Published: February 22, 2011 r2011 American Chemical Society 2234 dx.doi.org/10.1021/jp1116307 | J. Phys. Chem. B 2011, 115, 2234–2242 ARTICLE pubs.acs.org/JPCB Dielectric Relaxation Study of the Ion Solvation and Association of NaCF 3 SO 3 , Mg(CF 3 SO 3 ) 2 , and Ba(ClO 4 ) 2 in N,N-Dimethylformamide Anna Pzaczek, †,‡ Glenn Hefter,* ,† Hafiz M. A. Rahman, ‡ and Richard Buchner* ,‡ † Chemistry Department, Murdoch University, Murdoch, WA 6150, Australia ‡ Institut f€ ur Physikalische und Theoretische Chemie, Universit € at Regensburg, D-93040, Regensburg, Germany b S Supporting Information ABSTRACT: Solutions of sodium trifluoromethanesulfonate, mag- nesium trifluoromethanesulfonate, and barium perchlorate in N,N- dimethylformamide (DMF) have been investigated using broadband dielectric relaxation spectroscopy at 25 °C. All spectra were domi- nated by a solvent relaxation process centered at ∼15 GHz but also exhibited one (for NaCF 3 SO 3 ) or two (for the 2:1 salts) low- amplitude processes, centered at frequencies below 2 GHz, that could be attributed to the presence of ion pairs. Effective solvation numbers calculated from the solvent relaxation amplitudes indicated strong solvation of all three cations, with evidence for the formation of a second solvation sheath for Mg 2þ and possibly Ba 2þ . Detailed analysis of the solute-related processes showed that solvent-shared ion pairs (SIPs) were formed in NaCF 3 SO 3 solutions in DMF. The data for Mg(CF 3 SO 3 ) 2 and Ba(ClO 4 ) 2 solutions were not definitive but, consistent with the solvation evidence, favored the presence of double solvent-separated ion pairs and SIPs. Overall association constants, K A , were small for all three salts in DMF and increased in the order: NaCF 3 SO 3 < Ba(ClO 4 ) 2 < Mg(CF 3 SO 3 ) 2 . 1. INTRODUCTION Dielectric relaxation spectroscopy (DRS) is a widely applicable but relatively under-utilized technique for the study of electrolyte solutions. 1-3 In principle, it can provide detailed and quantitative information about both the dynamics and thermodynamics of such solutions and, in favorable cases, even structural insights. While most published DRS studies have been of aqueous systems, the technique is readily applicable to nonaqueous solutions. Molecular- solvent solutions investigated to date have included: propylene carbonate, 4,5 acetonitrile, 6,7 methanol, 8 formamide, 9,10 N-methyl- formamide, 9,10 and N, N-dimethylacetamide, 11 although such studies have in general been limited to a narrow range of monovalent (1:1) electrolytes. Liquid N,N-dimethylformamide (DMF) is a versatile solvent for organic and inorganic substances and has been employed extensively as a solvent for polymers and paints and as a reaction medium. 12 It has a reasonably high dielectric constant (ε = 36.71 at 25 °C) 13 and a sizable dipole moment (μ = 3.86 D). 13 With a Gutmann donor number of 26.6, 13 DMF is a strong electron- density donor and thus a powerful solvator of cations. On the other hand, it is a rather poor solvator of anions, with a Gutmann acceptor number of just 16.0 (similar to acetonitrile and propy- lene carbonate). 13 As might be expected from these generally favorable character- istics, the physicochemical properties of electrolyte solutions in DMF have been widely investigated. Such studies have included determinations of Gibbs energies, 14 enthalpies and entropies, 15 and viscosities, 16 although again most were restricted to relatively small numbers of 1:1 electrolytes. Recently, a comprehensive report of the molar volumes and heat capacities of a wide range of electro- lytes, including non-1:1 charge types, has appeared. 17 An ongoing problem for such studies is the reliable extrapola- tion to infinite dilution of experimental data measured at finite electrolyte concentrations to obtain the desired standard state properties. Because the extrapolation functions used for this purpose (such as the Redlich-Meyer and Pitzer equations) 18 assume that electrolytes are fully dissociated, this can lead to significant errors in the standard state values. 19 This difficulty is usually more pertinent to electrolyte solutions in nonaqueous solvents (because such solutions typically exhibit higher levels of ion association than their aqueous counterparts) 20,21 and be- comes acute when dealing with salts containing higher-charged ions. While theoretical treatments have been developed that make allowance for ion association, 22 the required equilibrium constants are almost invariably lacking for nonaqueous solutions, and the available methods for estimating such quantities 21 are too Received: December 7, 2010 Revised: January 11, 2011