Siliconized filter paper

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. This experiment was devised on the basis of a private communication from Prof. P. Jutzi, Bielefeld University.

Siliconized Filter Paper

1 Materials, Chemicals, Time Needed

Filter funnel tripod
2 fast-flow filter funnels
2 fluted filter papers
4 test tubes
Glass beaker, 50 m
Pasteur pipet with rubber cap

Copper sulfate, Xn, N
Dichloromethane (methylene chloride), Xn
Trichloromethylsilane, Xi, F

Allow about 10 minutes for silanizing the filter paper. The experiment itself takes less than 5 minutes.

2 Procedure and Observations

Using the pasteur pipet, wet one of the fluted filter papers with trichloromethylsilane in a fume cupboard. Ensure that the entire filter paper is wetted with trichloromethylsilane. Place the filter paper aside to dry.

While it is drying, dye approx. 15 ml water with copper sulfate. Fill two test tubes to a depth of around two centimeters with the colored water and add roughly the same amount of organic solvent (dichloromethane). Two phases form because the liquids are immiscible. Dichloromethane is denser than copper sulfate solution (colored water) and thus forms the lower layer (see Fig. 1).

Fig. 1. Test tubes containing test liquids

Fig. 2. Filtrates from the two filters

Now assemble the apparatus as shown in Fig. 2 and add the contents of one test tube to the “untreated funnel” and the contents of the other to the (dry) “siliconized funnel.” The water passes through the “untreated funnel” as usual, and the dicholoromethane remains in the funnel. In the case of the siliconized paper, the colored water remains in the funnel and the dichloromethane passes through (see Fig. 3).

Fig. 3. Residues on the two filter papers

3 Discussion of Results

Standard laboratory filter paper consists of cellulose, whose simplified structural formula is shown in Fig. 4.

Fig. 4: Schematic diagram of a cellulose molecule

Water consists of polar molecules. The molecules of the cellulose contain many hydroxy groups that can form hydrogen bonds and undergoes dipole-dipole interactions with water molecules. Water therefore penetrates into the hydrophilic cellulose and passes through the filter paper. The hydrophobic dichloromethane does not penetrate into the cellulose filter paper. When the filter paper is wetted with trichloromethylsilane, condensation reactions occur, with a large number of the hydrogen atoms in the hydroxy groups being replaced by methylsilyl groups. This also causes interlinking of the cellulose molecules (see Fig. 5 and 6). The product contains numerous nonpolar methyl groups that make it hydrophobic and lipophilic.

Fig. 5: Condensation reaction between cellulose and trichloromethylsilane

Fig. 6: Schematic diagram of siliconized cellulose

The resultant siliconized cellulose is no longer permeable to water. However, a nonpolar or weakly polar solvent that cannot form hydrogen bonds, e.g. dichloromethane, is able to flow through the siliconized cellulose.

4 Tips and Comments

Additional minor experiments could be performed to show that HCl is released when the filter paper is treated with trichloromethylsilane. This may be done by washing the paper with distilled water after siliconization and detecting hydronium ions in the water (with acid-base indicator) and detecting chloride ions with silver nitrate solution.
In the lesson, the pupils should suggest this identification reaction on the basis of the reaction scheme provided in Fig. 5, and should then perform it.
The mechanism of nucleophilic substitution may be discussed in connection with the reaction that occurs during siliconization (Fig. 5).
In addition to the variant described here, filter paper could also be siliconized with a) dichlorodimethylsilane and b) chlorotrimethylsilane. The observations made could then be discusssed in class.

5 Supplementary Information

Although this experiment bears a certain resemblance to the "Silicon-coated paper", there is a fundamental difference: When the filter paper in this experiment is siliconized, the cellulose reacts with the added trichloromethylsilane and the molecular structure of the paper is radically altered (see Figs. 4, 5 and 6). In the other experiment, when the paper is coated with silicone, the silicone has already been crosslinked and is applied in the form of a thin film; the vast majority of the paper’s molecular structure remains unchanged.