Journal of Molecular Liquids, 138 (2008) 51-54. doi:10.1016/j.molliq.2007.07.004 You can ask full-text to the following authors: https://www.researchgate.net/profile/Rafik_Besbes or : https://www.researchgate.net/profile/Habib_Latrous Self-diffusion coefficients and structure of the trivalent f-element ions, Eu, Gd, Am, Bk, Cf and Es in aqueous diluted and concentrated solutions H. Latrous , a , , R. Besbes a and N. Ouerfelli a a Faculté des Sciences de Tunis, 2092 El-Manar, Tunisia . Abstract Self-diffusion coefficients D of the trivalent ions in aqueous solutions of Eu and Gd have been determined in electrolyte support Gd(NO 3 ) 3 , and of Am, Bk, Cf and Es in electrolyte support Nd(ClO 4 ) 3 , in concentrated solutions at (pH = 2.5) HNO 3 , HClO 4 and at 25 °C using the open end capillary method (O.E.C.M.). This method measures the transportation time of ions across a fixed distance. In this paper we complete a measurement of self-diffusion coefficient at concentrated solution (1.5 M). We optimized the pH in order to avoid the hydrolysis, pairing and complexing trivalent 4f and 5f ions. The behavior of D and Dη are plotted versus √C. The similarities observed of self-diffusion coefficients of trivalent f-elements ions can be explained by a similar electronic configuration and same hydration number. The variation of D versus the square root of the concentration of the solution was found to be quasi-linear in the studied concentration range (C ≤ 1.5 M). Results of the diffusion for the two couples (Am–Eu) and (Bk–Gd) show us that 4f and 5f elements, having similar electronic configuration and/or sub shell half-filled configuration of trivalent ions, have the same behavior. We compared variation of D/D° versus √C for all studied 4f and 5f elements from concentration 0 to 1.5 M and we obtained the same variation with √C for all studied elements. We have estimated activity coefficients for both 4f and 5f trivalent ion elements using Nernst –Hartley expression. We deduced that we can achieve self- diffusion experience using 4f support electrolyte as Nd(ClO 4 ) 3 in acid medium and needing only some 5f element tracer element. This method is adequate for the study of 5f element which is not available as a macro element such as heavy 5f elements fermium Fm 3+ and mendelevium Md 3+ . Keywords: O.E.C.M; Self-diffusion coefficient; Trivalent 4f; 5f; Ions; Activity coefficient; Electronic structure; Support electrolyte; Tracer scale. 1. Introduction In a first study [1] , our measure of self-diffusion coefficient D i of the trivalent ions Am 3+ , Cf 3+ and Es 3+ at 25 °C and at pH = 2.5 in supporting electrolyte Nd(ClO 4 ) 3 with added HClO 4 , and 152 Eu 3+ and 153 Gd 3+ in supporting electrolyte Gd(NO 3 ) 3 with added HNO 3 allowed to show that the size of trivalent actinides increases slowly from Am to Cf –Es. Similar work on theses elements was presented by Fourest et al. [2] . In another work [3] , we have compared self-diffusion measurements of trivalent ions 152 Eu 3+ (4f) and 241 Am 3+ (5f) at pH = 2.5 in different concentrations, which showed that the ionic transport process for 152 Eu 3+ is similar to that for 241 Am 3+ . Moreover, it may be argued that 152 Eu 3+ (4f) ion in solution has the same structure as tripositive 5f ion in the absence of hydrolysis, pairing or complexing. Our measures [5] of self-diffusion coefficients for the trivalent ions of Berkelium, Americium and Europium for molar concentrations C (mol l − 1 ) 10 − 5 ≤ C ≤ 2.10 − 2 and in the same conditions, in absence of any phenomenon of hydrolysis and of complexing, show that the ions Am 3+ and Eu 3+ have the same behaviour of salvation. The measures of the self-diffusion coefficients D [4] and [5] of the ion Bk 3+ in 25 °C and at pH = 2.5, compared in the same conditions with those of the ions Am 3+ and Eu 3+ asserted the non-linear variation of D according to the square root of the concentration molar D = f (√C). The experimental results compared with the measures made by Weingärtner et al. [4] were satisfactory. Recent works [9] , carrying out measures Red/Ox of the couple Bk 4+ /Bk 3+ and distance Bk…OH 2 by RX show well that the ion Bk 3+ is hydrated by 9 water molecules; also Tilkens et al. [10] studied the crystalline structure of both complexes of hydrates of Europium and concluded that the number of hydration of the ion Eu 3+ fluctuated between 8 and 9.The research for the number of hydration of the trivalent ions Pu 3+ and Am 3+ , by dynamic calculation and by modelling gave also a value of 9. These values are comparable to the values deducted by absorbance and the spectroscopy diffraction for the ions Nd 3+ , Sm 3+ and Am 3+ . Moreover, Lindqvist-Reis et al. [11] have shown by measure by Laser fluorescence spectroscopy (TRLFS) that there is an equilibrium between two discernible species: “8-hydrated Cm 3+ ” and “9-hydrated Cm 3+ ”, with a dominance of the 9- coordinate species at room temperature. 2. Work presentation For the 5f elements, in literature, direct measurements of self-diffusion coefficients D concern only 5 elements. For the remaining 5f elements, D° are deduced from Nernst–Einstein expression by conductimetric or mobility measurements. Singular laboratories have at its disposal 5f radioactive elements.