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In many textbooks and chemistry curricula, we can see the description of large molecules (particularly in the discussion of hydrocarbons and other nonpolar compounds) as having "entanglement." However, I think the term is misleading, since the molecules don't really act like tangled shoelaces or two strings pulling against each other while interlocked. Can we try to clarify what actually occurs that we describe as entanglement?

Some things I think may happen:

The molecules are very large, and thus have many sites for IMFs, leading to high viscosity. By way of this size, the molecules are much harder pressed for movement and so take a while to move (the distinction is that in the first example the increased size begets more IMFs whereas the second causes "entanglement" just by the bulky molecules' shifting past each other).

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Well, very long hydrocarbons can become twisted around each other. This is definitely known in the polymer literature. Consider some branched polyethylene with ~1,000 repeat units. It's not too hard to imagine that multiple chains can become entangled in much the same way as shoelaces or strings when you mix a lot of them in a box.

You are also right that with large molecules, there are more sites for intermolecular interactions. Consider that there is a "combination" effect with lots of sites. That is, to pull two very long linear chains apart, you need to break lots of interactions.

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Notice that pulling one atom on the bottom away from its neighbor on the top chain will require the movement of all atoms on the bottom chain. So it's slightly harder to separate two long chains, not only because there are so many interactions, but also because of the combination.

Put simply, the difficulty in separating the two chains is slightly higher than simply the sum of the interactions.

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  • $\begingroup$ Why the down vote? $\endgroup$ – Geoff Hutchison Jun 4 '15 at 17:23
  • $\begingroup$ I'm extremely sorry, just edit your answer once and I'll remove my downvote. I was actually upvoting the answer. :| $\endgroup$ – Harshal Gajjar Jun 7 '15 at 12:42

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