(I guess) I understand the covalent bonding of water and the hydrogen bonding of water between two different molecules, but I would like to know which part is the part that that exposes itself to the air in the surface tension of water. There should be something different to the rest of the liquid, otherwise there would be no frontier, right?

Is it the electropositive part of hydrogen or is it the electronegative part of oxygen that faces the air? (or may be none of them).

Surface tension in water owes to the fact that water molecules attract one another, as each molecule forms a bond with the ones in its vicinity. At the surface, though, the outmost layer of molecules, has fewer molecules to cling to, therefore compensates by establishing stronger bonds with its neighbors, this leading to the formation of the surface tension.... more here: enter image description here


Thanks to all for your help (I'm in chemistry '101' for dummies so please try to be as basic as possible.

Video: https://www.youtube.com/watch?v=moITG5Q7zzI

(Hydrogen bonds https://www.khanacademy.org/science/high-school-biology/hs-biology-foundations/hs-water-and-life/v/hydrogen-bonding-in-water)

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    $\begingroup$ Hmm, I don't know if one orientation is more prominent on average. But I wanted to say that water molecules are moving all the time, faster than we might think. So they can't, for example, sit in one particular orientation with the oxygen always facing the air, and then stay there for good. But over time, on average, it seems plausible that you may have some imbalance in the orientation ("anisotropy"...?), or indeed you may have none. $\endgroup$ – orthocresol Aug 7 at 12:06
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    $\begingroup$ It may be either part; it does not matter. The molecules on the surfaces are no more ordered than those in the bulk. $\endgroup$ – Ivan Neretin Aug 7 at 12:06
  • $\begingroup$ They could be sideways. The surface is different from the bulk. You can see that it has a skin-like nature by overfilling a glass with water. This effect is more dramatic if you put some water in a dish, sprinkle some black pepper grains on it (no effect), then touch the surface of the liquid with a bar of soap. ZAP! The skin tears and is pulled to the edges of the water layer - pulling the pepper grains with it. $\endgroup$ – James Gaidis Aug 7 at 13:30

Surface tension may be described regardless if all / some / none of the water molecules expose one or both hydrogen atoms or the oxygen atom toward air, even if there are studies in this field (e.g., example, example).

Key points however are

  • the movement (translation, vibration) of the water molecules in the bulk of water droplet in restained, because of intermolecular attractive forces (e.g., hydrogen bonds), as well as intermolecular repulsion (molecules normally do not come closer to each other than the sum of the corresponding van der Waals radii). Like with humans, there is an optimal intermolecular distance where overall the energy passes a minimum.

    In extension of your drawing of the situation in the water droplet, I add the following figure where molecules are reduced to a black sphere; these are either in the inner of the droplet (light blue background), or at the interface (dark blue background) exposed to air. The black arrows indicate intermolecular attractive forces (responsible for cohesion).

    enter image description here

    (derived from wikipedia)

  • For a molecule in the inner of the droplet, surrounded completely by molecules of the same type, the net force on this molecule is zero.

  • For a molecule at the interface water / air, the attractive forces to molecules of water in the inner of the water droplet exceeds the attractive forces between this water molecule and air molecules. Thus, there is a net force pointing to the inner of the water droplet; this is symbolized by the red arrows.

    Because of this inward directed force, the droplet thus tends to contract, building an inner pressure until compensated by the earlier mentioned intermolecular repulsion.

Overall, the lowest energy of the water droplet would be achieved as a sphere. Sometimes, this is shown in microgravity (Scott Kelly at 8:28min). On earth, however, things like gravity, contact with a surface (adhesion), relative air speed (e.g., if a drop falls down the rain gutter) etc. lead to deviations from this ideal. However no, because the molecules in the inner of the water droplet are the same as the ones at the interface exposed to the air, there is no difference in the intermolecular, pairwise attractive forces between molecules of same type.

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