Lyotropic phase behaviour of n-octyl-1-O-?-D-glucopyranoside and its thio derivative n-octyl-1-S-?-D-glucopyranoside

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We have determined the phase diagrams of n-octyl-1-O-β-D-glucopyranoside and its thio derivative n-octyl-1-S-β-D-glucopyranoside in water, using polarizing microscopy and X-ray diffraction. Comparison of the two compounds gives us an insight into the complex interactions which determine phase behaviour. Both compounds form type I (normal) lyotropic phases, with the adoption of a fluid lamellar phase at low hydrations and a cubic phase at higher hydrations. However, the hexagonal phase observed in the n-octyl-1-O-β-D-glucopyranoside/water system adjacent to the micellar solution is suppressed in the n-octyl-I-S-β-D-glucopyranoside/water system. In addition, we have found that in both systems the cubic phase spontaneously grows into large monodomains, as revealed by faceting of entrapped air bubbles. Indexing of the monodomain diffraction patterns shows that in both cases the space group of the cubic phase is Ia3d (no. 230).