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    Cation binding properties of an anionic gemini surfactant monolayer

    Year: 2017

    Journal: Colloid Surf. A-Physicochem. Eng. Asp., Volume 522, JUN 5, page 536–543

    Authors: Rehman, Jeveria; Ponce, Concepcion P.; Araghi, Hessamaddin Younesi; Paige, Matthew F.

    Organizations: Natural Sciences and Engineering Research Council of Canada (NSERC); Canada Foundation for Innovation (CFI); Agriculture Development Fund; Saskatchewan Pulse Growers; Province of Saskatchewan; University of Saskatchewan

    Keywords: Gemini surfactant; Langmuir monolayers; Inorganic ions; Binding; Isotherms; Aggregates

    A new class of anionic, gemini surfactants based on N,N'-dialkyl-N-N'-diacetate ethylenediamine has recently been reported in the literature, with the compounds exhibiting controllable and, in some cases reversible, aggregation in response to solution pH and the presence of inorganic cations. In this work, the response of one member of this surfactant family (12 carbon long tail) to subphase Na+ and Ca2+ at basic pH was investigated at the air-water interface using Langmuir monolayer approaches. It was found that the anionic monolayer exhibited marked expansion in response to subphase cation concentration, with Ca2+ leading to significantly greater film expansion than Na+. The film's response to inorganic ions in the subphase was modeled using a simple ion-binding model, with association constants that were generally smaller than those reported for comparator systems such as phospholipids, and a 1:1 and 2:1 surfactant to cation binding ratio, respectively. However, Brewster angle microscope measurements revealed that monolayer film expansion was accompanied by the formation of multimolecular aggregates, tentatively assigned to the formation of poorly-dispersed deposits of cation chelated with the surfactant. The impact of this effect on the apparent magnitude of film expansion, ion-binding and controlling aggregation in these systems, and the potential for this system in terms of controlling surfactant aggregation is discussed. (C) 2017 Elsevier B.V. All rights reserved.
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