Coexistence curve of methanol+<i>n</i>-heptane: Range of simple scaling and critical amplitudes

Abstract

The coexistence curve of methanol+n-heptane has been determined. The difference between the volume fraction \ensuremath{\varphi} of coexisting phases has been found to be the correct order parameter. The results have been analyzed in terms of two thermodynamic fields, t=(T-${\mathit{T}}_{\mathit{c}}$)/${\mathit{T}}_{\mathit{c}}$ and t'=(T-${\mathit{T}}_{\mathit{c}}$)/T. While a simple scaling law fits the whole set of results when t' and \ensuremath{\varphi} are used, it is only valid up to t\ensuremath{\le}8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ when t is used with either \ensuremath{\varphi} or the mole fraction X, and one correction-to-scaling term is needed. The determination of the amplitudes of the correction-to-scaling terms is discussed in detail. The diameter of the coexistence curve has been found to show a (1-\ensuremath{\alpha}) anomaly when \ensuremath{\varphi} is used, while a 2\ensuremath{\beta} one when X is used. No improvement in the diameter analysis has been attained in terms of a crossover formulation. The pressure dependence of ${\mathit{T}}_{\mathit{c}}$ has been measured, the result being used to calculate the amplitude of the isobaric expansion coefficient.