Influence of counterions on lauric acid vesicles and theoretical consideration of vesicle stability.

The counterions, including inorganic cations, Na(+) and Cs(+), and organic cation, (C(2)H(5))(4)N(+), influence the phase behavior and self-assembled structures of the lauric acid (LA) in water. Dissolving LA in NaOH, CsOH, and (C(2)H(5))(4)NOH (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/L(1) (micelles) → L(1). 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/H(2)O and LA/TeAOH/H(2)O, while the phase behavior of the LA/NaOH/H(2)O 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/H(2)O and LA/TeAOH/H(2)O, 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/H(2)O, indicating that the bilayers are more rigid than those in the LA/NaOH/H(2)O and LA/TeAOH/H(2)O systems. Through the combination of the Helfrich curvature energy theory and the mass-action model, the effective bending constant K = 0.5 k(B)T in the LA/CsOH/H(2)O 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.