Attractive energy and entropy or particle size: the yin and yang of physical and biological science.

It is well known that equilibrium in a thermodynamic system results from a competition or balance between lowering the energy and increasing the entropy, or at least the product of the temperature and entropy. This is remarkably similar to the Taoist concept of yin, a downward influence, and yang, an upward influence, where harmony is established by balancing yin and yang. Entropy is due to structure, which is largely determined by core repulsions or particle size whereas energy is largely determined by longer range attractive interactions. Here, this balance between energy and entropy or particle size is traced through the theory of simple fluids, begining with Andrews and van der Waals, the subsequent developments of perturbation theory, theories of correlation functions that are based on the Ornstein-Zernike relation and the mean spherical approximation, electrolytes, and recent work on ion channels in biological membranes, where the competition between energy and size gives an intuitively attractive explanation of the selectivity of cation channels. Simulations of complex systems, including proteins in aqueous solution, should be studied to determine the extent to which these concepts are useful for such situations.