Shear Melting and Orientation of a Lyotropic Cubic Phase

(Received 13 December 1994, revised 28 February 1995, accepted 13 March 1995)

Abstract
A bicontinuous lyotropic cubic phase, composed of the nonionic surfactant pentaethylene glycol dodecyl ether (C ₁₂E ₅), heavy water (D ₂O) and tetradecane (C ₁₄H ₃₀), was investigated by small angle neutron scattering (SANS). The cubic phase is stable only in a narrow temperature and composition range. Upon increasing the oil content the phase melts to an L ₃ phase and at higher temperature it is in equilibrium with a lamellar (L ) phase. The scattering function of the powder cubic sample is dominated by a strong correlation peak at q=0.057 Å ^-1 in addition to a small hump at  Å ^-1. From the known area per C ₁₂E ₅ molecule at the polar/apolar interface the lattice parameter is calculated for different minimal surface model structures. The position of the dominating reflection from the cubic phase is found to be consistent with the spacing between the 211 planes of the Gyroid minimal surface structure. The effect of shear on the scattering pattern was investigated using a Couette shear cell. Under shear a weakly anisotropic scattering pattern was obtained. From a separate rheology experiments it was concluded that shearing the sample has the effect of melting the long range cubic order, presumably into a fluid disordered L ₃ phase. When turning off the shear, the cubic phase recrystallises with a preferred orientation. The diffraction pattern is consistent with the diffraction from 211 planes with one major and one minor population in the orientation of the <111> axis. A major population having the <111> axis parallel to the shear velocity (i.e. the tangent direction), and a minor component having the <111> axis parallel to the velocity gradient.