Focus: A Surface Attraction
Ions dissolved in water were once thought to keep their distance from its surface, but some negative ions are actually attracted to the air interface, according to recent experiments and simulations. In the 10 April and 18 December Physical Review Letters, researchers associate this phenomenon with the rearrangement of charge within the ions. They calculate various ions’ expected influence on surface tension and find values that agree with experimental data. There are other proposed explanations for the attraction, but the results reflect renewed scientific interest in a field that affects phenomena as diverse as protein folding and atmospheric chemistry.
Water molecules have a dipole–a “positive end” and a “negative end.” They can surround a negative ion with their positive ends pointing inward and easily remove it from a salt crystal. The ion moves through the liquid with this “shell” of water molecules attached, but its charge also influences the orientations of more distant water molecules. This long-distance influence has surprising effects. According to standard electromagnetic theory, any charge in such a non-conducting, “dielectric” medium creates a surface charge of the same sign at the boundary of the medium. So early-20th-century physical chemists proposed that ions are repelled from the air-water interface by this surface charge.
But recent experiments and simulations showed that some ions, especially negatively-charged halogen atoms such as bromine and iodine, are attracted to the interface. In atmospheric water droplets, this surface accumulation affects the way the halogens degrade ozone. Theorists have struggled with these contradictions but have yet to agree on a complete solution.
Yan Levin, of the Federal University of Rio Grande do Sul in Brazil, attributes the surface attraction to the mobility of charge within the relatively large halogen ions, a property called polarizability. Traditional models, which envision the ions as spheres with fixed charge, work well in the middle of the water, he says, but “this becomes a very bad model if you want to study ions near an interface.”
In his April paper Levin calculated charge motion within a spherical negative ion at the surface, with some of its volume protruding into the air. The ion’s protrusion reduces the volume of water it displaces, which cuts down on the energy cost of making a cavity amidst the loosely attractive network of water molecules. At the same time, the ion charge redistributes to the bottom of the sphere, where it is still in contact with water. This concentrated charge and the associated water molecules allow the ion to retain much of the energy benefit of the original, complete water shell. These effects lower the overall energy of the surface location and can lead to the ion being attracted to the surface.
In his latest paper, Levin and two colleagues account for positive ions in dissolved salts, as well as the negative ones. Because the small, positive ions polarize much less than the large negative ones, the calculations support the traditional view that they are repelled from the surface. The team calculated the surface tension–a measure of the energy needed to make more surface area–for water with various salts dissolved, and matched experimental results.
Listing the salts in order of the surface tension they generate exactly mirrors the “Hofmeister series,” which describes their effects on proteins in water and has been known for 120 years. Some salts make proteins precipitate, while others make proteins more soluble but make their three-dimensional structures unfold, by modifying their interactions with water. Levin says that including polarizability of ions could help explain these important effects. “We have a start on that,” he says, “but it will take some time before we can get the numbers.”
Phillip Geissler of the University of California, Berkeley, isn’t convinced that the polarization is the most important thing, since his group has found that changes in the shape of the interface around a surface ion can also stabilize it. More investigation will be needed to clarify which of the effects most naturally explains ions at surfaces, Geissler says. “It’s pretty premature to say we have the right simple picture.”
Don Monroe is a freelance science writer in Murray Hill, New Jersey.