1. Experiment is contained in the WACKER's Experimental Kit.

No

 2. Experimental procedure has been modified

/

 3. A separate experimental procedure has been devised

Yes

 4. Video clip available

No

 5. Flash animation available

No

 6. Other materials: Worksheet 9

Viscosity of Silicone Fluids/Variant B

TopDown 1 Materials, Chemicals, Time Needed
  • Retort
  • Retort clamps and sleeves
  • Magnetic hotplate and stirrer bar
  • Magnetic stirrer bar
  • Glass beaker, as large and tall as possible
  • Glass tube (50 cm x 5 mm diameter)
  • Rubber stopper
  • Steel ball (4 mm diameter)
  • Thermometer (100 °C)
  • Stop-watch
  • Magnet
  • Small funnel or a pipet for filling the tube
  • Fine permanent felt-tip marker
  • Angle-measuring scale (triangle)

A single series of experiments can be conducted in 15 minutes at room temperature.
Other series of measurements take more time because the tube has to be rinsed, dried and then equilibrated again each time. Fifteen minutes should be long enough to prepare for the next series.
If a measurement series is to be recorded at different temperatures, the time required depends chiefly on the time needed to heat up and cool down the water bath.

TopDown 2 Procedure and Observations

Set up the apparatus as shown in the diagram. Clamp the tube at a slight angle, but ensure that neither it nor the stopper make contact with the glass beaker. The precise angle is not critical. However, whatever angle is chosen, ensure that all measurements are made at that angle (the angle for precision measurements laid down in standards is 10°).

Make two marks on the glass tube, one about 3 cm beneath the water surface and the other 1.5 cm above the stopper. Fill the tube with the test liquid, ensuring there are no bubbles, clamp the tube in position and add the ball. The entire apparatus must be equilibrated in the measurement range for at least 5 minutes before the measurements are started. For the measurement, use the magent, which has also been equilibrated, to draw the steel ball up to about 1 cm below the water surface.

Then remove the magnet and measure the time for the ball to fall between the two markings. Viscosity measurements make it possible to distinguish between the different samples.

An interesting experiment is to measure the viscosity change that occurs on aging (use samples from the experiment "Rate of hydrolysis of chloromethylsilanes"). That provides evidence of progressive condensation. It is also interesting to compare the viscosity changes that occur in silicones and an organic oil as the temperature changes. The apparatus shown is ideal for distinguishing between the silicone fluids on the basis of their viscosity as measured in terms of the “fall time.” It turns out that silicone fluids with high molar masses have a greater viscosity and that the ball needs more time to cover the distance than it does in silicone fluids of low molar mass. When the hydrolysis products obtained in the experiment "Rate of hydrolysis of chloromethylsilanes" are used, the hexamethyldisiloxane is found to have a very low viscosity, whereas the dichlorodimethylsilane hydrolysis products generally have a higher and, more importantly, increasing viscosity as they age.

The viscosity of all samples decreases with rising temperature. However, the viscosity of olive oil is much more dependent on the temperature than that of silicone fluids. In the case of olive oil, the fall time is reduced by 80 % as the temperature is increased to 65 °C compared with a reduction of only 50 % for silicone fluids over the same temperature interval (see below).

TopDown 3 Discussion of Results

The experimental fall times for the ball correlate with the viscosity values of the fluids studied (the longer the fall time, the higher is the viscosity of the fluid). The higher the molar mass of the silicone fluids (i.e. the larger or longer the silicone molecules), the greater is their viscosity. This is shown, for example, by the hydrolysis product of dichlorodimethylsilane, whose viscosity increases with increasing age, i.e. with the degree of condensation.
The comparatively low temperature dependence of the viscosity of a silicone fluid, relative to that of olive oil, is clearly revealed by this experiment. Whereas the fall time for the ball is shortened by 80 % in olive oil, the corresponding figure for silicone fluid over the same temperature interval is just 50 % (see diagram).

TopDown 4 Tips and Comments

See Tips and comments under Variant A.

TopDown  5 Supplementary Information

See Supplementary information under Variant A.
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