Structural transformations in confined lamellar phases in oil-water-surfactant mixtures

Lamellar phases in oil-water-surfactant mixtures are studied in a slit geom etry. Within a lattice vector model we obtain the following results in a me an-field approximation. (i) In the case of swollen phases the lamellae are parallel to the walls for all distances L between them and for all surface- field strengths; the system responds elastically to the external stress. (i i) For shorter periods of the bulk phase (surfactant volume fraction rho (s )approximate to1/3) the lamellae are parallel to hydrophilic, and perpendic ular to neutral or very weakly hydrophilic surfaces. For a small range of t he surface-fields changes of L lead to a switch from the parallel to the pe rpendicular orientation, as found previously for confined block copolymers. (iii) For still shorter periods (rho (s)greater than or equal to1/2) the p erpendicular orientation is stable up to much higher hydrophilicity of the walls. Changes of L induce the switch between the two orientations for a la rge range of the surface fields. For rho (s)greater than or equal to1/2 the structure in the slit can no longer be described by one-dimensional densit y profiles even for the parallel orientation. Structural deformations simil ar to dislocations lead to an inelastic response to compression or expansio n. (C) 2001 American Institute of Physics.