X-ray structural studies of lyotropic-nematic phases in the ternary systemhexadecyl dimethylethyl ammonium bromide/water/n-decanol

The effect of n-decanol as a co-surfactant on structure formation in the te rnary system hexadecyl dimethylethyl ammonium bromide (C(16)Me(2)EABr)/wate r/n-decanol nias investigated by X-ray structural studies. Results show tha t the addition of n-decanol to the binary system C(16)Me(2)EABr/ H2O effect s a transition to structures with a smaller interfacial curvature throughou t the phase diagram. In ail phases, the incorporation of n-decanol in the m icelle causes an increase in electron density in the head group area of the micelle. Depending on surfactant concentration, the phase sequences that o ccur at T = 298 K are as follows: 15% w/w C(16)Me(2E)ABr S-->L-1*-->L-alpha-->Q(alpha) 32% w/w C(16)Me(2)EABr N-C-->N-D-->L-alpha-->Q(alpha) 50% w/w C(16)Me(2)EABr H-alpha-->L-alpha--> Q(alpha) 70% w/w C(16)Me(2)EABr Q(alpha)-->L-alpha (S=isotropic, micellar phase, L-1*=isotropic, shear-induced phase; L-alpha = lamellar phase, chain fluid; Q(alpha) = cubic phase; N-C = lyotropic-nema tic rod-like phase; N-D = lyotropic-nematic disk-like phase; H-alpha = hexa gonal phase, chain fluid). Anisotropic rod micelles are already present in the isotropic phase of the ternary system. In the N-C phase, the addition of n-decanol enlarges the di ameter of the cylindrical micelles. The length of the cylindrical micelles also increases. The structural properties of the N-D phase, which has a ver y narrow range of existence, are determined by the interaction of surfactan t parameters and water content.