This special topic contains a diverse collection of 40 articles that span the vast range of subjects that fall under the heading “Ions in Water,” a longstanding mainstay of chemical physics. The investigations reported herein employ state-of-the-art theoretical, computational, and experimental techniques, as well as combinations thereof, to provide new insights into the fundamental aspects of ion solvation and the important roles that ions play in mediating physicochemical processes occurring in solutions and at interfaces in a wide variety of settings relevant to biological, environmental, and technological applications.

Aqueous ionic solutions course through our veins, fill biological cells, cover our planet and percolate through pores in its soils and crust, and are prevalent in particles in the atmosphere that contribute to air quality and Earth’s radiative balance. They are also key components of materials that are essential in energy generation and storage applications and in the clean-up of the adverse effects of human activities on the environment.

In light of their ubiquity and widespread importance, for more than a century, scientists have been investigating the complexities of the structure, dynamics, thermodynamics, and chemical reactivity of aqueous ionic solutions. The solvent water possesses hydrogen bonding nuances that continue to make its liquid and solid phases fascinating. The addition of ions modifies the hydrogen bonding network of water and often has profound consequences for the physicochemical properties of the resulting solution. The vast subject of ions in water has high relevance to a broad array of scientific disciplines, including biophysics, biochemistry, materials science, ocean chemistry, geochemistry, and aerosol and cloud chemistry. Although, historically, the majority of studies have focused on the bulk properties of aqueous ionic solutions, there has been a surge of interest in interfacial properties in the last couple of decades. Recent advances in theoretical and experimental approaches have prompted new questions, and the knowledge gained by addressing these questions has advanced the understanding of the role played by aqueous ionic solutions in a multitude of disciplines.

The diverse collection of contributions to this Special Topic cover ion solvation and pairing in bulk solution and at simple to complex interfaces to shed light on the details and consequences of ion-solvent interactions. This issue includes articles that dig deeply into a wide range of fundamental aspects of aqueous ionic solutions, including structural and dynamical properties, glass forming properties, chemical potentials, single ion free energies, ion clustering, ion-mediated interactions between nanoparticles, ion interstratification in clays, ions under extreme solvent conditions, ice nucleation in salty water, salt crystal nucleation, interfacial versus bulk ion pairing rates, zeta potentials of microemulsions, binding and interfacial interactions of ions with lipids, relative permittivities in electric double layers, the chemistry of reactive ionic species, and water-separated ion pairs and their association with slow dielectric relaxation modes. In addition to research on the bulk and interfacial properties of aqueous salt solutions, some of the articles in this special topic report on how ion behavior in non-aqueous protic solutions differ from but also inform our understanding of aqueous ionic solutions.

The articles in this Special Topic highlight the growing body of knowledge and elucidate the complexities introduced by the presence of ions in water and other informing solvents. Given the vastness of the subject, the scope of this Special Topic is accordingly broad. We are pleased to have been able to gather an impressive collection of papers that span the broad scope of the subject of “Ions in Water,” and we are grateful to all of the authors for their excellent contributions.