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The pendant-drop method combined with axisymmetric drop-shape analysis was used to determine liquid−vapor and liquid−liquid interfacial tension of mixtures consisting of heptane (1), toluene (2), and N,N-dimethylformamide (3). The method of pendant drop with an efficient temperature control of the measuring cell allows precision measurements of surface tension and interfacial tension. Surface tensions and liquid densities of more then 20 ternary mixtures in the homogeneous region were determined at a normal pressure of 1 bar and over a temperature range from 287 K to 317 K. Inside the miscibility gap of this system, liquid densities in both liquid phases using buoyancy experiments dependent on temperature were obtained. Furthermore, the pendant-drop technique allows measurements of the interfacial tension between the two liquid phases even close to the critical solution point for the ternary mixture. The resulting interfacial tension and the density difference vs temperature up to the critical solution point could be well described using a Wegner-type expression.

The pendant drop method, combined with efficient temperature control of the measuring cell, allows high precision measurements of both surface tension and interfacial tension. The surface tensions and liquid densities of mixtures of cyclohexane + heptane, cyclohexane + propanone, and toluene + propanone have been determined at a normal pressure of 1 bar over the temperature range from 287 K to 317 K. The surface tension data could be parametrized with a cubic polynomial. Gibbs excess surface concentrations are derived from our experimental data, and the influence of activity coefficients is discussed. The high precision pendant drop apparatus allows accurate measurements of the interfacial tension in systems with liquid−liquid phase separation. In binary mixtures of heptane + N,N-dimethylformamide, heptane + N-methyl-2-pyrrolidone, and decane + N,N-dimethylformamide, the interfacial tension was measured over a wide temperature range close to the critical solution point. The densities of the corresponding liquid phases were obtained by buoyancy measurements. The accuracy of the density measurements is Δρ = ±0.0001 g·cm^-3 for the vibrating tube densimeter; for the buoyancy measurements we find Δρ = ±0.0005 g·cm^-3. The reproducibility of the surface tension measurements is generally better than Δσ = ±0.50% at the 95% confidence level. Standard deviations for all experimental values are given in the tables. The resulting data are parametrized using polynomial and Wegener expansion type equations.

The pendant drop method, combined with efficient temperature control of the measuring cell, allows high precision in surface tension measurements. The surface tensions of heptane, toluene, N,N-dimethylformamide, cyclohexane, N-methyl-2-pyrrolidone, and propanone were measured as a function of temperature using the pendant drop method. The results were compared with literature data. The surface tension and density of toluene + heptane and N,N-dimethylformamide + toluene at atmospheric pressure were measured over a temperature range. Gibbs excess surface concentrations are derived from the experimental surface tensions, and the influence of activity coefficients is discussed.