GB/T 11985-2026Surface active agents - Determination of interfacial tension - Drop volume method (English PDF)

5.1 Measuring Instruments

5.1.1 The apparatus for measuring interfacial tension using the drop volume method is shown in Figure 1.

5.1.2 A glass syringe, precisely calibrated, with a volume of

0.5 cm^3 or

1.0 cm^3, driven by a micrometer screw, its scale allowing for accurate measurement of droplet volume. Measure to ±0.0001 cm3.

5.1.3 Capillary tube, made of glass or stainless steel, with its end connected to the syringe, which can be disassembled for cleaning. The method for preparing capillaries is as follows:

a) Preparation of glass capillary tubes. Cut a 4cm length from a high-quality, small-diameter glass capillary tube of suitable and uniform diameter. One end of the segment is machined into a frosted nozzle that can connect to a glass syringe, while the other end is precisely ground on a glass plate coated with fine diamond powder. Grinding. To prevent the capillary tube from wobbling during polishing, it is embedded in a glass tube with an inner diameter of approximately

1.5 cm, containing paraffin (softening point approximately...). At 70°C. The capillary should be horizontal and perpendicular to its axis. Its circumference should be free of gaps.

b) Preparation of stainless steel capillary tube. Cut a section of approximately 4cm from a high-quality, small-diameter stainless steel capillary tube of suitable and uniform diameter. The long tube is cut at one end with a precision lathe to make it precisely right-angled with the tube shaft, and a polytetrafluoroethylene connector is fitted at the other end. Based on the interfacial tension value and density difference, the outer radius of the capillary should be selected between 0.1cm and 0.4cm.

5.1.4 Jacketed glass container, inner diameter 2.5cm~4cm, height greater than 5cm, temperature adjustable and capable of accommodating syringes and capillaries.

5.2 Super Thermostat The water bath temperature can be controlled accurately to ±0.5°C.

5.3 Instrument Installation The glass syringe (5.1.2) and capillary (5.1.3) shall be mounted on a support that allows them to move vertically, and the tip of the capillary shall be perpendicular to the ground. Its axis and diameter are uniform, and its edges are free of defects. The bracket is fixed to the metal plate of the vibration isolation platform. Since rotating the micrometer screw by hand will cause vibration, a micro motor can be used to drive the micrometer screw to reduce the vibration of the test device. The circulation of water causes small vibrations, so the thermostat should be turned off when the droplet is close to its maximum volume during measurement.

6.1 Preparation of Surfactant Solutions

6.1.1 Extra care must be taken when preparing the surfactant solution for testing. The water used in its preparation should meet the requirements specified in QB/T 1223. The requirement can be verified by measuring its surface tension. Cork should not be used in the structure of instruments for preparing water or in sealed containers for storing water, and even less so... Use a rubber stopper.

6.1.2 The temperature deviation of the solution should be kept within 0.5°C.

Note. Measurements taken near the critical dissolution temperature (Krafft temperature) or at the cloud point temperature of nonionic surfactants involved in the addition of ethylene oxide are highly susceptible to errors. Poor. It is recommended that the test be conducted above the critical dissolution temperature or below the cloud point temperature of the nonionic surfactant.

6.1.3 Since the mutual saturation of the two liquid phases and the adsorption of surfactants at the interface both require time, the interfacial tension will change with time. Furthermore, the time required varies across different systems, making it difficult to recommend a consistent standard time. Therefore, multiple measurements should be performed over a period of time to obtain a boundary value. The surface tension as a function of time is used to determine the time it takes for the interface to reach a static state, based on the flattening point of the curve.

6.1.4 Dust in the atmosphere or vapors emitted from solvents used nearby can easily contaminate the liquid surface; therefore, the indoor environment for the measurement should not be contaminated. Store or handle items containing volatile solvents. The entire instrument should be protected with a balance cover to minimize temperature fluctuations.

6.1.5 Because the surface of the solution is easily contaminated by insoluble particles and dust, it is advisable to use a pipette to draw a small amount of the test phase from the center of a large quantity of the liquid phase to be tested. Verify identity.

6.2 Cleaning of measuring instruments (5.1) For impurities that cannot be removed with a mixture of sulfuric acid and chromic acid, phosphoric acid, or potassium persulfate and sulfuric acid solutions, such as silicone, a specialized solvent (such as formaldehyde) can be used. Clean syringes, capillary tubes, and glass-jacketed containers with a solution of benzene, perchloroethylene, or potassium hydroxide in methanol or ethanol. If no such impurities are present, or if using... After solvent rinsing, the measuring instrument must be carefully washed with hot potassium persulfate or ferric ammonium persulfate sulfuric acid solution, and finally rinsed with freshly prepared pure water. (6.1.1) Rinse until neutral. Finally, rinse the syringe and capillary with water to reduce contamination. Before measurement, the measuring instrument should be completely dry.

6.3 Inspection of measuring instruments

6.3.1 Measurement of capillary radius Measure the capillary radius using a reading microscope (minimum division

0.01 mm) or other instruments with higher precision, and take 5-10 flat measurements. Mean.

Note. The capillary radius can also be measured using a liquid with known density and interfacial tension, according to Appendix A, but this is only applicable to microscopes without readings and micrometers.

6.3.2 Measurement of droplet volume To accurately obtain the droplet volume, the reading obtained from the movement of the micrometer screw is checked by weighing a few drops.

6.4 Measurement Connect the glass syringe (5.1.2) to the appropriate capillary tube (5.1.3) selected according to 6.5, fill the syringe with the aqueous phase, and then inject the organic phase. Inside the jacketed container. If the density of the aqueous phase is greater than that of the organic phase, immerse the capillary in the organic phase to a depth of more than

0.5 cm. To ensure the capillary is completely wetted, cover its tip with a drop of aqueous phase before immersing it in the organic phase. If the density of the aqueous phase is lower than that of the organic phase, the capillary tip should face upwards, and its orifice should be about 2 cm below the liquid surface to allow for the insertion of lighter droplets. form. Before measuring the static interfacial tension (i.e., time-independent interfacial tension) of the surfactant aqueous solution, the two surfactants were first subjected to a constant temperature water bath. The liquid reaches the desired temperature, and then, over 30 to 60 seconds, slowly forms a droplet close to its maximum volume at the capillary tip. The droplet volume increases slowly. When increasing the volume, continue for 2 to 5 minutes. Finally, the droplet should increase in size slightly and then fall off. If the droplet falls off without increasing in volume, a smaller initial volume should be selected; if the increase exceeds

0.001 cm3, the initial volume is too small.

Note. The droplet volume can be measured until it remains constant by increasing the droplet settling time (2 min, 5 min, 8 min, etc.). Automated instruments can more conveniently measure static volume. Interfacial tension. The droplet volume was continuously increased at different rates until the volume of the detached droplet became constant (see Figure 2). The readings from the micrometer screw scale were recorded. A fixed volume. Based on the interfacial tension value, a fully filled syringe can be used for several measurements. The volume of the first drop is negligible. At least four drops should be collected for each measurement. The average volume is used to calculate the interfacial tension.

7.1 Calculation Method

7.2 Precision The accuracy of interfacial tension measurement depends on the errors of V, Deltarho, r, f, and, and can be better than 1 mN/m. With further precision, the error can be reduced to. Approximately 1%. Limiting vibration, controlling temperature, and ensuring liquid saturation can effectively improve measurement precision. Note

1.For surfactant solutions with extremely low concentrations (e.g., below 10^-4 mol/L), the accuracy of this method will be affected if the droplet formation rate is not further reduced. Lower. Note

2.For the determination of interfacial tension between two obviously miscible liquids, both liquids must be fully saturated and equilibrated before the measurement is performed.