Influence of Counterions on Lauric Acid Vesicles and Theoretical Consideration of Vesicle Stability

Abstract

The counterions, including inorganic cations, Na⁺ and Cs⁺, and organic cation, (C₂H₅)₄N⁺, influence the phase behavior and self-assembled structures of the lauric acid (LA) in water. Dissolving LA in NaOH, CsOH, and (C₂H₅)₄NOH (tetraethylammonium hydroxide, TeAOH) solutions, respectively, we observed that the three systems totally exhibited the same phase behavior, from birefringent L_α phase/precipitates (P) → L_α phase → L_α phase/L₁ (micelles) → L₁. The temperature influence on phase behavior was investigated, and with an increase of temperature, we observed that less phase behavior change occurred in the systems of LA/CsOH/H₂O and LA/TeAOH/H₂O, while the phase behavior of the LA/NaOH/H₂O system exhibited an obvious change. Cryogenic transmission electron microscopy (cryo-TEM) images demonstrated that the different microstructures of L_α phase samples in the three systems existed. For the systems of LA/NaOH/H₂O and LA/TeAOH/H₂O, uni- and multilamellar vesicles coexist for L_α phase samples, as both the morphology and size of these vesicles are polydisperse. The curvatures of the bilayer membranes of the two systems are considered to vary from positive, zero, and even negative. However, only spherically unilamellar vesicles exist in the system of LA/CsOH/H₂O, indicating that the bilayers are more rigid than those in the LA/NaOH/H₂O and LA/TeAOH/H₂O systems. Through the combination of the Helfrich curvature energy theory and the mass-action model, the effective bending constant <i>K</i> = 0.5 <i>k</i>_B<i>T</i> in the LA/CsOH/H₂O system was obtained, demonstrating that the unilamellar vesicles are stabilized by thermal fluctuations. A primary discussion for the effect of the nature of counterions on the stability and deformation of the vesicles is presented