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

Product yields measured by high-performance liquid chromatography from chemical trapping of Cl^-, Br^-, and H<INF>2</INF>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<INF>m</INF>, and water, H<INF>2</INF>O<INF>m</INF>, 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<INF>m</INF> and H<INF>2</INF>O<INF>m</INF> 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<INF>m</INF> = [X<INF>w</INF>] and H<INF>2</INF>O<INF>m</INF> = [H<INF>2</INF>O<INF>w</INF>] in those solutions. The results show that X<INF>m</INF> and H<INF>2</INF>O<INF>m</INF> are functions of headgroup size, surfactant concentration, and aqueous counterion concentration and type. Plots of X<INF>m</INF> against [(CTRA)X] at a series of salt concentrations fall on separate curves. X<INF>m</INF> increases gradually with added (CTRA)X and almost incrementally with added (TMA)X. However, plots of X<INF>m</INF> and H<INF>2</INF>O<INF>m</INF> are essentially continuous functions of the aqueous counterion concentration, [X<INF>w</INF>], at constant degree of micelle ionization, α. Three factors affect the shapes of these profiles. (a) An initial rapid increase in X<INF>m</INF> is attributed to a salt-induced contraction of the micellar interfacial volume. (b) Above ca. 0.1 M [X<INF>w</INF>], an incremental (slope of 1) increase in X<INF>m</INF> 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<INF>m</INF> increases markedly for (CTMA)Br and (CTMA)Cl at their respective sphere-to-rod transitions ca. 0.1 M [Br<INF>w</INF>] and ca. 1.2 M [Cl<INF>w</INF>]. The increases in X<INF>m</INF> are accompanied by concomitant decreases in H<INF>2</INF>O<INF>m</INF>. 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<INF>m</INF> on [X<INF>w</INF>] contradicts assumptions in the original pseudophase ion exchange model for aggregate effects on chemical reactivity that can be corrected, in part, by setting X<INF>m</INF> equal to the sum the aqueous and interfacial counterion concentrations.