9. Procedure
9.1 Preparation of Test Specimens:
9.1.1 Obtain a representative test specimen of liquid, at least 25 mL, to be studied in accordance with Test Method D923.
9.1.2 Obtain a fresh aliquot of distilled water (50 to 75 mL) for interfacial tension measurements.
9.1.3 Test specimens should be covered, capped, or sealed immediately after they are obtained. Atmospheric dust and vapor can easily contaminate water, as can the growth of bacteria. Store test specimen in the dark at ambient temperature.
9.2 Determination of Test Specimen Density:
9.2.1 Determine test specimen density at 25 more or less 1°C using a liquid pycnometer or other density measuring device to more or less 0.001 g/cm3.
9.3 Interfacial Tension Measurement:
9.3.1 Introduce distilled water at a temperature of 25 more or less 1°C into a clean sample container to a depth of 10 to 15 mm. Place it on the tensiometer platform so that the ring comes down in the center of the container and is not close to any wall. Make sure no foam is present at the water surface. If foam is visible, discard the water. Foam indicates that the water is contaminated or that the vessel is not clean.
9.3.2 Clean the ring and mount it in the tensiometer. Follow the manufacturer's instructions on zeroing the ring in air.
9.3.3 Raise the platform until the ring is immersed to a depth not to exceed 6 mm into the water.
9.3.4 Slowly lower the platform and follow the manufacturer's instructions on how to adjust the tensiometer reading. Increase torque on the reading dial while maintaining the torsion arm in the zero position. As the film of liquid adhering to the ring approaches the breaking point, proceed slowly with adjustments. This will ensure that the torsion beam will be in the zero position when rupture occurs. Rupture the surface by continuing to increase the force and lowering the vessel. Record the reading when rupture occurs.
9.3.5 Surface tension is calculated as described in 10.2, using the value 0.997 g/cm3 for (D-d), the difference in density between water and air. A value of 71–73 mN/m must be obtained. If low values are found, clean and rinse the sample container thoroughly and obtain a fresh sample of water from a different source of supply. If low surface tension values are still obtained, check calibration of the tensiometer and examine the ring to ensure the plane of the ring is parallel to the water surface.
9.3.6 Carefully layer the oil on the surface of the water (with the ring submerged) until a depth of at least 10 mm is reached. One such procedure is to pipette the oil onto the surface slowly so that minimum mixing occurs and so that oil does not touch the surface of the submerged ring. Make sure that the oil is not so deep as to cover the ring cross bar.
9.3.7 Allow the oil-water interface to age for 30 more or less 1 s after the last of the oil has been layered onto the water.
9.3.8 Lower the platform as in 9.3.4 and record the value at rupture. Time this part of the measurement so that, as nearly as possible, 30 s are required to draw the ring through the interface. Proceed very slowly as the breaking point is approached, since the break is usually sluggish (due to viscosity of the oil). Complete the entire operation, from the time of pouring the oil onto the water until the interface ruptures, in about 60 more or less 10 s.
10. Calculation
10.1 Calculate the interfacial tension of the sample by means of the following equation;
Interfacial tension, mN/m = P x F
where:
P = scale reading when film ruptures, mN/m (Note 1), and
F = factor converting scale reading in mN/m to interfacial tension obtained as described in 10.2.
NOTE 1 - If the scale is not graduated in millinewtons per metre, or if either the ring or the platinum wire are of different diameters than those for which the scale is graduated, correct the scale readings to millinewtons per metre for the particular ring used.
10.2 Using the value of diameter ratio, R/r, specified by the manufacturer for the rings used, prepare a graph of correction factors, F, by means of the following equation; the graph should cover even increments of P/(D - d) from 0 to 800 and should give correction factors to three digits:
where:
P = scale reading, mN/m,
C = circumference of ring, mm,
D = density of water at 25°C, g/mL,
d = density of test specimen at 25°C, g/mL,
R = radius of ring, mm, and
r = radius of wire of ring, mm.