Introduction One of the ``Holy Grails'' of materials science is to achieve perfect control over the size, overall shape and morphology of crystals used in the production of technologically interesting materials [1]. With the revolution of nanotechnology a remarkable array of methods to control the size and monodispersity of small compact crystals has appeared. It has proved much more dicult to tailor the shape and morphology of crystals [1]. Because of a strong interfacial energy component to thefreeenergyofagrowingcrystal,thepreferredshapeis almost always compact spherical, cubic, normal poly- hedral). The use of soft colloids micelles, microemul- sions, Langmuir±Blodgett ®lms, lyotropic liquid crystal, polymer solutions and phases) as templates for inorganic crystallization has attracted considerable attention in recent years [2, 3, 4, 5]. Block copolymers [4] and polyelectrolyte±surfactant gels [5] have already been used with some success in this respect, and morphology control has been reported in some cases. We focus our present discussion on polymer±surfactant systems as novel media for inorganic crystal growth. We use the reaction between Pb 2+ and S 2) ions for the formation of PbS as a prototype reaction to examine the eect of the organization of the solution on the shape of the reaction product. PbS is in itself an interesting material, being a semiconductor with a wide range of potential application [6]. Recent work has shown that the shape and overall morphology of PbS nanocrystals can indeed be con- trolled by templating phenomena in systems in involving a strong interaction of Pb 2+ ions and a moiety of the template [7]. This investigation started some years ago, when we examined the size and shape variation of PbS nanocrys- tals formed in polyethylene oxide) PEO) sodium dodecyl sulfate SDS) solutions [8]. It was realized that the use of relatively low polymer and surfactant Progr Colloid Polym Sci 2001) 118:57±62 Ó Springer-Verlag 2001 POLYMER±SURFACTANT INTERACTIONS E. Leontidis T. Kyprianidou-Leodidou W. Caseri K. C. Kyriacou Surprising effects of polymer±surfactant solutions on inorganic crystallization processes E. Leontidis &) T. Kyprianidou-Leodidou Department of Chemistry University of Cyprus P.O. Box 20537 1678 Nicosia, Cyprus e-mail: psleon@ucy.ac.cy Tel.: +357-2-892185 Fax: +357-2-339063 W. Caseri Institut fuÈr Polymere, ETH Zentrum 8092 Zurich, Switzerland K. C. Kyriacou The Cyprus Institute of Neurology and Genetics, P.O. Box 3462 1683 Nicosia, Cyprus Abstract Polymer±surfactant solu- tions, in which the surfactant mole- cules interact strongly with the polymer chains forming polymer- bound micelles above the critical association concentration, are used as media for inorganic precipitation reactions. The formation of PbS is used as a prototype reaction with unexpected results. Under a wide range of conditions, the PbS crys- tallites initially produced evolve into a range of metastable structures composed of PbS and lead dodecyl sulfate. X-ray diraction and trans- mission electron microscopy serve as valuable tools to study the evolution of the crystallizing system in detail. The coexistence of three dierent colloidal particles polymers, sur- factant micelles and inorganic na- nocrystals) leads to extremely complex behavior. The present work highlights the signi®cance of cou- pling colloidal aggregation to ionic equilibria, and introduces polymer± surfactant solutions as a new med- ium for the study of inorganic crystallization reactions and the production of organic±inorganic nanocomposite materials. Key words Nanocrystals á Polymer±surfactant aggregates á Metastable structures á Electron microscopy