Published: August 25, 2011 r2011 American Chemical Society 7423 dx.doi.org/10.1021/ma201240k | Macromolecules 2011, 44, 7423–7429 ARTICLE pubs.acs.org/Macromolecules Phase Diagrams of Electrostatically Self-Assembled Amphiplexes Vesna Stanic, † Matthew Mancuso, † Waiken Wong, ‡ Elaine DiMasi, § and Helmut H. Strey* ,† † Department of Biomedical Engineering, Bioengineering Bldg, State University of New York at Stony Brook, Stony Brook, New York 11794-5281, United States ‡ Polymer Science and Engineering Department, University of Massachusetts Amherst, Amherst, Massachusetts 10003, United States § National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, United States ’ INTRODUCTION The ease with which discrete and independent entities assem- ble into ordered objects and arrays in nature has been the underpinning of a great deal of scientific inquiry and application. Self-assembly occurs in a great variety of systems, ranging from nano- to microscale sized, organic to inorganic, biologically functioning to inert. 1 Subject merely to the fundamental forces that govern the behavior of its participating components— including electrostatics, hydrophobic/hydrophilic interactions, and hydrogen bonding—this process provides researchers routes to structures with low dispersity in terms of size, shape, and spacing. These mechanisms consequently are opening doors to current and potential advancements in fields as diverse as data storage, photonics, biomimetics, and catalysis. 2 Opportunities lie not only in using self-assembled materials as they occur naturally but also in manipulation of the system to suit the needs of the scientist, made possible through an understanding of the under- lying physics. One heavily used and studied self-assembling system is that of polyelectrolyteÀsurfactant complexes (PSCs). 3À10 Especially amenable to experimental study because of their aqueous nature, PSCs have provided insights into binding interactions between polymers and small molecules, solution properties of those same components, and morphological behavior of soft matter. 11À14 Moving from fundamental science and toward functional materi- als, PSCs show promise in uses including encapsulation of small molecules, separations, and templating for various types of nano- structures. 15À18 The complex phase behavior of PSCs has been the subject of extensive study. 6,19À21 Various cubic symmetries, hexagonally packed cylinders, and lamellar stacks have all been observed and derive primarily from the action of the surfactant micelles and how they form long-rang ordered arrangements. 22 The PSC phase diagram has been studied as a function of polyelectrolyte charge density, 6,23,24 ionic strength and osmotic pressure. 5,6,25,26 The combination of solutions of surfactant micelles and poly- electrolyte at a condition of charge neutrality leads to precipita- tion of insoluble PSCs. The mesophases available to PSCs are numerous as experi- mental parameters such as surfactant/polymer identity and ratio are changed. However, for a given pair at a ratio in the insoluble regime, there is far less flexibility to tune the morphology and unit cell size, primarily due to geometrical constraints imposed by the surfactant’s molecular structure. The challenge becomes finding a method that allows, in a sense, to dial in a preferred morphology and unit cell size given the inherent spontaneous curvature and bending modulus of a particular species of surfactant micelle. As one considers tuning the phase behavior and morphology of micelles, manipulating the spontaneous curvature and bending modulus becomes key (e.g., ref 27); reducing the modulus increases the flexibility of the surfactant layer, and it is only with greater flexibility and reduced spontaneous curvature that the micelles will be able to swell. Here we will reduce the sponta- neous curvature of the surfactants using a cosurfactant as shown in Figure 1. We will present a new type of self-assembled entity that bears some of the characteristics of micellar solutions and emulsions but also has properties unique from those systems: electrostati- cally self-assembled amphiphilic complexes, with further abbre- viation of amphiphilic complex to amphiplex and the overall Received: June 1, 2011 Revised: August 10, 2011 ABSTRACT: We present the phase diagrams of electrostatically self-assembled amphiplexes (ESA) comprised of poly(acrylic acid) (PAA), cetyltrimethylammonium chloride (CTACl), do- decane, pentanol, and water at three different NaCl salt con- centrations: 100, 300, and 500 mM. This is the first report of phase diagrams for these quinary complexes. Adding a cosurfac- tant, we were able to swell the unit cell size of all long-range ordered phases (lamellar, hexagonal, Pm3n, Ia3d) by almost a factor of 2. The added advantage of tuning the unit cell size makes such complexes (especially the bicontinuous phases) attractive for applications in bioseparation, drug delivery, and possibly in oil recovery.