Glass is essentially a network of bonded Silicon and Oxygen atoms. Every once in a while there is an exposed OH or hydroxyl group . This hydroxyl group has a strong polarity just like water due to the fact that the oxygen is electronegative. Because the exposed hydroxyl groups are polar just like water, water is actually attracted to these groups. This attraction causes water to stick to glass and spread out instead of rolling off. Rain-X and Aquapel prevent this interaction allowing the water to roll right off. To fully understand how the works though the chemical structures must be examined.
Rain-x is made of mostly Polydimethylsiloxane (PDMS) , as seen below:
PDMS is a simple polymer made up of the silicon, oxygen, and carbon containing “methyl” groups. These carbon containing methyl groups make the polymer similar to oils due to the fact that these carbons are non-polar just like the carbons in oil.
Aquapel, one of the main competitors of rain-x is made of different compound call a fluoroalkylsilane (FAS). The most common FAS is known as FAS17 for the 17 fluorine atoms that it has on it :
The long chain of Carbon and fluorine’s create a similar non-polar chemical as Rain-X
The polarity of water causes it to repel substances like oil and Rain-X. When water (like a raindrop) lands on a hydrophobic surface (like a rain-x treated windshield), the water balls up because it repels the hydrophobic surface and collects into a sphere. This sphere is then touching very little of the windshield. This allows the bead of water to run off very easily as compared to normal glass which attracts the water instead.
To truly measure hydrophobicity a measurement called the contact angle is used. Physical chemistry is able to predict specific quantities . Knowing the contact angle allows physical chemists to predict the “surface tension” of the glass which is the same thing as its hydrophobicity. The contact angle also allows physical chemistry to calculate how easily the drop will slide off of the glass.
The more hydrophobic the material, the greater the contact angle, the greater the contact angle the easier the water will slide off. This makes sense because when the water has a large angle, there is less surface “holding” onto the water droplet.
According to a study by Kamitani , PDMS has a smaller sliding angle (which means it slides off easier) but a smaller angle of contact, while current FAS chemicals have a larger contact angle but a larger sliding angle. In this case the discrepancy is due to physical nature of the chemicals. The perfluorocarbon chains interact with the water molecule more causing to stay in place . The lab was also able to design a more effective FAS chemical that matches PDMS. This could possibly be used in the future to create a better windshield repellent.
Adhesion to Glass
To be able to bind to glass chemicals like FAS must be slightly modified. Oxygen containing group are excellent at binding to glass but only if the oxygen’s are exposed. In the case of FAS, all of the oxygen’s are bound up with carbons and not exposed. To expose these oxygens, the FAS17 monomer is exposed to specific amounts of nitric acid. This nitric acid cleaves the bond between oxygen and carbon allowing the oxygen groups to be exposed . The exposed oxygen then easily binds to the exposed hydroxyl groups on glass. A byproduct of this addition of acid is that the FAS monomers polymerize. Previous research has shown that dimers (two monomers put together) and trimers (three monomers put together) were most effective in bonding to the glass. With more polymerization, the polymer became too bulky to bond to the glass. The scheme below shows the process of activating the FAS chemicals :
Because FAS is chemically bonded to the glass it is very hard to remove. This allows the compound to stay active on the windshield over a long period of time .
Rain-X contains mostly PDMS ,. From the example above, PDMS has no possible ways to bind with glass. As with FAS, PDMS can also be activated by acid, which breaks down the polymer. In the case of FAS, dimers and trimers were produced. For PDMS though the polymer is broken down. This still leaves very large bulky groups which do not bind effectively to the glass . Rain-X also has chloroalkylsilanes left over from production. These chemicals have been reported to add to existing polymers to create difference types of polymers some of which are unwanted . This is important because larger polymers make the binding ability even lower . This means that wiper blades can remove PDMS over time, which causes it to lose its effectiveness. This is similar waxes that are applied to cars. As time goes on the waxes are removed by external forces, though this effect is more pronounced with Rain-X due to wiper blades being an almost constant force in rain.
In conclusion, Rain-X and PDMS based repellents have a greater water repellent effect but a much lower binding ability to glass. Aquapel and FAS based repellents have a greater lifespan but come with sacrificed effectiveness.
 Quantitative infrared spectroscopic measurement of hydroxyl concentrations in silica glass http://www.sciencedirect.com/science/article/pii/0022309396003304
 Rain-X MSDS https://www2.itap.purdue.edu/msds/docs/7756.pdf
 What’s Inside: Rain-X. Better Driving Through Alcohol
 PDMS Repeating Unit http://en.wikipedia.org/wiki/File:PDMS_repeat_unit.png
 Water-repellent coating films on glass prepared from hydrolysis and polycondensation reactions of fluoroalkyltrialkoxylsilane http://www.sciencedirect.com/science/article/pii/S0040609001009014
 Improvement on Hydrophilic and Hydrophobic Properties of Glass Surface Treated by Nonthermal Plasma Induced by Silent Corona Discharge http://www.springerlink.com/content/w020187756124126/fulltext.pdf
 Behaviour of water drops on different surfaces
 Development of Water-Repellent Glass Improved Water-Sliding Property and Durability http://www.springerlink.com/content/x72674302445h130/fulltext.pdf
 Coatings made by sol–gel and chemical nanotechnology http://www.springerlink.com/content/rq2l3h621r111275/fulltext.pdf
 Improvement on Hydrophilic and Hydrophobic Properties of Glass Surface Treated by Nonthermal Plasma Induced by Silent Corona Discharge http://www.sciencedirect.com/science/article/pii/S0040609001009014
 Easy-to-clean properties–Just a temporary appearance http://www.sciencedirect.com/science/article/pii/S0040609005011612
 Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces: Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon http://pubs.acs.org/doi/abs/10.1021/la000471z