Abstract
In preceding studies, we demonstrated that choline carboxylates ChC(m) with alkyl chain lenghts of m = 12 - 18 are highly water-soluble (for m=12, solubel up to 93 wt% soap and 0 degrees C). In addition, choline soaps are featured by an extraordinary lyotropic phase behavior. With decreasing water concentration, the following phases were found: micellar phase (L-1), discontinuous cubic phase ...
Abstract
In preceding studies, we demonstrated that choline carboxylates ChC(m) with alkyl chain lenghts of m = 12 - 18 are highly water-soluble (for m=12, solubel up to 93 wt% soap and 0 degrees C). In addition, choline soaps are featured by an extraordinary lyotropic phase behavior. With decreasing water concentration, the following phases were found: micellar phase (L-1), discontinuous cubic phase (I-1' and I-1"), hexagonal phase (H-1), bicontinuous cubic phase (V-1), and lamellar phase (L-a). The present work is also focused on the lyotropic phase behavior of choline soaps but with shorter alkyl chains or different alkyl chain properties. We have investigated the aqueous phase behavior of choline soaps with C-8 and C-10 chian-lengths (choline octanoate and choline decanoate) and with a C-18 chian-length with a cis-doubel bond (choline oleate). We found that choline decanoate follows the lyotropic phase behavior of the longer-chain homologues mentioned above. Choline octanoate in water shows no discontinuous cubic phases, but an extended, isotropic micellar solution phase. In addition, choline octanoate is at the limit between a surfactant and a hydrotrope. The double bond in choline oleate leads also to a better solubility in water and a decrease of the solubilization temperature. It also influences the Gaussian curvature of the aggregates which results in a loss of discontinuous cubic phases in the binary phase diagram. The different lyotropic mesophases were identified by the penetration scan technique with polarizing light microscope and visual observations. To clarify the structural behavior small (SAXS) and wide (WAXS) and X-ray scattering were performed. To futhre characterize the extended, isotropic micellar solution phase in the binary phase diagram of chline octanoate viscosity and conductivity measurements were also carried out.
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