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Polydimethylsiloxane (PDMS) is the main ingredient in Rain-X, a treatment that adheres to windshields.

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How does PDMS adhere to class? It can't form covalent Si-O bonds with glass, according to one source. Makes sense, as the Si is already coordinated with four groups in PDMS and none of them seem to be easily removable.

A similar windshield treatment chemical, FAS17, was noted by the same source as being able to form covalent bonds with glass in the presence of an acid to cleave the ether linkages.

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The source, however, didn't detail how exactly PDMS would adhere to glass. So what major forces would be involved in the adhesion of PDMS to glass?

Source:

http://hydrophobicsam.blogspot.com/2012/04/science-of-self-assembling-monolayers.html

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    $\begingroup$ If I had to guess, I'd go for Van der Waals interactions, which are the usual culprits when something is adsorbed but not forming covalent or ionic bonds with the surface. $\endgroup$ – Geoff Hutchison Jan 2 '16 at 23:41
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    $\begingroup$ This paper researchgate.net/profile/Jordan_Berg/publication/… also says Van der Waals forces. In order to get chemical bonding to occur a plasma treatment is required. $\endgroup$ – user1945827 Aug 18 '16 at 12:25
  • $\begingroup$ @user1945827 - I would suggest writing that up as a more thorough answer, including quoting relevant passages from the link, since links can break. (You may also want to use the DOI rather than a link to a particular website.) $\endgroup$ – Geoff Hutchison Aug 22 '16 at 16:56
  • $\begingroup$ From what I've read, it does bond to glass in the same way as FAS. However, since the addition of acid doesn't break down the polymers of PDMS into monomers, dimers, and trimers as it does for FAS, the bonds formed to the glass are weak, which explains why most customer reviews of Rain-X complain that it doesn't last very long. $\endgroup$ – KeatonB Aug 23 '16 at 23:13
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If you ask a chemist about PDMS and its adhesion to glass, she will immediately think of the work done in constructing micro-reactors. This is because one method of manufacturing micro-reactors is to stick a patterned piece of PDMS to a piece of glass; the PDMS is patterned such that it has reactor channels that are sealed when it is stuck to the glass. The glass is usually a micro-scope slide or something similar.

With the above in mind, in order to answer your question I began to search through the literature which describes the preparation of micro-reactors from glass and PDMS. What they were saying was that in order to form a bond between the molecules within the glass and the molecules within the polymer (PDMS) the glass had to be pre-treated,

"most of the research papers mention the use of oxygen plasma for developing chemical (siloxane) bonds be- tween the participating surfaces"

from "Studies on Surface Wettability of Poly(Dimethyl)Siloxane (PDMS) and Glass Under Oxygen-Plasma Treatment and Correlation With Bond Strength", DOI: 10.1109/JMEMS.2005.844746.

Since the manner in which your material is applied does not have any sophisticated pre-treatment then the only alternative method of adhesion (as pointed out earlier in this thread) is Van der Waals forces.

If this isn't quite the answer you expected, at least it is a good start.

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  • $\begingroup$ One reason for surface cleaning is that glass and most other surfaces accumulate "gunk" (various organic molecules) over time. A clean surface, naturally will produce better adhesion. $\endgroup$ – Geoff Hutchison Aug 23 '16 at 13:35
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As the above comments mention, van der Waals interactions are the primary types of intermolecular forces forces involved in the adsorption of any molecule onto a surface. Given that PDMS does not react with the glass surface chemically, it is pretty much certain that it is adhering through the combination of many weaker intermolecular forces (dipole/dipole, dipole/induced-dipole, and induced-dipole/induced-dipole interactions). You do not need covalent bonds for a molecule to adhere to the surface, given that there are enough weaker intermolecular forces holding it in place (covalent bond dissociation energies are on the order of hundreds of kcal/mol, whereas van der Waals interactions are an order to two orders of magnitude weaker). PDMS is a polymer, so given its length it should not be a problem to have many van der Waals interactions with the surface.

Indeed, if we consider the molecular structures of PDMS and glass (which is made from silica), we see that the Si-O bonds in PDMS and glass are permanent dipoles (oxygen is more electronegative than silicon). Therefore, dipole/dipole interactions are certainly at play. Given the rest of the structure, it is also safe to assume that dipole/induced-dipole interactions will be present, as well as induced-dipole/induced-dipole interactions.

However, from a quick search I learned that glass is not only composed of silicon and oxygen. Glass can also contain additional species such as calcium and sodium ions which are the result of treating silica with sodium carbonate and/or calcium carbonate during the manufacturing process. Compounds such as these are typically added to lower the melting temperature of silica as well as to keep the finished glass from deteriorating (source: http://www.cmog.org/article/what-is-glass). Given that there are these additional ions (and perhaps others) present in glass, it follows that ion/dipole and ion/induced-dipole interactions are also contributing to the adsorption of PDMS onto the glass surface, in addition to van der Waals forces.

Because these forces are directional, I suspect that PDMS adsorbs onto the glass such that the Si-O bonds are facing the glass surface. This way, it may maximize its interaction with the Si-O dipoles in the glass, as well as the calcium and sodium cations. As a result, the less polar methyl groups would be left pointing outwards from the surface, giving the coated surface its hydrophobic properties.

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The simple answer to the question is hydrogen bonding to the glass. Glass has silanol (>Si-OH) groups on the surface. by rinsing with an acid you can ensure that the proton is on the glass rather than other net positive species. In addition, perhaps some of the glass is etched or dissolved slightly, thereby cleaning the surface. If you look at the PDMS structure you can see that the oxygens are positioned with their lone electron pairs available to hydrogen bond with the glass. This allows the hydrophilic side of the PDMS linear molecule to associate with the glass while also directing the hydrophilic portion of the molecule outward. The result is a glass surface that appears to be very hydrophobic. The surface tension of the water is so strong that the water rolls off the windshield easily - the water beads on the low energy or hydrophobic surface.

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