Viscosity of oils

[Cap'n Refsmmat]

I was recently posed an interesting question:

 

Viscosity is generally related to the strength of intermolecular bonds in a fluid. Strong bonds lend to greater viscosity. So how is it that something like oil, which is non-polar and would have low intermolecular forces, can be more viscous than water, which experiences hydrogen bonding?

 

The only reasonable explanation I can think of is that oil, with its hydrogen content, actually does experience hydrogen bonding. But I don't think that's right. Is there any more logical reason?

[Mr Skeptic]

Maybe because the molecules are so big?

[UC]

London Dispersion Forces. Long hydrocarbon chains especially should give lots of space to interact between molecules. If you have tons of even weak bonds forming, the macroscopic results will be quite significant.

[hermanntrude]

for example, hydrocarbon chains twenty or so carbons long are waxes. ie they are solids, whereas water, with its hydogen bonds is still a liquid. london dispersion forces CAN be stronger than hydrogen bonds if there are enough electrons and enough polarisabiity

[ChemSiddiqui]

I was recently posed an interesting question:

 

Viscosity is generally related to the strength of intermolecular bonds in a fluid. Strong bonds lend to greater viscosity. So how is it that something like oil, which is non-polar and would have low intermolecular forces, can be more viscous than water, which experiences hydrogen bonding?

 

The only reasonable explanation I can think of is that oil, with its hydrogen content, actually does experience hydrogen bonding. But I don't think that's right. Is there any more logical reason?

 

I am not sure if this is right or not, but if i recall correctly double bonds also influence viscousity. There are more double bond in the organic oil while in water there are none, even though it is polar, oil is more viscous. Forgive me if i am wrong though!

[Mr Skeptic]

Can a solid be considered to have a viscosity?

[hermanntrude]

well if it can't, the hydrocarbon before the first solid one would be a liquid with a higher viscosity than water.

[amit]

hydrogen bonding in hydrocarbons is to such a least extent that it wont be wrong to say its not there. hence only due to london forces its viscous which in turn depends on size.

[hermanntrude]

hydrogen bonding in hydrocarbons is to such a least extent that it wont be wrong to say its not there. hence only due to london forces its viscous which in turn depends on size.

 

hydrogen bonding does not exist AT ALL in hydrocarbons.

[ChemSiddiqui]

hydrogen bonding does not exist AT ALL in hydrocarbons.

 

agreed. Say, just noticed you made it to 700 posts mark. keep contributing to chemical knoweldege of others and yourself and you will find that you are poster with a code name 007(read your number of posts backwards). licence to kill...wonders what that means for chemistry !

[amit]

i wont say h bonds don't exist in hydrocarbons as i read in a book that hydrogen bonding up to some extent is in every molecule but the bonding energy is so low at even very low tempretures can offset its effect

[hermanntrude]

hydrogen bonding does not exist in hydrocarbons. Your book was wrong, or you misinterpreted it. Hydrogen bonding only occurs in molecules with a bond between a hydrogen atom and a very electronegative atom such as F, O or N, and sometimes Cl. Carbon isn't electronegative at all and so any molecules containing only hydrogen and carbon cannot contain hydrogen bonding.

[amit]

try going on this link

http://www.ncbi.nlm.nih.gov/pubmed/17705268

what i under stood is that it can happen.

 

according to following article on wiki pedia

http://en.wikipedia.org/wiki/Hydrogen_bond

"Carbon can also participate in hydrogen bonding, especially when the carbon atom is bound to several electronegative atoms, as is the case in chloroform, CHCl3. The electronegative atom attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge."

[vedmecum]

hydrogen bonding does not exist in hydrocarbons. Your book was wrong, or you misinterpreted it. Hydrogen bonding only occurs in molecules with a bond between a hydrogen atom and a very electronegative atom such as F, O or N, and sometimes Cl. Carbon isn't electronegative at all and so any molecules containing only hydrogen and carbon cannot contain hydrogen bonding.

 

every element has some electronegativity .

 

i think there is bit confusion on defination of hydrocarbon :- amit you have given an example of CHCl3 , which you can't count as hydrocarbon .

[hermanntrude]

yes the common example of carbon participating in hydrogen bonding is when Chloroform and acetone are mixed. A hydrogen bond is formed between the C=O bond and the C-H bond. This happens due to the inductive effect of the three highly electronegative Cl atoms. However, neither of these compounds is a hydrocarbon.

 

As to whether carbon has any electronegativity, it depends how you define it. Personally i think of anything with a Pauling electronegativity of 2 or lower as being electropositive. In a hydrocarbon, the dipole is miniscule, and almost negligible. You are correct in saying carbon has some electronegativity, but you are labouring a useless point since it's such a small electronegativity difference when compared to hydrogen, that essentially no dipole exists, and certainly nothing remotely close to a hydrogen bond.

[ChemSiddiqui]

try going on this link

http://www.ncbi.nlm.nih.gov/pubmed/17705268

what i under stood is that it can happen.

 

according to following article on wiki pedia

http://en.wikipedia.org/wiki/Hydrogen_bond

"Carbon can also participate in hydrogen bonding, especially when the carbon atom is bound to several electronegative atoms, as is the case in chloroform, CHCl3. The electronegative atom attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge."

 

Well there you go you are contradicting yourself. You see carbon is electropositive element and wheh an electronegative atom is bonded to it , it takes the electron density away from carbon by iductive effect or mesomeric effect and there is a dipole created. however, the h-bonding can only occur with H atom and with highly electronegative atom such as O, N F etc.

[amit]

its fine i m more than happy if that book is wrong. but only thing is as carbon is more electronegative then hydrogen hence i also believe that there must be some intersaction although of no significance. as wikipedia says it is in carbon also "especially " in CCl4 etc... it means there must be cases in hydrocarbon also like if i cool it to temp say of liquid He

[hermanntrude]

the book isn't wrong. The book never said these things were hydrocarbons. You did that.

[jdurg]

Also, many oils have large, convoluted (e.g. not-linear) molecules and these molecules will "lock" into each other and thus make the liquid more viscous than one would think. I mean, octane is pretty non-viscous but it is a larger molecule than water. When you start introducing double bonds and methyl/ethyl groups onto the chain, it makes it a little less easier for the molecules to move past each other.

[amit]

do u even know which book i m talking about? actually i tried re reading that book and realised i goofed up things little badly. i found that book on google books here is a link :-

http://books.google.co.in/books?id=be0T64ZWp9EC&dq=inorganic+chemistry+,manku&printsec=frontcover&source=bl&ots=5NuxwrFxlK&sig=KvxdqrgDBsfR6jehkH4lrkpmZ8w&hl=en&ei=JeqXSdT1A4G86gP5uez4CA&sa=X&oi=book_result&resnum=1&ct=result

 

look page 215. actually there is the talk about h bonds of acetylene and data about C-H...O bonds due to which i thought that h-bonds may be in hydrocarbons also....

[hermanntrude]

the book introduces something I've never seen before, and it refers to it as being a hydrogen bond. That is, the bond between the triply-bonded acetylene and acetone. Now I could be prepared to believe that the Carbon in acetylene is more electronegative than most carbon atoms, but whether it counts as a hydrogen bond is another question... I guess that's a matter of semantics. Anyway thanks for showing me the exception to the rule.

 

Note, though that even though the acetylene has a strong dipole on the C-H bond, even that book doesn't claim it has hydrogen bonds on its own. Therefore my original statement holds true, with one addendum:

 

"no hydrocarbon on its own has hydrogen bonds"

[Kaeroll]

It's also worth adding that hydrogen bonds have a length which may or may not be greater than dispersion forces (I'm not sure off the top of my head). This is one reason ice is less dense than water, due to the lengths of the hydrogen bonds. Perhaps this is also a contributing factor?

 

Kaeroll

[nitric]

may i say that this was about the "viscosity of oils" and then it went on to this dispute on hydrogen bonding, and i would agree that its similar to how the hydrocarbons boiling temp heightens as the chain length lengthens(correct me if i am wrong, but it makes sense based on how a longer molecule would have more interaction points)

[GDG]

Viscosity is a measure of a liquid's resistance to sheer stress (OK, you can refer to the viscosity of a gas or solid, but I think the question is addressed mainly to liquids).

 

In a liquid without H-bonds, the primary intramolecular force is going to be van der Waals attraction, which increases with the size of the molecule. Long-chain hydrocarbons have boiling points higher than water, and thus presumably have stronger vdW forces between their molecules, and therefore one would expect higher viscosity. Note that short-chain hydrocarbons (e.g., hexane) have less viscosity than water, and also a lower boiling point.