A series of novel zwitterionic surfactants each with two hydrophilic and two hydrophobic groups in the molecule (so-called heterogemini surfactants) has been synthesized. One of the hydrophilic groups is a phosphodiester anion and the other is a quaternary ammonium salt. Two methylene groups separate the two headgroups. The critical micelle concentration values of the surfactants were determined using du Nouy tensiometry and steady-state fluorescence and are of the order of 10 -5 M. Very low surface areas per molecule were observed suggesting that the monolayer formed is extremely tightly packed. NMR self-diffusion measurements gave information about the micelle size distribution. A broad distribution of self-diffusion coefficients was observed and indicated that the time scale of monomer-aggregate and/or aggregate-aggregate exchange is slow compared to the NMR time scale used (100 ms). A mean aggregate size of about 55 nm is obtained for one sample. Adsorption of the gemini surfactants at hydrophilic and at hydrophobized silica was studied by reflectometry. The more symmetrical gemini surfactants gave very low values of surface area per molecule on the hydrophobic surface, indicating a very tight packing of surfactant molecules. At higher surfactant concentrations all gemini surfactant gave very high adsorbed amount, most probably due to formation of aggregates at the surface.

No abstract

When a giant vesicle, composed of neutral and anionic lipid (90:10 mol %), comes into contact with various poly-l-lysines (MW 500-29 300), ropelike structures form within the vesicle interior. By using fluorescence lipids and epi-fluorescence microscopy, we have shown that both neutral and anionic lipids are constituents of the ropes. Evidence that the ropes are also comprised of poly-l-lysine comes from two experiments: (a) direct microinjection of poly(acrylic acid) into rope-containing vesicles causes the ropes to contract into small particles, an observation consistent with a polycation/polyanion interaction; and (b) direct microinjection of fluorescein isothiocyanate (a compound that covalently labels poly-l-lysine with a fluorescent moiety) into rope-containing vesicles leads to fluorescent ropes. The results may be explained by a model in which poly-l-lysine binds to the vesicle exterior, forms a domain, and enters the vesicle through defects or at the domain boundary. The model helps explain the ability of poly-l-lysine to mediate the permeation of a cancer drug, doxorubicine, into the vesicle interior.

A gemini surfactant is able to promote columnar jointing in vitreous ice where long pillars, often of hexagonal cross section, are formed. This jointing is visible by cryo-high-resolution scanning electron microscopy (cryo-HRSEM), in which colloidal suspensions in bulk water are cooled rapidly in liquid ethane, thereby avoiding the potential artifacts with other types of EM. The jointing is proposed to arise from a new type of colloidal morphology where the surfactant self-assembles into hexagonal columns. Evidence for this mechanism comes from a cryo-HRSEM photo of an ice-free hexagonal "skeleton" composed of surfactant. Cryo-HRSEM, a method that is just beginning to realize its potential, would seem to have a promising future in the discovery of additional and as yet unimagined colloidal structures.