Testing the Lorentz–Lorenz relation in the near-critical binary fluid mixture isobutyric acid and water

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Abstract
The refractive index in each phase of the binary fluid mixture isobutyric acid and water was measured at temperatures below the system’s upper consolute point. This data was combined with existing density data to test the Lorentz–Lorenz relation in a near‐critical binary fluid mixture. The Lorentz–Lorenz relation is verified within experimental error (0.5%) when the volume change on mixing the components is taken into account. The density coexistence curve data is reanalyzed to determine the critical exponent β and amplitude B. By allowing the order parameter to be a definition of the volume fraction that includes volume loss on mixing, a very symmetric coexistence curve is obtained which can be described by simple scaling with β=0.326±0.003 and B=1.565±0.021. This exponent agrees with theoretical predictions while the amplitude, when combined with existing turbidity data, confirms two‐scale‐factor universality. The amplitude obtained by analyzing the coexistence curve when the refractive index is the order parameter also combines with turbidity data to confirm two‐scale‐factor universality, but does not require knowledge of the volume loss on mixing or the composition dependence of the refractive index.

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