Factors affecting formation and properties of liquid crystals from surfactants and lecithin

Abstract

Liquid crystals from surfactants and lecithin were studied regarding the formation and structures of liquid crystals, the effects of additives (trehalose, urea, sodium chloride and a-tocopherol) on the formation and structures of liquid crystals, and the possibilities of using the liquid crystalline systems as a drug delivery system The assessment was made by investigating the potential of the system to increase drug solubility, to control drug release, and to modify rate of water evaporation and by examining the stability of the system upon storage. Surfactants selected to study were Brij®72, Brij®721, Arlatone®2121, triethanolamine oleate and sodium dodecyl sulfate (SDS). Propylthiouracil (PTU) and triamcinolone acetonide (TA) were used as model drugs. In some systems, an oil (Arlamol®E or isopropyl myristate) or decanol was added to promote formation of liquid crystals. Partial phase diagrams were constructed from various combinations of surfactants or lecithin, oils or decanol, and water. Polarized light microscopy was used to detect the formation and to identify microscopic phases of the liquid crystals. Differential scanning calorimetry was used to investigate thermal behavior of some selected systems. The results showed that most surfactants and lecithin formed lyotropic liquid crystalline phases in the presence of water except for systems consisting of Brij®721 or Arlatone®2121 as the sole surfactant in the system. When the additives were added into the liquid crystalline systems, they influenced the formation and structures of liquid crystals as well as the gross physical appearance of the preparation when the concentration reached a certain level. Results from assessment of the possibilities of using liquid crystalline systems as drug delivery systems showed that the liquid crystalline systems from Brij and SDS could hold a maximum of 0.2% PTU. Maximal saturation concentrations of PTU in the systems of triethanolamine and lecithin were 1.1 and 0.5%, respectively. These values are more than the solubility of PTU in water (0.15%). On the contrary, the liquid crystalline systems รณdied could not accommodate any significant amounts of TA. The rates of water evaporation from all liquid crystalline systems were slower than that from bulk water. All of these systems could sustain the release of PTU. Rates of release from Brij®72, triethanolamine oleate, and nonionic cream base systems were diffusion-controlled, whereas rate of release from the SDS system followed first-order kinetics. Some of the liquid crystalline systems were not stable after 2 months. Due to high concentrations of surfactants required in these systems, future studies should be aimed at reducing surfactant concentration by inclusion of other non-irritating liquid crystal forming components into the system.