Are Nanoscale Ion Aggregates Present in Aqueous Solutions of Guanidinium Salts?

Abstract

A detailed investigation using broadband dielec. relaxation spectroscopy (DRS) has been made of the aq. solns. 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 they hydrophobically 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) solns. 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 mol. dynamics simulations and neutron diffraction. Possible reasons for this discrepancy are discussed. The present DR spectra and other soln. properties of GdmCl(aq) and Gdm2CO3(aq), such as apparent molar volumes and elec. conductivities, are shown to have strong similarities to those of the corresponding Na+ salts. However, such solns. also differ remarkably from their Na+ analogs (and all other simple electrolytes in aq. soln.) in that their av. water relaxation times correlate strongly with their bulk viscosities. The biol. implications of the present results are briefly discussed.