Issue Date: January 28, 2008
Ions in Interfaces
In "A Renaissance for Hofmeister," I was delighted to see credit given to Barry Ninham for bringing renewed focus to specific ion effects that are ubiquitous in biology and surface science and yet have remained unexplained since the time of Hofmeister (C&EN, Nov. 26, 2007, page 47). I would like to add a new perspective to this story.
The "congregation" of ions within interfaces is dramatic for aqueous association colloids such as micelles, microemulsions, and vesicles composed of ionic amphiphiles. In a micellar solution of 1 mM total amphiphile, the interfacial concentrations of headgroups and counterions are typically ≥ 1 M. The properties of ionic micelles depend on the nature of both the counterion and the amphiphile headgroup—for example, the critical micelle concentration, Krafft temperature, sphere-to-rod and other morphological transitions, and the rates of many micelle-catalyzed reactions.
Sphere-to-rod transitions are highly cooperative phenomena in which hundreds to thousands of amphiphiles act in concert, and their equilibrium structures are determined by a delicate balance of forces that can be shifted by changing amphiphile and counterion concentration and type. The hydrophobic effect is generally believed to drive aggregation. The conundrum is the balancing force(s) and the origin of ion-specific effects.
The chemical trapping method, reaction of a micelle-bound arenediazonium ion with weakly basic nucleophiles in the interfacial regions of association colloids, is providing new insight into the relationships between interfacial concentrations of water, halide ions, alcohols, urea, and peptide bond models and amphiphile aggregate morphology (Langmuir 2007, 23, 414). The results show that at the literature values of the sphere-to-rod transition concentrations of four different amphiphiles, the interfacial counterion concentration increases with a concomitant decrease in interfacial water concentration.
These concentration transitions are consistent with a model in which formation of headgroup-counterion pairs is accompanied by the release of excess water into the bulk aqueous phase, permitting tighter packing in the interfacial region and formation of rodlike aggregates. The water is released because the formally neutral, but polar ion pair has a lower demand for hydration than free headgroups and counterions. More weakly hydrated and/or more polarizable ions form pairs at lower concentrations and the morphological transitions occur at lower concentrations, that is, the Hofmeister series.
In sum, easily observable changes in micellar morphology are good reporters of specific ion effects and the interfacial region of micelles is a "laboratory" for studying ion specificity.
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