Systematical Characterization of Phase Behaviors and Membrane Properties of Fatty Acid/Didecyldimethylammonium Bromide Vesicles

Abstract

Fatty acids (FAs) are known to form vesicle structures, depending on the surrounding pH conditions. In this study, we prepared vesicles by mixing FAs and a cationic surfactant, and then investigated their physicochemical properties using fluorescence spectroscopy and dielectric dispersion analysis (DDA). The assemblies formed from oleic acid (OA) and linoleic acid (LA) were modified by adding didecyldimethylammonium bromide (DDAB). The phase state of FA/DDAB mixtures was investigated with pH titration curves and turbidity measurements. The trigonal diagram of FA/ionized FA/DDAB was successfully drawn to understand the phase behaviors of FA/DDAB systems. The analysis of fluidities in the interior of the membrane with use of 1,6-diphenyl-1,3,5-hexatriene (DPH) indicated that the membrane fluidities of OA/DDAB and LA/DDAB at pH 8.5 slightly decreased in proportion to the molar ratio of DDAB in FA/DDAB systems. The fluorescent probe 6-lauroyl-2-dimethylamino naphthalene (Laurdan) indicated that the LA vesicle possessed a dehydrated surface, while the OA vesicle surface was hydrated. Modification of LA vesicles with DDAB induced the hydration of membrane surfaces, whereas modification of OA vesicles by DDAB had the opposite effect. DDA analysis indicated that the membrane surfaces were hydrated in the presence of DDAB, suggesting that the surface properties of FA vesicles are tunable by DDAB modification.

Interaction forces and membrane charge tunability: Oleic acid containing membranes in different pH conditions

“…Previous studies have shown that the apparent pKa of oleic acid in different surfactant and lipid mixtures ranges from 6.0 to 9.5. For example, the pKa of pure oleic acid vesicles is 8.0- 9.5 [27,42,43]. While when mixed with monoolein, the pKa was 6.0-7.0 [24].…”

“…The pKa of an isolated carboxyl group in water is~4.8 [23], while the apparent pKa of oleic acid's carboxyl headgroup tends to be higher depending on the type of co-existing molecules in the mixture and structure of the assembly (i.e., pKa of~6.1 in the hexagonal phase of oleic acid and monoolein mixtures [24]; pKa of~6.5 in 4:6 oleic acid-DPPE vesicles [25]; pKa of~8.0-8.5 in pure oleic acid vesicles [23]; and, an even higher pKa of~9.85 for oleic acid monolayers at air-water interface [26]). The deprotonation of oleic acid in the system consequently alters molecular level lateral interactions and phase behavior of the mixture [23,27]. Likewise, deprotonation would significantly impact the interaction between membranes and oleic acid can be used to induce pH-dependent membrane fusion or membrane repulsion in model systems [25,28,29].…”

Synthesis of Non-Toxic Silica Particles Stabilized by Molecular Complex Oleic-Acid/Sodium Oleate

“…The formation of these bilayered vesicles was investigated through a simple technique: the titration of clear micellar solutions of alkaline salts with acids (usually inorganic acids such as HCl) [1,2,4,5,6,7,8]. The concentration of undissociated acid increases with acid quantity (RCOOH), and the initial transparent layout becomes opaque due to the formation of a vesicular supra-molecular structure [4,5,6,7,8].…”

“…The concentration of undissociated acid increases with acid quantity (RCOOH), and the initial transparent layout becomes opaque due to the formation of a vesicular supra-molecular structure [4,5,6,7,8]. The dispersion’s pH [4,5,6,7,8], the 13 C Nuclear Magnetic Resonance (NMR) signals [7], and the X-ray diffraction peaks [8] are modified due to vesicle formation.…”

“…Most of the potential applications of these aqueous dispersions are focused on the formation of alkyl double layer [1,2,3,4,5,6,7,8], and the presence of the hydrophobic components allows for the solubilization of various hydrophobic molecules (e.g., some superior alcohols [9,10], bioactive substances, drugs, antifungal, and dyes [1,2,5,9,10]).…”