Coacervation in mixtures of polyelectrolytes and surfactants with opposite charge is common in nature and is also technologically important to consumer health care products. To understand the complexation behavior of these systems better, we combine multiple experimental techniques to systematically study the polymer/surfactant binding interactions and the phase behavior of anionic sodium dodecyl sulfate (SDS) surfactant in cationic JR 400 polymer aqueous solutions. The phase-behavior study resolves a discrepancy in the literature by identifying a metastable phase between the differing redissolution phase boundaries reported in the literature for the surfactant-rich regime. Isothermal titration calorimetry analyzed within the framework of the simple Satake-Yang model identifies binding parameters for the surfactant-lean phase, whereas a calculation for polymer-bound micelles coexisting with free micelles is analyzed in the surfactant-rich redissolution regime. This analysis provides a preliminary understanding of the interactions governing the observed phase behavior. The resulting thermodynamic properties, including binding constants and the molar Gibbs free energies, enthalpies, and entropies, identify the relative importance of both hydrophobic and electrostatic interactions and provide a first approximation for the corresponding microstructures in the different phases. Our study also addresses the stability and metastability of oppositely charged polyelectrolytes and surfactant mixtures.

Oppositely charged polyelectrolyte-surfactant mixtures are ubiquitous in biology and the basis of numerous consumer healthcare products. Despite their broad use, however, a rational approach to their formulation remains challenging because of the complicated association mechanisms. Through compilation and analysis of literature reports and our own research, we have developed a semiempirical correlation of the binding strength of surfactants to polyelectrolytes in salt-free mixtures as a function of the polyion linear charge density and the surfactant hydrophobicity. We have found that the cooperative binding strength increases as the square of the polyelectrolyte's linear charge density and in proportion to the surfactant's hydrophobicity, such that a quantitative relationship holds across a broad range of polyelectrolytes. Deviations from the correlation reveal the role of system-specific interactions not considered in the analysis. This engineering relationship aids in the rational design of oppositely charged polyelectrolyte-surfactant formulations for consumer products and biomedicines by enabling the prediction of binding strengths in polyelectrolyte-surfactant mixtures based on mesoscale parameters determined from the chemical composition.

The microstructure and rheological properties of micellar solutions of Pluronic block copolymers (PEO x –PPO y –PEO x ) in deuterated ethylammonium nitrate (dEAN, a protic ionic liquid) are studied by small- and ultrasmall-angle neutron scattering (SANS and USANS), fluorescence microscopy, and shear rheology. A homologous series of three Pluronics with similar PPO block length (F127 (PEO106–PPO70–PEO106), P123 (PEO20–PPO70–PEO20), and L121 (PEO5–PPO70–PEO5)) are examined to explore the effect of the relative balance of solvophilic to solvophobic block size on the self-assembled microstructure. Both F127 and P123 form spherical micelles in dEAN that assemble into face-centered and body-centered cubic supramolecular crystals, respectively, upon increasing concentration or temperature. The transition from disordered to crystalline (ordered) structures manifests as a sol–gel rheological transition. A richer phase diagram is observed in the L121/dEAN system: due to its low solvophilicity in dEAN, L121 self-assembles into structures with low curvatures, namely, vesicles in the dilute regime, elongated (wormlike) micelles at intermediate concentrations and low temperatures, a nematic phase in concentrated solutions, and multilamellar phases at high temperatures. Highly viscoelastic solutions are created by the self-assembly of entangled L121 wormlike micelles. Structural length scales of the three Pluronic/dEAN systems in the various mesophases are determined from SANS and USANS.

The search for stable vesicular structures is a long-standing topic of research because of the usefulness of these structures and the scarcity of surfactant systems that spontaneously form vesicles in true thermodynamic equilibrium. We report the first experimental evidence of spontaneous formation of vesicles for a pure cationic double tail surfactant (didodecyldimethylammonium bromide, DDAB) in a protic ionic liquid (ethylammonium nitrate, EAN). Using small and ultra-small angle neutron scattering, rheology and bright field microscopy, we identify the coexistence of two vesicle containing phases in compositions ranging from 2 to 68 wt %. A low density highly viscous solution containing giant vesicles (D ~ 30 μm) and a sponge (L(3)) phase coexists with a dilute high density phase containing large vesicles (D ~ 2.5 μm). Vesicles form spontaneously via different thermodynamic routes, with the same size distribution, which strongly supports that they exist in a true thermodynamic equilibrium. The formation of equilibrium vesicles and the L(3) phase is facilitated by ion exchange between the cationic surfactant and the ionic liquid, as well as the strength of the solvophobic effect in the protic ionic liquid.

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