Ramsay-Shields equation

Ramsay-Shields equation An equation relating the molecular surface energy of a liquid with its temperature... 

It should be noted that Eotvos, Ramsay-Shields and Sugden s parachor equations are empirical in nature and their theoretical foundations are rather obscure. There have been several attempts to associate these equations with strict thermodynamical terms, but none have been successful. 

Herz11 calculated cr0 at the abs. zero by putting T=0 and v—l/q0, where 0=density at abs. zero, in the Ramsay and Shields equation ... 

MV which is equal to molecular volume can also be written as M/p where p is equal to the density of the liquid. The Ramsay and Shields equation then takes the form... 

Figure 3.6 Decrease in water surface tension with temperature. This dependency can be described by the Ramsay-Shields or the Eotvos equations fTc is the critical temperature) ...

The surface tension of most liquids decreases with increasing temperature in a nearly linear fashion (some metal melts being exceptional in this respect) and becomes very small in the region of the critical temperature, when the intermolecular cohesive forces approach zero. A number of empirical equations have been suggested which relate surface tension and temperature, one of the most satisfactory being that of Ramsay and Shields ... 

The accuracy of (5) is within the limits of experimental error for many liquids. As the linear relation does not usually hold near the critical temperature, (6) is less accurate, though it is preferable theoretically as containing one less arbitrary constant. Refinements of the linear equation have usually been made along one of two lines the first is to follow Eotvos s plan of introducing the twro-thirds power of the molecular volume, as in equation (1) of Ramsay and Shields, or Katayama s modification2... 

Since surface tension vanishes roughly 6 C above the critical temperature rather than at the critical temperature, Ramsay and Shields proposed the following equation for the temperature-dependence of surface tension ... 

For fused metals anomalous results for the association factor x below 1 are found, whilst fused salts give peculiar results, sometimes indicating an association factor of 10. (In such cases, equation (1) must be used, since the critical temperatures are unknown and must be very high.) These are, however, extreme cases of application. The abnormalities in general seem to be connected with the value of the critical temperature, At present, it is fairly generally agreed that, whilst low values of k for liquids about room temperature point to association, the calculation of an association factor x by Ramsay and Shields s method is unjustified. ... 

The above equation holds quite accurately for normal liquids at temperatures much nearer to the critical vedue than does the original form of Eotvos. In equation (4), according to Ramsay and Shields, the surface tension of the liquid becomes zero when the temperature reaches a value which is lesser than the critical temperature t, by 6°C. It follows that at critical temperature the surface tension of a normal liquid will become negative. The above prediction, however, is not universal. Several liquids have shown a value of zero only at the critical temperature and not earlier. For such liquids, Katayama s equation Is found to be the most suitable. It is expressed... 

Define surface tension. What are its imits Write the Ramsay and Shield s equation and explain the terms used in it. 

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