Drop Volume Method

The drop-volume method is used to measure surface and interfacial tensions of liquids by a detachment method. The liquid to be analyzed is pumped through the tip of a vertical capillary tube whose tip protrudes into air or an immiscible liquid (Figure 5.16). A droplet detaches itself from the tip of the capillary tube when the separation force (due to gravity) is balanced by the adhesion force (due to surface or interfacial tension). To measure surface tension, the tip should point downward into air, whereas to measure interfacial tension, the tip may point either upward or downward depending on the relative densities of the two liquids being analyzed. If the liquid in the tip has a higher density than the surrounding medium, the opening of the tip faces down and the drop moves downward once it becomes detached. On the other hand, if the liquid in the tip has a lower density, then the tip faces up and the drop moves upward when it becomes detached.

The surface or interfacial tension can be related to the volume of the detached droplet by analyzing the forces which act on it just prior to detachment (Couper 1993):

^DROP Apg

nd where d is the diameter of the tip, Ap is the density difference between the liquid being analyzed and the surrounding liquid, FDROP is the volume of the droplet, and g is the gravitational constant. The volume of the droplets can be determined using a graduated syringe or by weighing them using a sensitive balance (once the liquid density is known).

As with other detachment methods, the surface area of the droplet expands rapidly during the detachment process, and therefore the method is unsuitable for analyzing liquids that contain emulsifiers whose adsorption time is comparable to the detachment time. To obtain reliable results, it is normal practice to bring the droplet slowly to the detachment process. The accuracy of this technique has been reported to be about 0.1% (Couper 1993).

FIGURE 5.16 Drop-volume method of determining the interfacial and surface tension of liquids.

Spinning Droplet

Aqueo Phasi

Oil Drop!

Spinning Droplet

Aqueo Phasi

Oil Drop!

Spinning Capillary Tube

FIGURE 5.17 Spinning-drop method of determining the interfacial tension of liquids.

Spinning Capillary Tube

FIGURE 5.17 Spinning-drop method of determining the interfacial tension of liquids.

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