Understanding self-assembly of silica/surfactant mesostructures through multi-scale modelling

Nanoporous silica materials are produced by templated synthesis, whereby a surfactant liquid-crystal acts as template, and thus allow control over pore size by changing synthesis conditions [1]. They are thus excellent... [ view full abstract ]

Nanoporous silica materials are produced by templated synthesis, whereby a surfactant liquid-crystal acts as template, and thus allow control over pore size by changing synthesis conditions [1]. They are thus excellent candidates for computational design, which would have tremendous benefits in areas like gas separation, catalysis, and drug delivery. Accomplishing this goal, however, requires models that can accurately predict how silicate precursor and surfactant templates interact in aqueous solutions, giving rise to ordered mesostructures. This is a very challenging task, given the wide range of time and length scales that are relevant to the self-assembly of these structures. In this study, we present results of a new multi-scale simulation approach to understand porous silica synthesis at the molecular and mesoscale levels, based on constructing a chain of models that range from the quantum-mechanical level, through classical atomistic simulations, up to a mesoscale coarse-grained (CG) description of the system [2]. Our CG model was validated by accurately reproducing the phase diagram and average micelle aggregation number of cationic surfactant solutions, in remarkable agreement with experimental measurements.

Using this newly developed CG model, we have performed extensive molecular dynamics (MD) simulations in the microsecond time scale to probe in detail the phase diagram of silica/surfactant aqueous solutions, changing several key synthesis conditions such as temperature, pH and degree of silica polymerisation [3,4]. Our results provide answers to several hitherto contentious points regarding the synthesis of nanoporous silica materials: i) silicate monomers replace bromide counterions and strongly adsorb on the surface of surfactant micelles; ii) monomeric silica adsorption leads to micelle size increase and eventually to a sphere-to-rod transition; iii) silicate oligomers are necessary to promote formation of hexagonal liquid crystals, by forming “charge bridges” linking different surfactant micelles; iv) nanoporous silicas form through a co-operative templating mechanism controlled by charge-matching interactions between silicates and surfactants; v) the postulated liquid-crystal templating mechanism, where silicates are mere spectators, is not viable for this class of materials under typical synthesis conditions. Our modelling approach thus provides unprecedented insight into how these materials actually form in solution, and paves the way for computational material design efforts.

[1]   Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L., J. Am. Chem. Soc., 114, 10834 (1992).

[2]   Pérez-Sánchez, G.; Gomes, J. R. B.; Jorge, M., Langmuir, 29, 2387 (2013).

[3]   Pérez-Sánchez, G.; Chien, S.-C.; Gomes, J. R. B.; Cordeiro, M. N. D. S.; Auerbach, S. M.; Monson, P. A.; Jorge, M., Chem. Mater. 28, 2715 (2016).

[4]   Chien, S.-C.; Pérez-Sánchez, G.; Gomes, J. R. B.; Cordeiro, M. N. D. S.; Jorge, M.; Auerbach, S. M.; Monson, P. A., J. Phys. Chem. C, 121, 4564 (2017).