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Water molecules on the surface of an air-water interface have preferred orientations that lower their energy. This implies that these molecules are not uniformly distributed in orientation space, implying that the entropy is somewhat reduced when molecules are on the surface.

How much of water's surface tension is entropic? IE, the surface tension is the free energy change per unit area of surface created. How important is the $TS$ term compared to the $U$ term in this free energy? Can this be extracted from the surface tension as a function of temperature?

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  • $\begingroup$ The entropy per unit area is minus the partial derivative of the surface tension w.r.t. temperature at constant pressure and surface area. $\endgroup$ – Count Iblis Jun 1 '16 at 0:39
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If the water-air system is a closed system, when surface tension reshapes the interface shape between water and air, entropy of air increases due to increasing in volume and entropy of water decreases due to decreasing in volume. The heat to make this happen will reduce the air temperature. The water pressure will increases and air pressure will decreases. Because temperature change is very small, internal energy change can be neglected. TS is more important. So at high temperature, available free Gibbs energy is less than that at lower temperature.

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  • $\begingroup$ Where is the heat associated with the entropic change from? $\endgroup$ – user115350 May 31 '16 at 19:52
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all of it is entropic. when you smush stuff it gets hotter. when you expand it it gets colder. higher density smushed and lower unsmushed. very disordered and very much heating and cooling takes place.

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