A bile acid-induced aggregation transition and rheological properties in its mixtures with alkyltrimethylammonium hydroxide

Abstract

A bile acid, lithocholic acid (LCA)-induced aggregation transition and rheological properties in the mixtures of LCA and alkyltrimethylammonium hydroxide were investigated. In the salt-free system of tetradecyltrimethylammonium hydroxide (TTAOH) and LCA at equimolar ratio, unilamellar vesicles were observed initially and then transferred to a suspension of tubes within several hours at 25 °C, even at a very low total concentration. When an equimolar amount of NaBr was added into the solutions at a higher temperature, huge vesicles with a diameter of tens to hundreds of micrometres were formed initially and crystals quickly emerged inside the vesicles when cooling to room temperature spontaneously. In the longer chain system of cetyltrimethylammonium hydroxide (CTAOH)/LCA, vesicles were also observed after the dissolution of LCA in CTAOH micelle solution, but the vesicles were transferred to helical ribbons after several days at higher concentrations and a lower temperature, and gel was obtained after a longer time of aging. The huge vesicles were more stable than those of the TTAOH/LCA system when an equimolar amount of NaBr was added. The tube suspensions of the TTAOH/LCA system at a high concentration formed a weak gel with relatively low viscoelastic properties, indicating a weak interaction between the aggregates. The tube and helical ribbon suspensions can quickly transfer to viscoelastic vesicular solutions when heated at 30 °C, indicating a reversible transition process.