Surfactants are molecules composed of a hydrophilic (water-loving) part and a hydrophobic (water-hating) part, which makes them align in water/oil interfaces or form a fascinating range of nanostructures in solution, depending on their concentration, the temperature of the system, and the presence of additives in solution. Materials science exploits the use of surfactants to create self-assembled nanostructures, and a good example is the synthesis of templated mesoporous silica based materials, introduced by Kresge et al. . Creating inorganic materials with well controlled pore sizes and pore connectivity can be important in many fields, from catalysis and separations, to sensors, electronic applications, and biomaterials. [2-4]
Understanding the formation of these materials has not been easy, and although significant progress has been made over the last decade, from an experimental [4-8] and modelling [9-14] and theoretical  perspectives, many issues are not well understood. One of the major challenges that the formation of surfactant templated materials poses is that it is an inherently multiscale problem.
At the smallest scale we have the condensation reactions of the inorganic precursor, whose mechanism is strongly affected by the pH of the solution. Some understanding of these processes comes from the extensive work that has been done to understand the formation of zeolites [16-18], although understanding how the presence of surfactants affects such processes is a problem that has not been addressed.
The next scale deals with the interactions between small inorganic oligomers and surfactant self assemblies. It is now understood that the presence of silica oligomers is necessary to drive the phase separation that leads to the formation of templated materials [10,19], and simulations also confirm that strong interactions between inorganic and surfactant heads lead to a phase separation where a sufficiently high surfactant concentration can form ordered phases. Even when some approximations can be made for systems at equilibrium, understanding the evolution of these structures is far from understood. No reports have been made for any successful coarse graining approach that will successfully describe the structural evolution observed in these materials, as state of the art molecular dynamics simulations can not reach sufficiently long times .
At the largest scale we have the effects of hydrodynamics (stirring as reported in experiments), which is known to have some effect that can favour or not the ordering of nanostructures, but our understanding of such phenomena is not sufficient to model or predict it.
A parallel problem to understanding the synthesis of surfactant templated materials is to describe the structures formed with sufficient accuracy to understand their applications and limitations. Extensive work has been done in trying to determine pore size distributions of materials containing cylindrical pores, using theory and simulations [21-24], but more limited work has addressed the effect of surface roughness and variations of pore size in the axial direction of a given cylinder that leads to constrictions [25-27].
We consider that the importance and potential impact that surfactant templated materials can have in science and technology demand a better understanding of their synthesis and characterisation, which requires the development of appropriate models and simulation approaches that can deal with the different scales involved.
(Universidade do Porto)
(University of Manchester)