Are Nanoscale Ion Aggregates Present in Aqueous Solutions
of Guanidinium Salts?
J. Hunger, S. Niedermayer, R. Buchner, and G. Hefter,
J. Phys. Chem. B 114 (2010) 13617-13627.
A detailed investigation using broadband dielectric relaxation spectroscopy (DRS) has been made of the aqueous
solutions of guanidinium chloride and carbonate, GdmCl(aq) and Gdm2CO3(aq), at 25 °C.
The spectra indicate that Gdm+ ions, C(NH2)3+, do not bind
strongly to water nor are theyhydrophobically hydrated; rather they appear to have a most unusual ability to dissolve in
water without altering its dynamics. Although DRS is particularly sensitive to the presence of ion pairs,
only weak ion pairing was detected in Gdm2CO3(aq) solutions and none at all in GdmCl(aq).
Surprisingly, no evidence was found for the existence of the higher order homo- and heteroionic nanoscale aggregates that
have been identified in recent years by Mason and co-workers using molecular dynamics simulations and
neutron diffraction. Possible reasons for this discrepancy are discussed. The present DR spectra and
other solution properties of GdmCl(aq) and Gdm2CO3(aq), such as apparent molar volumes
and electrical conductivities, are shown to have strong similarities to those of the corresponding Na+
salts. However, such solutions also differ remarkably from their Na+ analogues (and all other simple
electrolytes in aqueous solution) in that their average water relaxation times correlate strongly with
their bulk viscosities. The biological implications of the present results are briefly discussed.