Structural relations between lyotropic phases in the vicinity of the nematic phases

Abstract

« Nematic lyotropic » phases are anisotropic fluids made of aggregates of amphiphilic molecules dispersed in water. Their nematic behaviour (flow, orientation in a magnetic field) is related to the orientational correlations between the aggregates which are anisotropic particles of finite dimensions. Therefore they are examples in the field of lyotropic liquid crystals of intermediate states of organization between the well known ordered lamellar, hexagonal or cubic phases and the totally disordered micellar phases. This paper presents an attempt to determine the conditions governing the occurrence of nematic phases in the system sodium decyl sulfate/1-decanol/water. For this purpose the ternary phase diagram is investigated in the vicinity of the nematic phases, first the various lyotropic phases are recognized according to their textures, then their structures are determined through small angle X-ray scattering. From these observations the effects induced by changing the two most obvious parameters, the concentrations of decanol and water, are as follows. When the phase diagram is crossed towards increasing decanol concentration the curvature of the amphiphile-water interface decreases (an hexagonal phase made of infinite cylinders is replaced by a lamellar phase, intermediate phases made of infinite ribbons and in some cases « nematic » phases may appear in between). On the other hand, when the water content is increased, the curvature of the amphiphile-water interface increases and the stacking of the aggregates becomes more disordered (a lamellar phase is first replaced by a phase made of infinite ribbons, thereafter by a « nematic » phase made of prolate spheroids, then by a « nematic » phase made of oblate spheroids or discs, and finally by a fully disordered micellar phase). Therefore decanol and water control the shapes, sizes and packing of the aggregates along antagonistic ways and the « nematic » phases, which occupy a very limited area in the phase diagram may be considered as resulting from the delicate interplay of these two parameters.