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I am currently studying interface engineering, a field sometimes referred to as surface tension engineering. In the course, it has been mentioned that surface energy, also known as surface tension in the context of simple fluids, is considered a form of Gibbs free energy. I am attempting to grasp whether this classification is purely a matter of terminology or if there is a deeper connection between surface energy and Gibbs free energy. The Gibbs free energy (G) is expressed in various forms, and one common representation is: G = U + PV − TS. My question pertains to whether surface energy could be conceptualized as a component of the PV term in the Gibbs free energy formula. Since surface energy represents the energy required to create a unit area of interface, is it plausible to interpret this as a form of work performed in establishing space for the material, thus linking it to the PV term?

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I believe this is just a matter of terminology rather than a direct connection. The Gibbs free energy (G) is a thermodynamic potential that combines enthalpy (U), pressure-volume work (PV), and entropy (TS) to describe the maximum non-expansion work that can be extracted from a system at constant temperature and pressure. In simple words, it is a measure of the system's ability to do useful work.

On the other hand surface energy is the excess energy associated with the creation of a new surface or interface between two phases. It represents the energy required to increase the interfacial area by a unit amount. For example, in the case of a liquid-gas interface, surface tension is a measure of the surface energy per unit area.

While surface energy does involve work in the sense that energy is required to create an interface, it is not directly related to the PV term in the Gibbs free energy equation. The PV term in the Gibbs free energy equation typically refers to the mechanical work done by or on the system due to changes in volume under pressure. It does not specifically account for the work associated with the creation of an interface.

However, there is a connection between surface energy and the thermodynamic concept of excess free energy. Excess free energy (ΔG_excess) is a measure of the departure of the Gibbs free energy from its ideal behavior in a non-ideal solution. In the case of a binary liquid mixture, for example, the excess free energy can be related to the interfacial area and composition of the mixture.

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  • $\begingroup$ One can happily add terms for any other energies to G. For example, magnetic terms are commonly needed for many element crystal structures. A surface energy term is quite appropriate. $\endgroup$
    – Jon Custer
    Dec 29, 2023 at 3:05

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