Probing the Hofmeister series beyond water: Specific-ion effects in non-aqueous solvents

Abstract

We present an experimental investigation of specific-ion effects in non-aqueous solvents, with the aim of elucidating the role of the solvent in perturbing the fundamental ion-specific trend. The focus is on the anions: CHCOO>F>Cl>Br>I>ClO>SCN in the solvents water, methanol, formamide, dimethyl sulfoxide (DMSO), and propylene carbonate (PC). Two types of experiments are presented. The first experiment employs the technique of size exclusion chromatography to evaluate the elution times of electrolytes in the different solvents. We observe that the fundamental (Hofmeister) series is observed in water and methanol, whilst the series is reversed in DMSO and PC. No clear series is observed for formamide. The second experiment uses the quartz crystal microbalance technique to follow the ion-induced swelling and collapse of a polyelectrolyte brush. Here the fundamental series is observed in the protic solvents water, methanol, and formamide, and the series is once again reversed in DMSO and PC. These behaviours are not attributed to the protic/aprotic nature of the solvents, but rather to the polarisability of the solvents and are due to the competition between the interaction of ions with the solvent and the surface. A rule of thumb is proposed for ion specificity in non-aqueous solvents. In weakly polarisable solvents, the trends in specific-ion effects will follow those in water, whereas in strongly polarisable solvents the reverse trend will be observed. Solvents of intermediate polarisability will give weak specific-ion effects.

Theoretical and Computational Insight into Solvent and Specific Ion Effects for Polyelectrolytes: The Importance of Local Molecular Interactions

“…With regard to this point, some ion pairs are more stable than others which also depends on the chosen solvent and further components of the solution. Corresponding experimental and simulational approaches suggested that this behavior is universal and occurs for all ions in aprotic as well as protic solvents due to different solvation interactions [18,20,34,74,[90][91][92]. Previous explanations for these effects often cite the law of matching water affinities (LMWA), which states that ion pairs with comparable water affinities reveal the highest ion pair formation stabilities [93].…”

Properties of Ion Complexes and Their Impact on Charge Transport in Organic Solvent-Based Electrolyte Solutions for Lithium Batteries: Insights from a Theoretical Perspective

“…In contrast to simple electrostatic mean-field approaches, recent numerical and experimental findings indeed revealed the occurrence of various ion complex states. Hence, ions and solvent molecules aggregate within the Bjerrum length to ion-solvent complexes with distinct stoichiometries, as influenced by the ion species, the ion concentration, and the molecular properties of the solvent molecules [45,53,63,[67][68][69][70][71][72][73][74][75][76][77][78].…”

“…For some solvents and specifically for water, differences in the complex formation tendencies for distinct ions are observed, which can be rationalized by the influence of specific ion effects [38,54,63,68,70,75,[83][84][85][86]. These effects are often observed in aqueous solutions, whereas studies of specific ion effects in organic solvents are rather sparse [30,[76][77][78]87]. A series of seminal studies [35][36][37] regarding the solubility of various lithium salts in acetonitrile reveals the following order, LiPF 6 < LiFSI < LiTFSI ≤ LiClO 4 < LiBF 4 LiCF 3 CO 2 , which coincides with the observed tendency of ion complex formation.…”

“…Moreover, it has to be noted that the LMWA approach reveals some similarities with the famous hard/soft acids and bases (HSAB) principle, which states that hard Lewis acids as electron acceptors prefer to coordinate with hard Lewis bases as electron donors, and soft acids prefer to coordinate with soft bases [91]. As already briefly mentioned, few studies are devoted to the occurrence of specific ion effects in non-aqueous solutions [30,38,[76][77][78]83]. With regard to the corresponding results, it has been observed that the LMWA approach is also applicable for several organic solvents, such that it can be generalized to a law of matching solvent affinities [83].…”

“…With regard to the corresponding results, it has been observed that the LMWA approach is also applicable for several organic solvents, such that it can be generalized to a law of matching solvent affinities [83]. Recent experimental results [78,83] thus reveal the applicability of this concept for various organic solvents such as methanol, formamide, PC, and dimethyl sulfoxide (DMSO).…”