Building a Hofmeister-like series for the maximum in density temperature of aqueous electrolyte solutions.

Abstract

The temperature of the maximum in density (TMD) at room pressure is experimentally evaluated for aqueous solutions of a set of halides containing F−, Cl−, Br−, I−, Li+, Na+, K+, Rb+, Cs+ and Mg2+ at a 1 m concentration. The measurements were performed by monitoring the density- temperature profiles and tracking the temperature-dependent position of the meniscus, in a capillary glass tube. Adding salts diminishes the TMD of the solutions with respect to pure water, being the magnitude of the change dependent on the nature of the electrolyte. The experimental values of the shift in the TMD can be split into individual ion contributions. From this information we were able to establish a rank of ions (i.e. a Hofmeister-like series) according to their efficiency in shifting down the TMD. The experimental results are also compared to simulation values obtained via Molecular Dynamics using the Madrid-2019 force field that assigns non-integer charges for the ions and is parametrized for the TIP4P/2005 water model. Finally, since the TMD is a fingerprint property of water, we will discuss the impact of ions on this maximum in relation with the way different ions modify the structure of water.