STRUCTURE OF LYOTROPIC PHASES FORMED BY SODIUM DODECYL-SULFATE IN POLAR-SOLVENTS

The lyotropic liquid crystals formed by sodium dodecyl sulfate in various polar solvents (water, formamide, glycerol, ethylene glycol, N-methylformamide) were investigated by using low-angle X-ray diffraction and optical microscopy. The method for rapid and continuous observations by X-ray diffraction of the surfactant-solvent system throughout their transitions from the micellar to the lamellar phase is described: the micellar and lyotropic phases are observed along the line of equilibrium with the crystalline phase. Complex phases were observed for the sodium dodecyl sulfate (SDS)/water system which displays an original sequence of two-, three-, and one-dimensional ordered phases: hexagonal (p6m) <--> centered rectangular (cmm) <--> rhomboedral <--> centered tetragonal <--> lamellar. The rhomboedral structure turns into a cubic phase Im3m. In the other polar solvents the phase sequence is very different. The SDS/formamide system exhibited the following sequence: hexagonal <--> cubic (Ia3d) <--> lamellar. Only lamellar structures were observed in the other less structured solvents. This work shows the importance of geometric constraints and head polar-solvent interactions.