Characterization of polymerizable surfactant vesicles by nuclear magnetic, absorbance and fluorescence spectroscopy

Abstract

The formation of vesicles and polymeric vesicles from unsaturated surfactants was discussed in regards to conditions for formation (sample size and sonication time), polymerization, substrate entrapment, hydroxide ion permeability, detection of phase transitions and determination of vesicle and polymeric vesicle stability. The optimum sample size and sonication time for each surfactant were stated. Three methods were utilized to polymerize the surfactant vesicles: (1) free radical initiation using azoisobutyronitrile (AIBN); (2) utilizing a spacer group (crotyl alcohol or acrylonitrile), along with AIBN; and (3) involved ultraviolet irradiation of the vesicle solution. The time for polymerization ranged from 6 hours with AIBN, 8-12 hours with uv irradiation, to over 20 hours with AIBN and spacer group. Demonstration of polymerization was made using proton NMR. Not all surfactant vesicles (10, 21-25) polymerized using these methods. Vesicle sizes were determined either by electron microscopy (EM) or dynamic laser light scattering. The range in vesicle size was from 500 to 1000 (ANGSTROM). The EM and laser light scattering indicated no change in vesicle size or shape upon polymerization. To prove the existence of water cores, cationic probes were entrapped in the cationic vesicles and anionic probes were entrapped in anionic vesicles. The change in the cationic probes fluorescence with pH determined the rate of permeation of hydroxide ion across the bilayer. The rate of permeation decreases in going from nonpolymeric vesicles, to the ultraviolet light polymerized vesicles. Two methods were utilized to detect phase transition phenomena: (1) the change in turbidity method and (2) the change in the NMR method. The phase transition of the polymeric vesicles were below those of the non-polymeric vesicles. Vesicle stability was determined; in vitro, to lysis by alcohol, to electrolytes, to halide ion and to drying the thermal shock. In all cases the polymeric vesicles were more stable.