measured by the pendant drop technique. Later studies with an EOR spinning drop interfacial tensiometer gave much information on middle phase films, as shown in the photographs made with a camera mounted on our tensiometer. 

The adsorption of a surfactant at an interface between CO2 and a second fluid, such as water, may be determined directly from measurement of the interfacial tension (change in Gibbs free energy with surface area), y, versus surfactant concentration. A novel tandem variable-volume pendant drop tensiometer has been developed to measure equilibrium and dynamic values of y as a function ofT.p and time (Figure 2.4-1) [21]. An organic [21] or aqueous phase [18] is preequilibrated with CO2 in the first variable-volume cell (drop-phase cell). A droplet of this liquid is injected into the second variable-volume cell, with two windows at 180° mounted on a diameter, containing either pure CO2 or CO2 and surfactant. 

Figure 2.4-1 Tandem variable-volume view cell tensiometer for measuring the interfacial tension by the pendant drop technique TC = temperature controller PG = pressure gauge.

Alternatively, interfacial tension can also be measured continuously with area changes, with an expanding or contracting pendant-drop instrument adapted to oscillatory measurements as described by Bhardwaj and Hartland (74). Dilational data are obtained without introducing shear as the interfaee is expanded and contracted. This is also often achieved with an expanding drop volume or bubble-pressure tensiometer (65). Nikolov et al. (170) have recently developed this technique and an instrument to study oiLwater systems. 

As preliminary tests to show surfactant properties of asphaltenes were performed interfacial tension measures in a Kriiss Tensiometer DSAIOO using the pendant drop method. The analysis time was 90min and a needle with a diameter of 1.463 mm and a 500pL syringe used. 

The surface tension of the surfactant solutions was measured by the Wil-helmy plate method, whereas the surface tension of the oils was measured by the Du Nouy ring technique on a Krtiss KIOT digital tensiometer. The interfacial tension of the oil-solution interfaces was measured by the pendant drop method. 

Important techniques to measure the surface tension of liquids are the sessile drop method, the pendant or sessile bubble method, the Du-Notiy ring tensiometer, and the Wilhelmy-plate method. 

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