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I read that addition of soluble impurities increase the surface tension of water. I initially thought it was because when soluble impurities dissolve in water it leads to the dissociation of that impurity into ions and ions being a fully developed charge have a stronger force of attraction. But i don't see how it increases the surface tension between the liquid surface. How does it work? What is the mechanism behind it?

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Consider surface tension as a force per unit length acting within the plane of the (infinitesimally thin) liquid surface. Any component that is co-located in that immediate surface plane and increases attractive forces between the constituent atoms, molecules, or ions in that immediate surface plane will act to increase surface tension. Essentially, when the attractive forces are larger, we require more force per unit length (surface tension) to pull the constituents apart from their equilibrium separation distance.

As an added note, soluble components can include everything from solid ionic salts to liquid organic acids or alcohols.

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Surface tension corresponds to an energy penalty from adding surface area for a given volume. The reason is that bonding is generally better satisfied in the bulk, when material is available from all sides, than at the surface, where material is missing on one side.

(A nonpositive surface tension corresponds to a gas; here, there’s no energy penalty for the liquid forming arbitrarily small droplets, down to the molecular scale.)

There are two reasons an impurity might dissolve: (1) the bonding is less favorable than that of pure impurity and pure host material (e.g., water), but entropy drives some dissolution because it makes additional molecular arrangements possible, and (2) the bonding between the impurity and the host material is more favorable than that of the pure substances, and so energy minimization drives mixing.

In the former case, the bonding in the bulk is worse upon mixing, and so the energy penalty of introducing a surface is lessened. The surface tension is therefore lowered.

In the latter case, the bonding in the bulk is stronger upon mixing, and so the energy penalty is introducing a surface and interrupting this bonding is heightened. The surface tension is therefore increased.

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My intuition suggests the point chemomechanics made above. Which is that the nature of the impurity should determine whether the surface tension at an interface is lowered or increased by the addition of an impurity.

Surface tension exists at surfaces or rather interfaces separating two different kinds of media - which is where molecules of either kind have fewer similar bonding partners than they do inside the bulk of the material and that makes the molecules at the interface more volatile in a sense. For example at a liquid-gas interface the liquid molecules on the liquid surface have some missing binding energy due to the missing molecular partners and experience forces pulling them back into the bulk of the liquid.

OTOH an impurity mixed in should impact the surface tension differently depending on its relation to the bulk binding energy contributions (I think that is correct).

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