Critical Liquid-Vapor Interface in SF6. I. Thickness of the Diffuse Transition Layer

Below the critical point the liquid-vapor interface of a simple fluid is characterized by a thin transition layer across which the values of density vary between the two phases. This transition layer can be described by a density profile characterized by an effective interface thickness $L$. As the temperature $T$ approaches the critical temperature $T_c$, $L$ diverges as ${\epsilon }^{-{\nu }^{\prime }}$, where $\epsilon =1-\frac{T}{T_c}$. From the measurement of optical reflectivity of the liquid-vapor interface of sulfur hexafluoride, we have deduced the interface thickness and the critical index $\beta$ for the coexistence curve. In the temperature range $9.73\times {10}^{-5\mathrm{}}\le \epsilon \le 5.33\times {10}^{-2\mathrm{}}$ we find ${\nu }^{\prime }=0.620\mathrm{}\pm 0.01$ if an error-function profile is assumed as proposed by Buff, Lovett, and Stillinger, or ${\nu }^{\prime }=0.663\mathrm{}\pm 0.02$ if the density profile proposed by Fisk and Widom is used. For both profiles $\beta$ is found equal to 0.333 ± 0.008. These indices, when combined with the critical index ${\gamma }^{\prime }$ for the isothermal compressibility and $\mu$ for the surface tension, are used to test various scaling relations.