Piezo‐ and elasto‐optic properties of liquids under high pressure. III. Results on twelve more liquids

Precision optical interferometric measurements at high pressures have been made on twelve liquids. The liquids studied are chloroform, carbon disulfide, o‐xylene, m‐xylene, 1‐3‐5‐trimethyl benzene, cumene, methyl cyclohexane, ethyl bromide, ethyl iodide, isobutyl alcohol, ethyl acohol, and deuterium oxide. The piezo‐ and elasto‐optic properties of these liquids are found to be similar to those of other liquids described in Papers I and II of this series. For all these liquids, the data reveal pronounced nonlinear piezo‐optical (Δn vs P) behavior, but linear elasto‐optical (Δn vs e, when e is the Eulerian strain) relationship. Thus, the Eulerian representation of strain is advantageous in describing the elasto‐optic behavior of liquids, irrespective of the shape, size, and composition of the molecules of the liquid. The linear relationship between Δn and Eulerian strain can be used to discriminate between the various equations of state. It may also provide a simple empirical expression for the computation of density from the accurately measured values of the changes in refractive index of liquids. The various expressions prevalent in the literature for the relationship between refractive index and density were analyzed by least‐squares analysis technique. It is shown that the Drude and Lorentz–Lorenz equations (which are most widely used in the literature on such studies) give either too low or too high values of Δn. Kirkwood–Brown and Omini equations which were derived using detailed statistical mechanical approach, appear to yield values in moderately good agreement with experimental values.