I've seen partial answers to my question on Wikipedia and several other questions on this site, but I'm not quite grasping how intermolecular forces lead to the creation of a membranous boundary between, for example, water and air.

I can grasp that with no balancing forces at the surface of a water droplet, surface molecules tend to be attracted to the interior of the droplet, but fail to see how that leads to a stronger attraction between the molecules at the surface. Wouldn't the perpendicular forces (and their imbalance) be irrelevant to the effect surface molecules have on each other?


1 Answer 1


here is one way to look at it which might help:

every water molecule that is completely surrounded by other water molecules exhibits a certain amount of affinity for its nearest neighbors. But at a free surface, the water molecules occupying it have only half as many nearest neighbors with which to commune. as a result, the strength of the intermolecular attraction between molecules at the free surface is to first order about twice what it is in the interior of the body of water, which makes the free surface act like a stretched membrane that wants to snap back.

  • $\begingroup$ Thanks for your response. I haven't studied physics in a couple of years, so maybe I'm forgetting some basic fact, but how does halving the number of neighbors double the force between them? $\endgroup$ Jan 29, 2018 at 4:15
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    $\begingroup$ each molecule apportions its capacity for attractive force equally between its nearest neighbors. if the number of nearest neighbors is halved, then there's twice as much attractive force to go around, and the surface molecules cling to each other harder. $\endgroup$ Jan 29, 2018 at 4:31
  • $\begingroup$ Thanks, I follow that. I'm curious about the chemistry and physics behind a molecule even having a defined capacity for attractive force, can you point me to somewhere I could learn more about that? Is this a property specifically of intermolecular forces? In an environment of equally-charged particles, for example, I wouldn't think that there was any maximum or minimum to the amount of Coulomb force a particle could exert. $\endgroup$ Jan 29, 2018 at 4:34
  • $\begingroup$ what you want to look into are what are known as the colligative (sp?) properties of water- the behavior of large numbers of them in interactive proximity. most important being van der waals forces. if you googled surface tension or searched it on wikipedia, you will probably find everything you need. $\endgroup$ Jan 29, 2018 at 18:05

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