Do Amphiphile Aggregate Morphologies and Interfacial Compositions Depend Primarily on Interfacial Hydration and Ion-Specific Interactions? The Evidence from Chemical Trapping

Abstract

Surface-active amphiphiles aggregate spontaneously in water to form association colloids such as micelles, microemulsions, and vesicles. The hydrophobic effect drives aggregation, but the opposing forces that provide balance and determine equilibrium morphologies are not understood, in particular, how specific ion effects, which often follow a Hofmeister series, affect the properties of association colloids. We have harnessed the competitive trapping of arenediazonium ions by weakly basic nucleophiles such as halide counterions, anionic headgroups, alcohols, urea, and water, to estimate their concentrations in the interfacial regions of association colloids from reaction product yields. In the chemical trapping method, product yields are proportional to the concentrations of water and other nucleophiles within the interfacial region, not their stoichiometric concentrations in solution. Changes in the balance of forces controlling aggregate structure are reflected in changes in interfacial concentrations of water and other components in association colloids as reported by the chemical trapping method. Significant changes in interfacial water and counterion concentrations are observed during structural transitions. Specific ion effects on sphere-to-rod transitions of cationic amphiphiles are interpreted in terms of the strengths of headgroup and counterion pairing and ion hydration interactions. Trapping results also provide important information on interfacial compositions of microemulsions, vesicles, nonionic micelles and macroemulsions, reverse micelles, micelles in aqueous urea, and anionic polyelectrolytes. Identifying relationships between aggregate morphology and interfacial composition by chemical trapping has just begun.

Using lanthanides as probes for polyelectrolyte–metal ion interactions. Hydration changes on binding of trivalent cations to nucleotides and nucleic acids

“…There is increasing evidence for the importance of counter ion dehydration on binding to polymer and surfactant systems [32,33,64,80], possibly involving the dehydration entropy [81].…”

A Semi-Empirical Spectrophotometric (SESp) Method for the Indirect Determination of the Ratio of Cationic Micellar Binding Constants of Counterions X^|^ndash; and Br^|^ndash; (KX/KBr)

“…The effects of the inert counterionic salts on the rates of some ionic micellar-mediated reactions have been quantitatively explained in terms of pseudophase ion exchange PIE model 21 . The chemical trapping method of Romsted has been used to determine the value of K X Cl as 2.6 for X Br 11 which is similar to ones determined by various methods including SEK method 24 .…”

Study of Cationic and Nonionic Mixed Micelles with NaBr and 3,5-Cl₂C₆H₃CO₂Na by Use of Probe Nucleophilic Reaction of Piperidine with Ionized Phenyl Salicylate

“…Nonetheless, it has been accepted from previous reports 43,47 that the appearance of these maxima under such experimental conditions is a sign of the presence of wormlike/twisted wormlike micelles in a solution. Contrarily, there was no existence of maxima Although specific ion effects on structural features of aqueous ionic surfactant aggregates have been known for the last nearly more than ten decades and thousands of papers have published, the basic forces responsible for such specific effect remain unclear both theoretically and experimentally 48 . It is considered to be useful to develop reversible micelle-vesicle conversions by changing a physical parameter such as pH, temperature or pressure of the medium.…”