An Investigation of the Surface Tension of Liquids Near the Critical Temperature

The surface tension-temperature relations of methyl ether and propylene have been investigated by the capillary rise method over a range of temperatures approaching more closely to the critical temperature than has hitherto been done. From the data obtained, it has been concluded that the molecular surface energy does not become equal to zero when the meniscus is no longer visible, since the surface tension-temperature curves obtained apparently did not become asymptotic to the temperature axis at the critical temperature, the latter being considered as the temperature at which the meniscus is no longer discernible by the wave-lengths of visible light. The angle of interception of the surface tension-temperature curve with the temperature axis has been interpreted as indicating a discontinuity in properties at the critical point.The data obtained have also been applied to the examination of various relations involving surface tension. The Katayama equation has been shown to be considerably more accurate than that of Ramsay and Shields. Sugden's equation, has been found valid over the ranges of temperature investigated. The Macleod relation has also been examined, and an increase in the Macleod constant with increase in temperature shown to occur in the case of propylene, while no marked or progressive increase was noticed in the case of methyl ether. Calculation of the parachors served to emphasize the difference in behavior of the two substances in this respect. By a consideration of available data on benzene, chlorobenzene, and carbon tetrachloride, together with those obtained in this investigation, the increase of parachor in the case of those substances having unsaturated linkages in their molecular structure has been ascribed to increased electronic interaction with increased temperature. This is analogous to an increase in unsaturation.An attempt was made to adapt the ring method to the investigation of surface tension in the critical region, by measuring the force necessary to effect removal of the ring from the surface of the liquid by means of a calibrated quartz spiral. Although the method was unsatisfactory for the problem at hand, the feasibility of the method has been demonstrated. From the data obtained it has been possible to verify the zero angle of contact between methyl ether and glass.