The liquid–liquid coexistence curve and the interfacial tension of the methanol–n-hexane system

Abstract

At atmospheric pressure, the bulk density and liquid–liquid coexistence curve for the binary system of methanol and n-hexane have been measured as a function of temperature in the range 293–308 K. Comparison with the most reliable of the sparse literature data shows good agreement. A video image processor designed for automatic digital measurement has been used to determine the interfacial tension by the pendant drop technique. Results into the 0.01 mN m⁻¹ range have been readily obtained. The coexistence envelope has been analysed in terms of near critical extended scaling theory using non-asymptotic Wegner corrections to indirectly determine the upper critical solution temperature at 309.5 K, in excellent agreement with contemporary work. Of the possible order parameters considered (mole, mass and volume fraction), the volume fraction difference proved the most satisfactory, yielding a critical exponent consistent with 3D Ising universality. Simple scaling relations for the interfacial tension are inadequate for reduced temperatures beyond 10⁻².