As Aaron suspected it is true that glass is naturally hydrophylic. We find that it becomes hydrophobic from organic molecules carried to it by particulate “dirt” carried in the air. Check out this interesting video.
It can become hydrophobic as well from the leaking or spread of carbon based molecules that come from the edge gaskets. This condition is confined to the edges. As stated in the video Titanium Dioxide nanocoatings will “burn off” oily residue leaving clean surfaces which are hydrophylic. Wouldn’t it be nice if we could apply such coatings to glass ourselves? I am working on it. Probably a sol/gel technique. I should first look at that forum.
This post however is really about the ability of hydrophylic coatings to stand up to the attack of mineral deposits or hard water spots. The truth is simply that superhydrophylic coatings are better equipped than hydrophobic coatings to do this for one very simple reason. They do not allow the formation of water drops. Water drops concentrate the deposition of minerals as they dry. This also increases the pH of the water in the drop. That pH reaches its highest number just before the drop poofs away into oblivion. This highly alkaline water will eat right through most hydrophobic sealants. But a hydrophylic sealant will spread out the water so that it will sheet off the window. Virtually no mineral deposits will be left behind. There are no drops to evaporate. Just a very thin film of water. Now it is true that certain hydrophylic coatings are more resistant to alkali attack than others. In this case I put forth the idea that titanium dioxide coatings used to effect what are called Self Cleaning surfaces might actually be one of the most interesting coatings out there if indeed they are very resistant to alkali attack. They are already hydrophylic. Further titanium dioxide coatings are photocatalytic so they burn off thin organic greasy films using the energy from the Suns UV rays. So they might also be very resistant to pH values higher than 7 which is nuetral. Such that hard water deposits would be easier to remove from such a surface. I would like to experiment with my Slow Release Compounds to see if SRC pads could easily remove hard water spots from self cleaning surfaces without doing any damage to such surfaces.
Henry
