Molecular attraction and repulsion

Pressure—a reflection of the number of molecules present and their motion, temperature—a reflection of the kinetic energy of the molecules, and molecular attraction and repulsion. Pressure and molecular attraction tend to confine the molecules and pull them together. Temperature and molecular repulsion tend to separate the molecules. 

Surface tensions arise from the unbalance of molecular attractive (and repulsive) forces that result at an interface from the different nature of the materials that come in contact. Surface tensions are more readily observed in liquids—between immiscible liquids or between a liquid and a gas. Surface tensions are also present on the surface of solids, but the rigidity of the solid structure prevents their manifestation in an observable way, except in combination with the surface of a liquid in contact with the solid surface. Thus, while the surface tension and the surface free energy in a liquid are the same, this is not the case on solid surfaces. 

At the theta state, the contributions of the intra-molecular attraction and repulsion to the coil size compensate with each other. Flory first gave a thermodynamic treatment to the theta state (Flory 1953). He assumed that the solvent molar mass was Nj, and the excess chemical potential = dAF IdN] contained two parts of contributions, i.e., enthalpy and entropy... 

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