Test Method for Surface Tension of Petroleum Products and Water
Surface tension of a liquid is the tension acting along any boundary on the liquid surface, which is caused by the imbalance of molecular attraction in the surface layer of the liquid. Simply put, the liquid surface behaves like a stretched elastic film, tending to contract to the minimum surface area.
It is caused by the fact that the distance between molecules on the liquid surface is larger than that between molecules inside the liquid. This causes the surface molecules to be subjected to an attractive force pointing to the interior of the liquid, thus generating a contractile force. Common examples include water droplets forming a spherical shape and small insects being able to walk on the water surface.
Purpose of the Experiment：
Evaluate Oil Product QualityJudge the concentration of impurities (such as surface-active substances) in the oil through the interfacial tension value, so as to indirectly reflect the oil purity.
Optimize Process ParametersGuide the adjustment of parameters such as settling time and temperature during the oil refining process to improve oil-water separation efficiency.
Prevent Equipment FailuresAbnormal interfacial tension may indicate the risk of colloid and asphaltene deposition, requiring advance cleaning or process adjustment.
Monitor Oil AgingThe interfacial tension changes rapidly during the initial aging stage, which can be used to quickly judge the sludge formation trend.
Ensure Equipment SafetyFor example, the decreased interfacial tension of transformer oil will make it prone to water absorption, which impairs its insulating performance.
Experimental Instruments & Samples
Experimental Samples：Petroleum Products,water
Experimental Instrument：SH107 Automatic interface tension meter，compliant with GB/T6541 standard。
Testing method
I. Sampling and Preparation
Use a dry syringe or pipette to take an appropriate amount of the oil sample to be tested, avoiding secondary contamination or introduction of air bubbles.
Inject deionized water into the test cell with a volume sufficient to cover the operating range of the ring.
Temperature Control: Start the thermostatic device and stabilize the ambient temperature to the set value (e.g., 20°C). Proceed with the operation only after the temperature fluctuation has stabilized.
II. Interfacial Tension Measurement Process
Interface Formation: Slowly inject the oil sample into the test cell to make it contact with deionized water, forming a clear and stable oil-water interface.
Ring Method Operation: Vertically immerse the measuring ring into the oil-water interface, ensuring that the plane of the ring is parallel to the interface. Detect the upward pulling force (caused by tension) exerted on the ring at the interface via the instrument sensor. Record the value after the pulling force stabilizes, and repeat the measurement 2–3 times to improve accuracy.
Data Recording: Immediately record the pulling force value and ambient temperature after each measurement. If the data is abnormal (e.g., excessive fluctuation), clean the ring again and repeat the above steps.
III. Result Calculation and Reporting
Calculate Interfacial Tension: Based on the pulling force value output by the instrument and combined with the geometric parameters of the ring (e.g., circumference), substitute the data into the standard formula to calculate the interfacial tension coefficient.
Example Formula (Simplified): Interfacial Tension = Pulling Force × Correction Factor / Ring Circumference
Experimental Results：
The experimental result is 32.5 mN/m (millinewtons per meter), which reflects the intensity of intermolecular forces at the oil-water interface. This value is consistent with the typical range (30–35 mN/m) for similar oil products reported in standard literature, verifying the reliability of the experiment.