Arenediazonium Salts:  New Probes of the Interfacial Compositions of Association Colloids. 6. Relationships between Interfacial Counterion and Water Concentrations and Surfactant Headgroup Size, Sphere-to-Rod Transitions, and Chemical Reactivity in Cationic Micelles

Abstract

Product yields measured by high-performance liquid chromatography from chemical trapping of Cl -, Br -, and H 2 O by an aggregate-bound arenediazonium ion in cetyltrialkylammonium halide {(CTRA)X, R = Me, Et, n -Pr, and n -Bu; X = Cl, Br}, micelles are used to estimate, simultaneously, interfacial counterion, X m, and water, H 2 O m, concentrations as a function of [(CTRA)X] and tetramethylammonium halide concentrations, [(TMA)X]. The results are interpreted by using a two-site pseudophase model. Values of X m and H 2 O m are estimated by assuming that when the product yields from reaction of a long-tail aggregate-bound arenediazonium ion in micelles are the same as the product yields from reaction of its short chain analogue in an aqueous quaternary ammonium ion salt solution, then X m = [X w ] and H 2 O m = [H 2 O w ] in those solutions. The results show that X m and H 2 O m are functions of headgroup size, surfactant concentration, and aqueous counterion concentration and type. Plots of X m against [(CTRA)X] at a series of salt concentrations fall on separate curves. X m increases gradually with added (CTRA)X and almost incrementally with added (TMA)X. However, plots of X m and H 2 O m are essentially continuous functions of the aqueous counterion concentration, [X w ], at constant degree of micelle ionization, α. Three factors affect the shapes of these profiles. (a) An initial rapid increase in X m is attributed to a salt-induced contraction of the micellar interfacial volume. (b) Above ca. 0.1 M [X w ], an incremental (slope of 1) increase in X m with added counterion for (CTEA)Br, (CTPA)Br, and (CTBA)Br micelles is attributed to free movement counterions and co-ions between the interfacial region of the micelles (up to the micellar core) and the aqueous pseudophase. (c) X m increases markedly for (CTMA)Br and (CTMA)Cl at their respective sphere-to-rod transitions ca. 0.1 M [Br w ] and ca. 1.2 M [Cl w ]. The increases in X m are accompanied by concomitant decreases in H 2 O m . Specific salt-induced rod formation is attributed to dehydration (partial) and tight ion pair formation between surfactant headgroups and counterions. Phase separation of (CTBA)Br micelles in 0.5 M (TMA)Br occurs when the interfacial water concentration is too low to maintain micelle stability. The dependence of X m on [X w ] contradicts assumptions in the original pseudophase ion exchange model for aggregate effects on chemical reactivity that can be corrected, in part, by setting X m equal to the sum the aqueous and interfacial counterion concentrations.