Comment on “ On the relationship between the entropy and enthalpy of interfacial segregation ” by L . Rubinovich and M . Polak

The paper of L. Rubinovich and M. Polak [Eur. Phys. J. B 22, 267 (2001)] contains some controversial items. The notation “excess” in connection with various thermodynamic quantities describing solute segregation on the basis of the Langmuir-McLean isotherm is used in a meaning contradictory to general thermodynamics. The approach used to interpret the linear relationship between segregation entropy and enthalpy results in rather strange conclusions. These items are discussed, and it is shown how a formulation with standard terms is in accordance with general thermodynamics. PACS. 05.70.Np Interface and surface thermodynamics – 75.50.Bb Fe and its alloys – 64.75.+g Solubility, segregation, and mixing; phase separation Recently, Rubinovich and Polak published a contribution to the presently discussed relationship between enthalpy and entropy of interfacial segregation [1]. Besides an original look at this correlation, their paper demonstrates an ambiguity which exists in the terminology of thermodynamic description of segregation and which is likely to result in confusion of the reader. The purpose of this comment is (i) to point out the difficulties that may arise with such an approach, and (ii) to elucidate other discrepancies of their work, and (iii) to enhance the basic understanding of segregation phenomena. Although the term “excess” free energy of segregation is sometimes used in papers devoted to solute segregation, its use is not consistent with general thermodynamics. An example of such an incorrect application is demonstrated by Rubinovich and Polak in paper [1] as well as in their extended review [2]. The approach used by them is based on the idea that the total change of Gibbs free energy due to segregation, ∆GI , that is equal to zero in equilibrium, can be decomposed into two parts, a configurational term and a remaining term [3]. In principle, this approach is correct, however, sometimes an inappropriate notation of these terms is used in the literature as demonstrated in references [1,2]: the configurational and the rea e-mail: [email protected] maining terms are called ideal, ∆G I , and excess, ∆G ex I , Gibbs free energy of segregation, respectively. Using this terminology, the Langmuir-McLean segregation isotherm can be expressed as cp 1− cp = c 1− c exp ( − ex I