(676b) Explicit MOLECULAR-BASED Analysis of the EFFECT of Urea ON the ION PAIR ASSOCIATION IN METAL Sulfates AQUEOUS-Urea Solutions

The effect of added urea on the properties of surfactant aggregates and proteins in aqueous solutions has been the target of intense research effort toward the interpretation of their stability and the elucidation of the underlying mechanism. The outcome of these studies can be summarized in terms of two possible mechanisms: (a) one based on the favorable H-bonding interactions between urea and water that enhances solute solubility by the replacement of water from the hydration shell of hydrophobic sites, and (b) the other based on the energetically favorable water re-structuring by the presence of urea around the solutes that facilitates dissolution of hydrophobic moieties.  Regardless of their validity, these two seemingly different mechanisms share in principle a common microscopic reality, i.e., the urea-induced change of polarity of the local environment around dissolved species, and the corresponding effect on the dielectric permittivity of the resulting fluid mixtures. 

In the context of a ‘continuum’ (primitive) picture, the added urea becomes a dielectric modifier, i.e., a means to manipulate the screening of electrostatic interactions.  Obviously, the actual aqueous-urea solvent does not behave as a continuum dielectric, and consequently, we cannot draw any unambiguous link between the species polarity, the solvent composition, its dielectric constant, and the stability of the solutes in solution, unless we have a detailed description of the system’s microstructure. In this context molecular-based simulation can aid in the search for the mechanism behind the urea effect on the stability of aqueous electrolytes, through the molecular-based characterization of the local environments around the ions at the prevailing global state conditions, to link that microstructural information to the macroscopic behavior under investigation.

Here we analyze, by means of isobaric-isothermal molecular dynamics simulations, the hydration behavior of aqueous-urea nickel sulfate, at ambient conditions and constant ionic strength, to determine explicitly the urea effect on the extent of the ion-pair formation according to Poirier-DeLap formalism ¹.  The first goal is to gain valuable insights into the potential link between the dielectric permittivity of the mixed-solvent, the degree of water-water and water-urea hydrogen bonding, the degree of ion-pairing, and consequently, the role played by urea in the stabilization of moieties in solution.  The second goal is to exploit the generated microstructural data in the exploration of alternative first-order difference routes to extract structural information from Neutron Diffraction with Isotopic Substitution (NDIS) experiments in order to mitigate the problems associated with the partial contributions of ion pairing to the neutron-weighted distribution functions ².

¹                  J. C. Poirier and J. H. DeLap, Journal of Chemical Physics 35, 213 (1961).

²                  A. A. Chialvo and J. M. Simonson, Molecular Physics 100, 2307 (2002); A. A. Chialvo and J. M. Simonson, Journal of Chemical Physics 124 (15) (2006); A. A. Chialvo and J. M. Simonson, Collec. Czech. Chem. Commun. 75 (4), 405 (2010).