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

Yes

 2. Experimental procedure has been modified

Yes, additions

 3. A separate experimental procedure has been devised

Partly

 4. Video clip available

Yes (as wmv or as mov)

 5. Flash animation available

Yes

 6. Other materials: Worksheet 2, Worksheet 9, Slide DH5

Hydrophobic Properties of Silicone Fluids/Variant A

TopDown 1 Materials, Chemicals, Time Needed
  • Dropping pipet
  • Various clean surfaces:
    - Glass
    - Copper panel
    - Beer mat
    - Paper tissue
    - Polyethylene panel
    - Polyvinyl chloride panel
    - Wooden panel
  • Candle
  • Glycerol
  • Silicone Fluid AK 5000 from WACKER's Experimental Kit
  • Methylene blue

The experiment will take around 10 minutes for each surface.

TopDown 2 Procedure and Observations
Uniformly coat part of the various surfaces with the respective oils. Then apply a layer of candle wax to a piece of card. Apply a drop of water to both the treated and the untreated surfaces.

As may be seen from the photos, the water drops on the siliconized surfaces and the card coated with candle wax form fairly spherical droplets and do not spread out. Viewed from the side, the drops on the treated surfaces are much higher than those on the untreated surfaces.

Untreated
glass surface
Siliconized
glass surface
Glass surface treated
with glycerol
Card coated with
candle wax

TopDown The water on the siliconized surfaces is absorbed much more slowly than that on the untreated surfaces. It is not possible to calculate the contact angle with the naked eye.

The water drops on the surfaces treated with glycerol spread out extensively and are quickly absorbed. Thus, the water-repellency of a copper panel that has been thoroughly cleaned with ethyl alcohol is substantially reduced by glycerol. In contrast, the silicone fluid actually increases the water repellency.

Untreated
copper panel
Copper panel treated with glycerol
Copper panel treated
with silicone fluid

TopDown 3 Discussion of Results

As the results show, the surfaces treated with silicone fluid have much better water repellency than the untreated surfaces. This is reflected in the marked increase in the contact angle. The silicone molecules spontaneously arrange themselves on surfaces that have polar centers, e.g. glass, building materials and textiles, in such a way that the methyl groups protrude from the surface. They are responsible for the hydrophobic (water-repellent) effect of the treated surfaces.
TopDown The structural similarity of the silicone fluids and the surfaces of the glass, building materials and textiles are the reason why the fluids adhere well to the surfaces. In the case of glass, electrostatic forces of attraction cause bonds to form between the negatively charged oxygen atoms of the silicone molecules and cations within the glass. With paper, the polar oxygen atoms form hydrogen bonds with the –OH groups in the cellulose (see both diagrams).
Interactions between cellulose and silicone fluid

TopDown The card coated with candle wax is water repellent because the candle wax is actually solid paraffin wax, i.e. it is a mixture of saturated aliphatic hydrocarbons. These are nonpolar molecules that, like the methyl groups in the silicone fluid, have hydrophobic properties. They interact very weakly with the water molecules and so the water stays in the drop and does not wet the paraffin wax.
Paraffin wax is considered to be the most hydrophobic water-repellent agent. However, paraffin waxes do not adhere as well as silicone fluids to surfaces made of stone, glass or paper, and can be scratched off.
Another major advantage over paraffin waxes is that silicone films have a very low surface tension, a fact which makes them highly spreadable and which enables hydrophobic effects to be achieved with much smaller quantities. Unlike paraffin waxes, silicone fluids have hardly effect on surface properties, such as their appearance and “ability to breathe.”

The increase observed in the hydrophilic properties of surfaces treated with glycerol may be explained in terms of the structure of glycerol. The glycerol molecule C3H5(OH)3 has three hydroxyl groups that form hydrogen bonds with water molecules. Glycerol can therefore be mixed in any proportion with water.


TopDown Extension of experiment

With the aid of a camera, a computer and image-processing software, it is possible to calculate the contact angle, as shown on the right.
The following results were obtained in a reference experiment:
Type of surface Contact angle for water droplet Literature value
Siliconized glass surface
Approx. 103° 100-110°
Copper panel Approx. 74 °  
Polyethylene Approx. 95° Approx. 95°
Paraffin waxes Approx. 103° Approx. 105°
Glass (degreased) Approx. 35°
Aside from the results for glass, the results agree well with the literature values. The discrepancy in the case of glass might be dirt on the glass surface.

TopDown 4 Tips and Comments

  • Use a camera to calculate the contact angle accurately. The contact angle can be calculated fairly accurately with the aid of a computer.
  • Color the water droplet with methylene blue dye to accentuate the contours of the water droplet and to make observations easier. That will greatly help determining the contact angle with the aid of the camera.
    The silicone fluid can be baked into the glass and then rubbed off with a paper towel so that the coating is almost invisible. Nevertheless, the water-repellent effect can still be observed.
    Baking can be done by thoroughly cleaning the silicone-treated glass and dry heating it at over 100 °C for 15 to 20 minutes.
  • A qualitative version of the experiment (without a camera) is easy and quick to perform. The experiment is thus ideal for demonstrating the water-repellent properties of silicones, and especially of silicone fluids. Qualititative determination of the contact angle with a camera is fairly time-consuming and offers little in the way of new insights, as the result can be interpreted qualitatively.
  • Comparing the molecular structures of silicone fluid, glycerol and paraffin wax is a very good way of explaining the different properties. This affords a good way of incorporating the important relationship between a material’s particle structure and its properties into the lesson.
  • In secondary school chemistry classes, the experiment could be used, for example, to cover the terms hydrophobic and hydrophilic. It could also be used as part of a project.
    Advanced chemistry students could draw on their knowledge of particle structure and properties (water repellency of alkanes) to interpret the results and deepen their understanding. In the construction industry, the water-repellent property of silicone fluid is a key element of monument conservation ("Silicones for construction")
  • Since the experiment is safe and simple, it is ideal for the classroom. To save time, the pupils should be divided into groups, with each group investigating a different surface.

TopBottom  5 Supplementary Information

The organic methyl groups contained in silicones are very hydrophobic (water repellent), because there is extremely little interaction between the methyl groups and the water molecules.
A measure of the hydrophobic properties of a material is its tendency to cause wetting, i.e. the ability of liquids (in this case, water) to form a boundary with solids.
The wetting tendency can be derived from the contact angle θ formed between the water and the solid surface. The higher the contact angle, the less interaction there is between the solid and the water.
Contact angle  of a water drop on a (water-repellent) surface
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