Browsing by Author "Drexler, C. I."

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Open Access
Local electric fields in aqueous electrolytes
(American Chemical Society, 2021-07-27) Drexler, C. I.; Cracchiolo, O. M.; Myers, R. L.; Okur, Halil İbrahim; Serrano, A. L.; Corcelli, S. A.; Cremer, P. S.
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 Mg2+ 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.
Open Access
Molecular mechanism for the ınteractions of hofmeister cations with macromolecules in aqueous solution
(American Chemical Society, 2020) Bruce, E. E.; Okur, Halil I.; Stegmaier, S.; Drexler, C. I.; Rogers, B. A.; van der Vegt, N. F. A.; Roke, S.; Cremer, P. S.
Ion identity and concentration influence the solubility of macromolecules. To date, substantial effort has been focused on obtaining a molecular level understanding of specific effects for anions. By contrast, the role of cations has received significantly less attention and the underlying mechanisms by which cations interact with macromolecules remain more elusive. To address this issue, the solubility of poly(N-isopropylacrylamide), a thermoresponsive polymer with an amide moiety on its side chain, was studied in aqueous solutions with a series of nine different cation chloride salts as a function of salt concentration. Phase transition temperature measurements were correlated to molecular dynamics simulations. The results showed that although all cations were on average depleted from the macromolecule/water interface, more strongly hydrated cations were able to locally accumulate around the amide oxygen. These weakly favorable interactions helped to partially offset the salting-out effect. Moreover, the cations approached the interface together with chloride counterions in solvent-shared ion pairs. Because ion pairing was concentration-dependent, the mitigation of the dominant salting-out effect became greater as the salt concentration was increased. Weakly hydrated cations showed less propensity for ion pairing and weaker affinity for the amide oxygen. As such, there was substantially less mitigation of the net salting-out effect for these ions, even at high salt concentrations.