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 headgr oup has been synthesized: 2-(4-butyloctyl)malonic acid (BOMA). From th e geometry of the surfactant, reflected in a packing parameter (P), it was anticipated that the surfactant would preferably aggregate in bil ayers. This expectation was borne out in practice by the aggregation b ehavior, as studied by transmission electron microscopy (TEM) with neg atively stained samples and freeze-fracture samples of aqueous vesicle dispersions. The structure of the surfactant headgroup has been varie d by changes in pH of the vesicle dispersions. Over a wide pH range (p H 2.8-12.8) small unilamellar vesicles with a size of 20-50 nm were ob served. Multilamellar vesicles were observed as well in the pH range 2 .8-5.1. Phase-penetration experiments using an optical polarization mi croscope showed the birth and growth of myelins and multilamellar vesi cles 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-le an phase. The physical and colloidal stability of the vesicular aggreg ates is high: up to one year as confirmed by TEM. The fusogenic proper ties of BOMA vesicles were examined. Upon the addition of Ca2+ ions, a ggregates tended to become larger, but the formation of Ca2+ surfactan t crystals was also observed.