Synthesis and aggregation behavior of 2-(4-butyloctyl) malonic acid in aqueous solution. The formation of physically and colloidally stable vesicles by a branched-chain malonate

A new surfactant with a branched monoalkyl chain and a malonate headgroup has been synthesized: 2-(4-butyloctyl)malonic acid (BOMA). From the geometry of the surfactant, reflected in a packing parameter (P), it was anticipated that the surfactant would preferably aggregate in bilayers. This expectation was borne out in practice by the aggregation behavior, as studied by transmission electron microscopy (TEM) with negatively stained samples and freeze-fracture samples of aqueous vesicle dispersions. The structure of the surfactant headgroup has been varied by changes in pH of the vesicle dispersions. Over a wide pH range (pH 2.8-12.8) small unilamellar vesicles with a size of 20-50 nm were observed. Multilamellar vesicles were observed as well in the pH range 2.8-5.1. Phase-penetration experiments using an optical polarization microscope showed the birth and growth of myelins and multilamellar vesicles over a pH range 1.5-13.1. At pH < 2.8, a phase separation occured into a surfactant-rich, optically isotropic phase and a surfactant-lean phase. The physical and colloidal stability of the vesicular aggregates is high: up to one year as confirmed by TEM. The fusogenic properties of BOMA vesicles were examined. Upon the addition of Ca2+ ions, aggregates tended to become larger, but the formation of Ca2+ surfactant crystals was also observed.