Micellization of water-soluble complex salts of an ionic surfactant with hairy polymeric counterions Ana Maria Percebom, a John Janiak, b Karin Schill´ en, b Lennart Piculell * b and Watson Loh * a For ionic surfactants in general, a change from simple to polymeric counterions leads to increasing attraction between micelles, condensing them in a concentrated phase. In the present study, two novel “complex salts” were prepared in which the cationic surfactant hexadecyltrimethylammonium was neutralized by two different copolyions, both having poly(methacrylate) main chains randomly decorated with oligo(ethylene oxide) side chains. The presence of hydrophilic side chains in the polyion backbone is proposed as a strategy to stabilize the complex salt aggregates in aqueous solutions and prevent them from separating out in a concentrated phase. Surface tension experiments reveal that the complex salts form soluble nano-aggregates by surfactant ion self-assembly at a distinct critical micellization concentration (cmc), similar to the micellization of a conventional ionic surfactant. This is the first time that cmc values have been determined for complex salts in the absence of all other ions. The physicochemical nature of the aggregates formed was investigated by dynamic light scattering, nuclear magnetic resonance self-diffusion measurements and steady-state fluorescence spectroscopy. Much larger aggregates are formed when the temperature is increased, but the small aggregates reform at room temperature, suggesting that the soluble aggregates are equilibrium structures, much like the micelles of conventional surfactants. Introduction Polyelectrolytes can drastically change the self-assembly and phase behavior of aqueous solutions of oppositely charged ionic surfactants, usually leading to an associative phase separation in a more or less wide composition range around the charge- stoichiometric ratio. 1 To avoid the phase separation phenom- enon, the present study involves the synthesis of stoichiometric complex salts (ionic surfactant + polyion) which are soluble in water, and the investigation of the system formed in dilute aqueous solutions free of simple ions. Many recent investigations have, with varying success, focused on preparing water-soluble ionic surfactant–polyion complexes with an aim to extend the application of these systems to aqueous solutions, e.g. for drug delivery. Nonionic surfactants with ethoxylated headgroups have been added to electroneutral complex salts of DNA and the cationic surfactant dodecyltrimethylammonium (DNA–DTA). 2 However, while the nonionic surfactants enter the complex salt aggregates and generate a rich phase behavior, they do not increase the solubility of the DNA complex salt in water. In contrast, the same family of nonionic surfactants was recently shown to increase the solubility of complex salts formed by polyacrylate and hexadecyltrimethylammonium (C 16 TAPA), but only in a limited range of composition. 3 Neutral polymers (including poly(ethylene oxide)) have been used to stabilize nanoparticles composed of complexes of poly(ethyleneimine) and sodium dodecyl sulfate (PEI–SDS) in water in excess surfactant ions. 4–6 To the best of our knowledge, the earliest work on designing the polymer architecture to achieve stable aqueous dispersions of ionic surfactant–polyion complexes was performed by Kabanov et al. 7–13 By covalently attaching a neutral hydrophilic chain polymer to the polyion chain, these authors prepared stable dispersions in water over a wide range of ratios between surfactant and polyion, including stoichiometric complexes. They found that poly(ethylene oxide)-block-poly(sodium meth- acrylate) associates with cationic surfactants due to the elec- trostatic attraction, forming aggregates stabilized in water by a hydrophilic neutral corona. From freeze fracture electron micrographs, they suggested that stoichiometric complexes spontaneously form vesicles in water. 8 These systems have been used to evaluate the aggregates’ capability of solubilizing drug molecules, which gives them the potential to be used in phar- maceutical applications. 9 Other groups have used the same strategy and synthesized different hydrophilic diblock copolymers consisting of a a Institute of Chemistry, University of Campinas (UNICAMP), Caixa Postal 6154, Campinas, SP, Brazil. E-mail: wloh@iqm.unicamp.br; Fax: +55 19 3521 3023; Tel: +55 19 3521 3148 b Division of Physical Chemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00, Lund, Sweden. E-mail: Lennart.Piculell@ em1.lu.se; Fax: +46 46 222 4413; Tel: +46 46 222 9518 Cite this: Soft Matter, 2013, 9, 515 Received 20th July 2012 Accepted 4th October 2012 DOI: 10.1039/c2sm26683k www.rsc.org/softmatter This journal is ª The Royal Society of Chemistry 2013 Soft Matter , 2013, 9, 515–526 | 515 Soft Matter PAPER Published on 29 October 2012. Downloaded by Lund University on 02/10/2013 23:09:53. View Article Online View Journal | View Issue