The variation of the superficial excess of entropy in relation to the structure of water in sodium fluoride solutions

Abstract

The mercury-NaF aqueous solutions interfacial tension has been measured at varying concentrations and temperatures. These results, associated with those of other authors [1,2], enable one to determine, without ambiguity, and for all accessible values of the electrode charge, the sign and the order of magnitude of the variation with the temperature of the superficial excess of entropy. The curves showing the variations of the superficial excess of concentration with the charge indicate a very slight specific adsorption of the fluoride ions at low temperatures which leads to a contribution of about 0.007 erg K −1 cm −2 to the entropy. These results have been compared with those forecast theoretically by Bockris and Habib [3] who schematically consider the interfacial layer of water as being composed of monomers and dimers, the latter being unaffected by the electrical field. Taking into account the variation with the temperature of the various factors contributing to the interfacial entropy, as envisaged by these authors, one finds a variation of −0.04 erg K t-1 cm −2 as compared with +0.14 erg K −1 cm −2 found experimentally. This reveals the shortcomings of the proposed model in spite of the good agreement obtained at one temperature for the variations with the charge. This model can therefore only be considered as acceptable if supplementary hypotheses are made as for example assuming interactions between the different molecular forms. One can also consider that this type of dimer or even larger aggregate such as those envisaged by Frumkin [4] and Parsons [5] for the interfacial water, consistutes a somewhat over simplified approach of the problem. The experimental results obtained and their examination in the light of recent theories show that the important problems of the structure of the interfacial water still remains unsolved. There is therefore a need for a statistical approach to these questions based the theories suggested for the volume, and also taking into account the influence of the substratum.