Hydrophobicity of Cyclic Siloxanes

The first variable I altered in trying to find the optimal time and temperature to make a surface hydrophobic using cyclic siloxanes was time. I allowed the reaction to run at 9 different time intervals: 15 minutes, 30 minutes, 1 hour, 2 hours, 6 hours, 24 hours, 48 hours, 72 hours, and 168 hours. I had hypothesized that the longer the reaction was allowed to run, the more hydrophobic the silicon surface would become, and the thicker the siloxane layer coating the silicon would become. This set of reactions was done at 100°C with about 150 mg. of hexamethylcyclictrisiloxane present in each vile.

After running these reactions and gathering data through ellipsomerty and goniometry, I discovered that generally speaking, my hypothesis was right in regards to the change in hydrophobicity. The 15 minute reaction resulted in an average advancing contact angle of 68° and an average receding contact angle of 63°. By the time the reaction ran for 6 hours, the hydrophobicity increased to new advancing and receding contact angles of 96° and 89°, respectively. The additional 162 hours the reaction proceeded for only resulted in a 10° increase for both advancing and receding contact angles. What this data tells me is that most of the increase in hydrophobic qualities occurs within the first 6 hours under these conditions, and gradually plateaus soon after.

The results gathered regarding the change in thickness of the siloxane layer was different than what I had hypothesized. What I found is that, over the first 72 hours, the thickness has sharp increases and decreases. The thickness changed from 9.63Å to 3.27Å in 15 minutes, than went up to 7.67Å in just an additional half hour. This rise and fall in thickness continued until the third day, and after 72 hours there was a steady decline in thickness, which began to plateau.

The next variable I would like to alter after gathering this data is the temperature of the oven in which the reaction takes place. I am interested to see how the patterns from this experiment being performed at 150°C for the same time intervals will compare and differ to the patterns I found at 100°C.

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