Structure of Mesh Phases in a Cationic Surfactant System with Strongly Bound Counterions

• Published in: Langmuir Vol. 23; no. 7; pp. 3606 - 3614
• Main Authors: Ghosh, S. K, Ganapathy, R, Krishnaswamy, R, Bellare, J, Raghunathan, V. A, Sood, A. K
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 27-03-2007
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Water; Ternary system; Hydrocarbon; Rod; Chain length; Cationic surfactant; Sodium bromide; Surfactant; Ionic surfactant; Condensed benzenic compound; Quaternary ammonium compound; Naphthalene; Salt effect; Organic salt; Water content; Bicyclic compound; Carboxylate; Counter ion; Aggregate; Structure; Space group; Low temperature
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la0631466

We report the observation of an intermediate mesh phase with rhombohedral symmetry, corresponding to the space group R3̄m, in a mixed surfactant system formed by the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN). It occurs between a random mesh phase ( ) and a lamellar phase (Lα) at low temperatures; at higher temperatures, the phase transforms continuously into the Lα phase with an increasing surfactant concentration (φs). To separate the effects of salt and φs on the phase behavior, the ternary system consisting of cetyltrimethylammonium 3-hydroxy-naphthalene-2-carboxylate (CTAHN), sodium bromide (NaBr), and water was studied. The intermediate mesh phase is found in this system at high NaBr concentrations. The micellar aggregates, both in the intermediate and random mesh phases, are found to be made up of a two-dimensional network of rod-like segments, with three rods meeting at each node. The average mesh size increases with φs, and the transition from the random mesh phase to the intermediate phase is found to occur when it is approximately 1.5 times the lamellar periodicity. The intermediate mesh phase is absent in the equimolar dodecyltrimethylammonium bromide (DTAB)−SHN system, indicating the role of the surfactant chain length in the formation of this phase. This system exhibits a random mesh phase over a very wide range of water content, with the average mesh size decreasing upon an increasing φs, contrary to the trend seen in the CTAB−SHN system.