This is a study of 10 asymmetric gemini surfactants that self-assemble into vesicles which, in turn, self-assemble into gels. The geminis have the following general structure: long-chain/phosphate/2-carbon spacer/quaternary nitrogen/short-chain. Dynamic light scattering and transmission electron microscopy (TEM) demonstrate that in dilute aqueous systems these compounds self-assemble into vesicles. The vesicles are cohesive as proven by cryo-high resolution electron microscopy (cryo-HRSEM) images that reveal a "pearls on a string" morphology. These strings of vesicles create a complex network that rigidifies the water. The one gemini in the study that does not form a gel is also the only vesicle system that, according to cryo-HRSEM and TEM, assembles into clumps rather than chains. It is proposed that the vesicles are cohesive owing to protrusion of short chains from the vesicle surfaces, thereby creating hydrophobic "patches" whose intervesicular overlap supersedes the normal membrane/membrane repulsive forces. Analogous geminis having two long chains, neither of which are thought capable of departing from their bilayers, also form vesicles, but they are noncohesive (as expected from the model). Rheological experiments carried out on the gels show that gelation is mechanically reversible. Thus, if an applied torque breaks a string, the string can rapidly mend itself as long as the temperature exceeds its calorimetrically determined T(m) value. Gel strength, as manifested by the yield stress of the soft material, was shown to be particularly sensitive to the structure of the gemini. All three individual components of the systems (geminis, vesicles, and gels) have widespread practical applications.

A zwitterionic gemini surfactant forms a coacervate which is "elementary" in the sense that it consists of a single solute as opposed to the multicomponents (e.g., cetylpyridinium chloride/hexanol/water/NaCl) common in the coacervate literature. The gemini dissolves in water but then quickly separates as oily droplets which, despite the high water content of 83 wt %, are immiscible with water. Cryogenic temperature high-resolution scanning electron microscopy (cryo-HRSEM) examination of the droplets shows a distinct "sponge" structure. Although previously proposed for coacervate phases, a sponge morphology has never before been clearly depicted. The absence of previous electron microscopy (EM) pictures of the coacervate network is attributed to artifacts associated with transmission electron microscopy (TEM) methods, to the fragility of the coacervate toward physical perturbations, and to possible compositional changes with complex mixtures during sample preparation. When the gemini coacervate was exposed to mild shear, the honeycomb structure disappeared and was replaced by a lamellar phase.

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.

We report the first case of a pharmaceutical cocrystal formed between an inorganic acid and an active pharmaceutical ingredient (API), which enabled us to develop a stable crystalline and bioavailable solid dosage form for pharmaceutical development where otherwise only unstable amorphous free form or salts could have been used.

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