Líquidos: condensados, desordenados, e às vezes complexos

Liquids: Condensed, disordered, and sometimes complex

David Chandler1

Department of Chemistry, University of California, Berkeley, CA 94720

Aliquid can be isotropic like a gas yet dense like an ordered solid. Its very existence depends on a delicate balance between adhesive intermolecular interactions that cause it to condense and entropic forces that prevent it from crystallizing. It is remarkable that such phases exist, and remarkable, too, that quantitative molecular principles and techniques are available for their study.

It wasn’t always this way. Before the 1970s, it was generally held that nothing substantive could be said about the microscopic origins of liquid properties, like equations of state and temperature and density dependences of liquid transport coefficients. The delicate balance between entropy and energy seemed to be too intricate and too system-specific, making generally applicable and simple-to-use principles elusive. What changed was the introduction of experimental probes of microscopic structure and dynamics like neutron scattering and pressure tuning spectroscopy plus the introduction of computer simulations of liquid matter.

These tools provided unambiguous tests of molecular theories so that inaccurate approximations could be avoided and sound principles could be established. This set the stage for what Jerome Percus called a ‘‘quiet revolution’’ (1) and for Benjamin Widom to write a few years later ‘‘we have indeed . . . a realistic, quantitatively reliable theory of simple liquids’’ (2).

Widom was referring to reliable analytical theory for dense homogeneous fluids and fluid mixtures composed of rare gas atoms and small molecules like acetonitrile, benzene, octane, and carbon tetrachloride. For systems like those, the statistics of fluctuations in local arrangements of molecules are limited in size by dense molecular packing, but not so limited to be highly structured or rare or intermittent. As such, probability distributions for these fluctuations are reasonably simple and easy to characterize with quantitative formulas.

But the story does not end there, in part because there are many ways by which liquid matter exhibits large length scale heterogeneity, and when this heterogeneity is the result of order–disorder phenomena, the small fluctuation theory of simple liquids is
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