Local Electric Fields in Aqueous Electrolytes

Abstract

Vibrational Stark shifts were explored in aqueous solutions of organic molecules with carbonyl-and nitrile-containing constituents. In many cases, the vibrational resonances from these moieties shifted toward lower frequency as salt was introduced into solution. This is in contrast to the blue-shift that would be expected based upon Onsager's reaction field theory. Salts containing well-hydrated cations like Mg 2+ or Li + led to the most pronounced Stark shift for the carbonyl group, while poorly hydrated cations like Cs + had the greatest impact on nitriles. Moreover, salts containing I − gave rise to larger Stark shifts than those containing Cl − . Molecular dynamics simulations indicated that cations and anions both accumulate around the probe in an ion-and probe-dependent manner. An electric field was generated by the ion pair, which pointed from the cation to the anion through the vibrational chromophore. This resulted from solvent-shared binding of the ions to the probes, consistent with their positions in the Hofmeister series. The "anti-Onsager" Stark shifts occur in both vibrational spectroscopy and fluorescence measurements.

Expulsion of Hydroxide Ions from Methyl Hydration Shells

“…Our EFP-MD simulations further indicate that the Na + countercation population is located primarily near the hydroxy head group of TBA, and its hydration-shell concentration is correlated with that of the anion, which is more nearly uniformly distributed around TBA, with an elevated I – concentration and depleted OH – near TBA methyl groups. This counterion correlation is similar to that recently inferred from experimental and simulation studies of other amphiphilic solutes in aqueous salt solutions . Comparisons of the present results with prior measurements of the solubilities of nonpolar solutes (including H 2 , O 2 , methane, n-butane, and benzene), ,, as well as recent simulations of methane in various aqueous solutions, confirm that the decreased affinity of anions for nonpolar hydration shells with decreasing anion size also extends to amphiphilic solutes such as TBA.…”

“…Additionally, comparisons of ion distributions relative to the TBA CC, CM, and OT atoms indicate that the cation and anion have some preference for residing in different parts of the TBA hydration shell, with Na + often located near the TBA hydroxyl head group and both OH − and I − having a slight preference for the hydration shell of the nonpolar (methyl groups) of TBA, although OH − is primarily present in the second hydration shell of TBA. This counterion localization behavior is similar to that recently found for other amphiphilic solutes in aqueous salt solutions 63. Our results further reveal that the local Na + concentration is anion dependent and increases with the affinity of the anion for the TBA methyl groups.…”

“…This counterion correlation is similar to that recently inferred from experimental and simulation studies of other amphiphilic solutes in aqueous salt solutions. 63 Comparisons of the present results with prior measurements of the solubilities of nonpolar solutes (including H 2 , O 2 , methane, n-butane, and benzene), 16,17,65 as well as recent simulations of methane 19 in various aqueous solutions, confirm that the decreased affinity of anions for nonpolar hydration shells with decreasing anion size also extends to amphiphilic solutes such as TBA. The expulsion of OH − from the three neighboring methyl groups of TBA, combined with the increased air−water surface tensions in aqueous NaOH, 66 and the increase in alkane−water interfacial tension in other salts containing small anions, including NaCl, 67 suggest that the expulsion of OH − from nonpolar interfaces occurs at both molecular and macroscopic length scales.…”

“…This counterion localization behavior is similar to that recently found for other amphiphilic solutes in aqueous salt solutions. 63 Our results further reveal that the local Na + concentration is anion dependent and increases with the affinity of the anion for the TBA methyl groups.…”

Vibrational Stark Effect Mapping of Polaron Delocalization in Chemically Doped Conjugated Polymers

“…We then argue that the behavior of the F 4 TCNQ – nitrile stretching modes can be quantitatively described by using the framework of the vibrational Stark effect (VSE). There is a rich existing literature using the vibrational Stark shifts for a variety of nitrile, carbonyl, and other pendant groups to directly measure the local electric fields experienced in biological systems, − self-assembled monolayers, − solvated ion pairs, − and other applications. − Thus, by measuring the vibrational Stark shift of the nitrile stretches on the F 4 TCNQ anion across a range of doped P3HT environments, we can directly infer the anion–hole separation distance. We also show that there is a direct correlation between the electric field experienced by the dopant anion and the degree of polaron coherence, allowing us to directly infer the degree of polaron delocalization.…”