The aqueous phase behavior of polyion–surfactant ion complex salts mixed with nonionic surfactants

Abstract

The aim of this work was to study intermolecular interactions in systems containing charged polyion (polyacrylate, PA⁻), charged surfactant (C₁₆TA⁺) and nonionic surfactant (C₁₂E₅ or C₁₂E₈). To achieve this we have created four different phase diagrams using two different so-called complex salts, C₁₆TAPA₂₅ and C₁₆TAPA₆₀₀₀, both consisting of positively charged surfactant (C₁₆TA⁺) with polyacrylate (PA⁻) as counterions (no simple salt). The difference between the salts is the length of the polyion (25 or 6000 monomers). Both are insoluble in water. The results revealed that decreasing polyion length and increasing the PEO chain length of the nonionic surfactant were important factors for increasing the solubility of the complex salt. We also found that the curvature effects are quite small at low water content when gradually exchanging C₁₂E₈ for either one of the complex salts while there is a gradual change in curvature for the systems containing C₁₂E₅. Another interesting observation was the possibility for relatively large amounts of complex salt to be incorporated into a V₁ (Ia3d, bicontinuous) phase in the C₁₂E₈-containing systems. This gives rise to several questions regarding arrangements and dynamics of the polyion in this phase. In the dilute regime several different liquid crystalline phases can coexist with a dilute liquid phase containing the nonionic surfactant.