Abstract:

A review. Polar lipids and certain surfactants are known to form thermodynamically stable lyotropic liq. crystals (LLC) when mixed with water. The major phases are lamellar (La), normal and reverse hexagonal (HI and HII) and cubic bicontinuous and discontinuous structures (VI VII and II, III, resp.). Theor., the transformation sequence of the phases with increasing water content is III → HII → VII → La → VI → HI → II. Lyotropic liq. crystal transformation was extensively studied and was found to take place also upon surfactant modifications (head or tail), addn. of a guest mol. (hydrophilic or hydrophobic), co-surfactant or electrolyte and varying the temp. The phases are of growing scientific and industrial interest because of their structural resemblance to human membranes through which drug passage of lipophilic compds. (vitamins, fats, oil and cholesterol) occurs and because of their high surface area and solubilization capacities. The variations in the phase formations, the phase behavior and phase transitions are of significant importance when designing a potential application for these systems. This chapter presents studies related to phase behavior and phase transitions as a function of different phys. and chem. conditions. The relationship between surfactant geometry that includes tail vol., tail length and area per head group and the corresponding phase formation is stressed. Research demonstrating dependence of the phase behavior on the addn. of a third component such as hydrophobic, hydrophilic guest mol. or co-surfactant is summarized. A brief overview of the instruments used in the above is also presented, illustrating their functionality in detecting the phase transitions and the unique information potentially extd. from each instrument. The main findings show successful control of lyotropic liq. crystal structure by altering the surfactant geometry in various respects: unsatn. site or degree, head or tail chain length and by an addnl. component such as linear or branched oils or alcs. [on SciFinder(R)]