2.2. Test Method and Principle of Interfacial Tension

The common methods for measuring the interfacial tension of liquid–liquid are the ring removal method, drop volume method, rotating drop method, hanging piece method, bubble pressure method, stop drop method, hanging drop method, etc. In this paper, the rotating drop method is used to measure the interfacial tension of liquid–liquid (surfactant solution and oil).

The basic principle of the rotating droplet method is to fill a rotating sample tube C with a liquid B of high density ρ, add a drop A of low density ρ to suspend it in liquid B, and install it in the sample tube of the rotating droplet instrument in a closed manner.³⁴⁻³⁷ At this time, the sample tube is parallel to the rotating axis D. After starting the machine, take the central axis of the sample tube as the rotating axis and carry the liquid to rotate at an angular velocity ω. Under the action of centrifugal force, gravity, and interfacial tension, a long spherical or cylindrical droplet is formed in the high-density liquid, whose shape is determined by the rotation speed and the interfacial tension between the two liquids.²⁷ Assuming that droplet A coincides with the axis of rotation after stretching and neglects the influence of gravity and buoyancy, when the angular velocity ω is large enough, it can be regarded as a cylinder with two semicircle bottoms. Based on the principle that the total energy of droplet A is the smallest when the volume of the droplet is constant, Vonnegut³⁰ derived the approximate calculation formula of interfacial tension.³³

In this paper, the liquid–liquid interfacial tension is measured by a rotary drop interfacial tension tester Tx-500c, as shown in Figure ​Figure22. The Tx-500c meter/interface tensiometer is controlled and measured by a computer control system.

Rotating drop interfacial tension tester.

Prepare 20 mL of white oil and then prepare 10, 15, 20, 25, 30, 35, and 40% of rinse solution with water as the reference volume;

The interfacial tension means measuring the interfacial tension between the liquid phase and the incompatible liquid phase; in this experiment, the surfactant solution of different concentrations and white oil are used as the measurement objects;

Use the small volumetric flask of the interfacial tension tester and fill a certain surfactant solution in it, then place the volumetric flask horizontally, inject 1–2 drops of oil into the small volumetric flask through a syringe, place the volumetric flask in the interfacial tension tester, at room temperature of 25 °C, rotate for 20 min at a rotating speed of 5000 rpm, adjust the angle of the volumetric flask until the oil drops appear in the display, take photos to measure the surface tension of the oil drops, and measure the oil drops in the forward and reverse directions four times, and take the average value;

According to the above steps, measure the interfacial tension between surfactant solution of different concentrations and white oil, for a total of seven groups.