Correlation between functional group and viscosity of an organic compound?

Question

I'm looking to explore the correlation between the type of functional group on an organic molecule and the viscosity of the compound. I know that viscosity is affected by intermolecular forces (Ion-induced dipoles, Ion-dipole, Hydrogen bonding, van der Waals), and that the type of forces present depends on the functional group. I'm aiming to explore viscosities of compounds with the following groups:

• Alkenyl
• Alkynyl
• Hydroxyl
• Ether
• Aldehyde
• Carbonyl
• Carboxyl
• Amide
• Amine
• Nitrile
• Phenyl

How do the (absolute) viscosities of compounds with each of these groups (assuming single-functional group compounds) differ, and why (in terms of intermolecular forces present)? Are there any databases with values for viscosities that I could use?

Thank you for the help.

Answer 1

There are empirical methods that can give you the viscosity from the structure. For lower temperatures, Orrick-Erbar equation, and for higher temperatures, Letsou-Stiel equation is what you are looking for. Orrick-Erbar maybe more suitable for you, because it is a group contribution method, and by looking at the values assigned to functional groups, you can develop an intuition about how each group affects viscosity. Letsou-Stiel uses acentricity factor, because at high temperatures, secondary bonds become less important.

As for database, NIST may be useful. Or if not, look at some supplier webpage (I use SigmaAldrich for this).

Answer 2

This is a great chance for you to do some searching through the literature and draw conclusions on your own. Make a list of molecules containing each functional group, group them by trends, and compare their viscosities. (alkyl: ethane, alkenyl: ethylene, alkynyl: acetylene, etc... to see the effect of adding pi bonds, for example). Most, if not all of this data is freely available through the CRC Handbook of Chemistry and Physics.

Once you've searched and found the trends on your own, you should be able to tell from a structural standpoint why these trends exist. To measure viscosity, liquids are sheared between two solid surfaces, and the viscosity is proportional to the force required to move the surfaces. At the molecular scale, parts of different molecules must move past each other under shear, and the "ease" of molecule sliding past one another is what determines viscosity. One can imagine that molecules with stronger intermolecular forces will have higher viscosities, since those forces must be overcome when molecules slide past one another. For example, consider acetone and acetic acid, which have viscosities of 0.306 and 1.056 mPa-s at 25 C, respectively. Acetic acid has much stronger intermolecular forces because of the dipoles present (so much that it forms dimers in the gas phase), and as a result has a higher viscosity.