SURFACE RELAXATION OF A HEXAGONAL LYOTROPIC MESOPHASE

The influence of the free surface on the structure of the hexagonal ly otropic mesophase (H-1) of the binary surfactant system pentaoxyethyle ne-n-dodecylether + water was probed by grazing incidence X-ray diffra ction. The reflected diffraction patterns consist of hexagonally arran ged discrete spots showing that the orientation of the (01) lattice pl anes is parallel to the surface. This orientation extends several micr ometers into the bulk. We report on experimental evidence for the coex istence of two lattices with different d-spacings. In the reflected di ffraction patterns observed at a limited penetration depth of the inci dent beam, the Bragg spots split into two well-resolved peaks, one of which disappears as the penetration depth is increased. Analysis of th e diffraction data reveals the existence of a surface layer of hexagon ally packed micelles with a lattice constant smaller than the correspo nding bulk value.