Surfactants for Self-Assembly Materials
Introduction
Surfactants are generally employed in materials science to generate self-assembly structures and play a critical role in stabilizing interfaces and nanostructures. Self-assembly of surfactants creates hydrophobic and hydrophilic structural domains, creating opportunities to use surfactant systems as mediators for the synthesis of self-assembled materials with unique properties.
Surfactant mediated ceramics synthesis
Some interesting ceramic architectures through surfactant mediation have been described. The use of amino acid based surfactants represent novel trends in inducing silica self-organization to meso structured materials. Further application of biologically relevant templates involve the use of tobacco mosaic virus liquid crystals in silica synthesis. Biologically relevant ceramics such as calcium carbonate have been synthesized in the presence of surfactant-dendrimer complexes. The dendrimers with long alkyl tails are able to incorporate both cationic and anionic surfactants, thereby facilitating a systematic study of surfactant mediated calcium carbonate crystallization.
Polymer architectures templated by surfactants
It has often been stated that goal of such polymerization is to solidify and preserve the morphology of self-assembly, thereby creating structures that are ordered over several length scales. Doping self-assembled surfactant microstructures with monomers to synthesize nonparticulate polymers with unusual morphologies remains a fascinating objective. For example, the polyelectrolyte-surfactant complexes are doped with vinyl monomers subsequent to which free-radical polymerizations are conducted. The resultant polymers show wire and cigar shaped morphologies which appear related to the structure of the complex. Doping hydrophobic monomers in vesicles to produce hollow polymer shells also represents continuing efforts to induce morphology into polymer structures through surfactant templating mechanisms.
Surfactants and nanocrystal synthesis and assembly
Templated crystallization of inorganic nanoparticles in surfactant systems has progressed in the last few years. Spherical nanoparticle synthesis in water-in-oil micro emulsions continues to be a much-studied area with new variants. Au/Pd and Au/Ag bimetallic particles have been synthesized in AOT water-in-oil microemulsions, which show the formation of true bimetallics and not particles of individual species. The use of surfactant systems to synthesize nonspherical inorganic nanoparticles is also of interest since shape anisotropy may have pronounced effects on nanoparticle function.
Application
The fact that surfactant self-assembly leads to nanoscale hydrophobic and hydrophilic domains has allowed synthesis of hybrid functional materials such as mesoporous ceramics containing polymers within the pores, nanoparticles in polymer matrices for device applications, etc. A notable example is the incorporation of conjugated polydiacetylenes in meso porous silicas through co-synthesis of polymerizable surfactants that provide the template for silica condensation. The conjugated polymers exhibit novel chromatic changes in response to thermal, mechanical and chemical stimuli and the silica provides the mechanical integrity for device fabrication.
Reference
- John, V. T., Simmons, B., McPherson, G. L., & Bose, A. Recent developments in materials synthesis in surfactant systems. Current opinion in colloid & interface science, 2002, 7.5-6: 288-295.