Selective Adsorption of Ions at Liquid Electrolyte Interfaces
Abigail Miller, Dale Otten, Robert Onorato, Gregory Dallinger
Textbook descriptions hold that the liquid-air interface of an electrolyte solution is essentially devoid of ions. This is considered to be a necessary consequence of the Gibbs adsorption equation, which relates the increase in surface tension observed for all inorganic salts to a negative surface excess of ions. Onsager and Samaras provided the theoretical rationalization in their famous model, which concludes that image charges of the ions repels them from the interface. This primitive model has been refined and extended over the years but still predicts the outermost liquid layer to be essentially devoid of ions. Motivated by observations in atmospheric chemistry of otherwise inexplicable chemical reactions of sea salt aerosols, recent computer simulations predict a dramatically different behavior for “soft” ions (like large anions), finding the concentrations of some anions to be significantly enhanced at the liquid-vapor interface, and attributing this to the preference of highly polarizable ions for the surface, with its unbalanced electric fields. Others have questioned this proposed mechanism, and suggested relative dehydration energy as the most crucial factor influencing surface activity[1,2].
We have recently verified predicted surface enhancements for a number of small ions, including iodide[3], azide[4], thiocyanate[5], ferrocyanide[6], nitrate, and nitrite in a series of femtosecond UV second harmonic generation experiments, which demonstrated Langmuir adsorption behavior of the ions and yielded free energies of adsorption that were in agreement with predicted values.
A related phenomenon is known as the Jones-Ray Effect. In the 1930’s, experiments revealed that some salts did not produce the monotonic increase in surface tension that had become the expected behavior, but actually exhibited a minimum in the surface tension at milimolar concentrations, in clear violation of the Onsager-Samaras model. This result has since been reproduced by other groups, but has never been satisfactorily explained. We have verified the Jones-Ray Effect for alkali iodide and potassium ferrocyanide solutions by UV femtosecond SHG experiments[3,6], which reveal an initial surface enhancement, followed by a depletion at higher concentrations.
A particularly interesting case is the potential existence of hydroxide and/or hydronium at the interface, thus establishing the surface pH of water, where conflicting interpretations are reached from macroscopic and molecular-scale studies. We will present second harmonic generation (SHG) results for hydroxide and hydriodic acid solutions, which support surface enhancement of hydronium, but give no indication of preferential adsorption of hydroxide at the outermost liquid layer of the interface. These results are in agreement with other molecular-scale experiments and simulations, but disagree with conclusions based on macroscopic experiments[7,8].
Given that two-thirds of the Earth is covered by aqueous solutions, these new findings could have an important influence on both ocean and atmospheric chemistry, as well as electrochemistry and biology.