Ionic surfactants with polymeric counterions Lennart Piculell a, ⁎, Jens Norrman a , Anna V. Svensson a , Iseult Lynch a , Juliana S. Bernardes b , Watson Loh b a Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, Box 124, SE 22100 Lund, Sweden b Institute of Chemistry, State University of Campinas (UNICAMP), Caixa Postal 6154, Campinas, SP, Brazil abstract article info Available online 8 October 2008 This review summarizes recent progress in our understanding of aquoeus “complex salts” of ionic surfactants with polymeric counterions. Complex salts are simplified versions of the much-studied mixtures of oppositely charged polyelectrolytes and surfactants, and are also good model systems to study the fundamentals of polyion-mediated forces in colloidal systems. Comparisons are made with conventional ionic surfactants, which have monomeric counterions, and with surfactants having oligomeric counterions containing two, three or four charged groups. Complex salts form similar aggregates as conventional ionic surfactants, but as the degree of polymerization of the counterion increases, the interaction between the surfactant aggregates becomes attractive rather than repulsive. Introducing uncharged comonomers in the polyions affects both the shape and the organization of the surfactant aggregates. © 2008 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 2. The “complex salt”— a surfactant with polymeric counterions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 2.1. Methods to produce complex salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 2.2. Polymeric versus monomeric counterions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 3. A walk through parameter space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 3.1. The polyion length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 3.2. The surfactant tail length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 3.3. The polyion charge density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 3.4. Remarks on water-soluble complex salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 3.5. Added co-surfactant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 3.6. Other complex salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 4. Concluding remarks and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 1. Introduction The aim of the present review is to summarize recent progress in our understanding of the self-assembly of ionic surfactants that are neutralized by polymeric counterions, as opposed to conventional monovalent counterions. Oligomeric counterions, where the number of ionic groups is 2–4, are also included. In a series of recent studies from our laboratories [1–9], we have investigated how various molecular parameters affect such surfactant systems in an aqueous environment, and it now seems timely to collect these observations in a single comprehensive review. In practical applications, as well as in most scientific work, systems containing ionic surfactants and oppositely charged polyions are obtained by mixing two different salts in water: a surfactant salt (a surfactant ion with its simple counterion) and a polyelectrolyte (a polyion with its simple counterions); see Fig. 1 . Typically, mixtures of this kind phase separate, under a more or less wide range of system compositions, into one concentrated phase and one dilute phase [10– 12]. The phase separation can be viewed as an ion exchange process, since the concentrated phase is enriched in what we have chosen to call the complex salt (the surfactant ion with the polyion as counter- ion), whereas the dilute phase predominantly contains the simple salt (a combination of the two simple ions), see Fig. 1 . The separation of a phase rich in complex salt is the reason why oppositely charged polyelectrolyte/surfactant mixtures are used in a wide range of Advances in Colloid and Interface Science 147–148 (2009) 228–236 ⁎ Corresponding author. Tel.: +46 46 2229518;fax +46 46 2224413. E-mail address: lennart.piculell@fkem1.lu.se (L. Piculell). 0001-8686/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2008.09.009 Contents lists available at ScienceDirect Advances in Colloid and Interface Science journal homepage: www.elsevier.com/locate/cis