The structure of intermediate ribbon phases in surfactant systems

Abstract

Intermediate phases consisting of elongated rods with a non-circular cross section (i.e. ribbons) that pack on a deformed hexagonal lattice are often formed in surfactant systems. The crystallographic lattice of such a ribbon phase can be either of oblique (or two dimensional monoclinic), primitive rectangular, centred rectangular or hexagonal symmetry, all of which are observed according to the literature. We have studied a ribbon phase that is formed in the dodecyl-1,3-propylene-bisamine/HCl/water system, by means of SAXS experiments, and a centred rectangular structure is obtained. In addition, we have reviewed, and partially reinterpreted, previously published results on the structure of different ribbon phases. 21 scattering data sets for ribbon phases of lower than hexagonal symmetry have been analysed. Both the centred rectangular (cmm) and the primitive rectangular (pmm) symmetry fit 10 of the data sets, whereas only a centred rectangular lattice can be fitted to the other 11 data sets. There is no experimental indication that supports the existence of an oblique ribbon (p2) phase. The underlying physical reasons for the observation that the centred rectangular structure is favoured are discussed in terms of a cell model approach. The energetically most favoured cell model has a centred rectangular symmetry (cmm), in accord with the experimental data, and is termed the hexagon-rod model. This model can be used to evaluate the dimensions of the deformed rods of the centred rectangular ribbon phase from scattering data and NMR data separately. One major advantage of the hexagon-rod model is that the smallest dimension of the aggregate is not required as an input parameter in the calculations. Axial ratios between 1·2:1:1-2:1 for the aggregates are obtained when this model is applied to SAXS, SANS and NMR data for the centred rectangular ribbon phase for none different systems.