mooney1el wrote:Simon (SEJohnson95) - As one chemist to another, I think a good challenge for you would be to investigate: What is Rain-X? How does it work? Are there pitfalls in the application procedure? Could the product be improved?
Hint; think surface tension/ surface energy and "molecular bonding".
Richard Gray
OK then, here goes (I hope this makes sense from a chemical point of view and I'm not about to ridicule myself) Take a deep breath everyone!
Background
Glass is composed of a network of silicon and oxygen atoms bonded together, with hydroxyl (OH) groups present at various places, these are arranged in a tetrahedral manner around a central silicon atom to create a large 3D structure. (Glass is amorphous, not crystalline) Image shown below of a representation of the structure after adding Sodium Carbonate which makes the glass less brittle, hence the Na+ counter ions(this is the sort of glass used in windows.)
Hydrogen Bonding
Water is a very polar compound owing to the large difference in electronegativity between its two constituent elements, the oxygen operating what is known as a -I inductive effect on the hydrogen, pulling the electron density through the sigma bonded framework of the molecule. This causes the hydrogen atom to become
deshielded , hence it carries a delta+ (slight positive charge) because the electron density is not shared equally between the two atoms in the bond.
Water has some unusual properties for a molecule of its size anyway (high boiling point, its solid form is also less sense than its liquid form, etc.) because of hydrogen bonding. This is the attraction between a de-shielded/electron deficient hydrogen on one water molecule and a lone pair of electrons on another oxygen in a neighbouring molecule, i.e. an
intermolecular interaction.
Since glass has hydroxyl groups present, Hydrogen bonding between the water on the windscreen and the surface of the glass can occur, causing a greater interaction between the two, which is why water tends to stick to windscreens.
Polysiloxanates
Rain-X's primary active ingredients are known as polysiloxanates, more specifically Polydimethylsiloxane (PDMS), which is a Silicon based polymer with two methyl groups attached. I.e. two organic functional groups attached to a inorganic chain. This makes the compound very non-polar, so it will be repelled by the water droplets. The monomer is shown below: (one uses many monomer to make a polymer, e.g. styrene and poly(styrene) )
PDMS binds to the hydroxyl group of the glass surface and thus inhibits the interaction of water molecules with the surface of the glass. The two methyl groups provide low surface tension, and this serves to repel high-tension fluids such as water. (Think about what happens when you skim a stone across water, or do a belly flop in a pool, it's the surface tension caused by Hydrogen bonding across molecules on the surface which allows this to happen.)
Glass is also very porous owing to its slightly random 3D structure and the Rain X coating also serves to fill in these pores to to minimise the surface area that the water can make contact with. It increases what is known as the contact angle, and 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.
This forces the water to bead and roll off the windscreen.
Rain X is a type of superhydrophobic coating (water hating), it works at the molecular level to inhibit the interaction.
Adhesion to Glass
Oxygen groups are good at bonding to glass (see above explanation) but only when they are exposed. In a polymerised chain there is a lot of
steric bulk (the large amount of space taken up by the rest of the molecule) which reduces their ability to bond to the glass.
In order to make these sort of polymer adhere to glass, the oxygen groups need to be exposed. Consider a similar product to Rain X, known as FAS17. This a fluoroalkylsilane (long groups of singly bonded C-C atoms with fluoro substituents instead of the typical hydrogen substituent as in a simple alkane) with three silyl ether functional groups attached to the Silicon.
FAS17 can be made to adhere to glass better by breaking down the compound with a catalyst (in this case
acid catalysed hydrolysis -the breaking down of a substance using water , an organic mechanism that I learned this year), to produce a Silanol (a silicon with an alcohol functional group) followed by a step known as condensation, in which effectively water is to bridge the Silanol and the OH group on the inorganic substrate, in this case the surface of the glass. You then apply a large amount of heat (indicated by the delta H) and polymerise the Silanols, which gives you something like the starting product but now with a more permanent adhesion to the glass surface instead of a coating.
Because the FAS is chemically bonded to the glass it is very hard to remove. This allows the compound to stay active on the windscreen over a long period of time as opposed to the Rain X (PDMS) which can be removed over time by external factors such as wiper blades which is the reduction in effectiveness that you all observe (hence the need for a washer additive to bridge the gap in between applications) however since the compound has been altered chemically through various syntheses, its effectiveness is reduced.
Conclusion
In conclusion, Rain-X and PDMS based repellents have a greater water repellent effect but a much lower binding ability to glass and so need to be replenished. Rival FAS based repellents have a greater lifespan but come with sacrificed effectiveness.
I hope I'm on the right lines with that lot!
Oh and on a final note if I may mention my first-year results briefly (which I received in June but kept quiet about) 87.3% overall, with 89,90 and 86% in Inorganic, Organic and Physical chemistry respectively. The cherry on top was receiving the Harding Prize for the top undergraduate year 1 laboratory marks out of a 120 strong cohort. I'm still a little in shock to be fair!
I was never that good at A level
