Bilayer structures in dioctadecyldimethylammonium bromide/oleic acid dispersions

Abstract

This paper reports on the properties of bilayers composed of dioctadecyldimethylammonium bromide (DODAB) and oleic acid (OA) at various molar ratios. The mole fraction of OA, X OA, was varied in the range of 0–1 and the total lipid content was constant and equal to 10 mM. The DODAB/OA dispersions were extruded at a temperature higher than that of the gel–liquid transition of DODAB. The morphology of bilayer structures formed in the dispersions was inspected using a cryogenic transmission electron microscopy (cryo-TEM) and a differential interference contrast microscopy (DIC). The observations revealed that the incorporation of OA into DODAB bilayer results in a decrease of the membrane curvature. Anisotropy measurements using 1,6-diphenylhexatriene (DPH) as a rotator probe demonstrated that the DODAB/OA membrane microviscosity decreased considerably for X OA > 0.4. The thermal behavior of DODAB/OA membranes has been studied by differential scanning calorimetry (DSC). In the case of the systems in which X OA < 0.8, the DODAB/OA membranes are in the gel phase at room temperature. Additionally, Langmuir monolayer experiments of the DODAB/OA mixtures showed that due to the electrostatic interactions between the oppositely charged head groups of DODAB and OA they get close to each other, which results in a decrease of the mean area per molecule. The results were next discussed based on the packing parameter concept. The reduction of the mean area per head group ( a) in the DODAB/OA systems leads to subsequent increase in the so-called packing parameter ( S), which governs the morphology of surfactant aggregates.