By combining traditional SANS with SEMSANS for the first time, the international collaboration was able to investigate the mechanism behind the initial growth of mesostructured silica-surfactant particles at the nano- and micro-metre scales simultaneously. The study, published in ACS Applied Materials & Interfaces, provides valuable insights on the formation of these particles, which are the precursors to thin films that grow at the solution interface. The work has also been chosen for the front cover of the July issue of the journal.
A mesoporous material is one that contains pores with diameters between 2 and 50 nm. Creating thin films that are mesoporous is an area that has attracted a lot of scientific interest because the ability to manipulate the structure of the film, and therefore the pore size, gives the films potential applications in a wide variety of fields including catalysis and drug-delivery.
The focus of this experiment was on self-organising silica-surfactant systems, as these can self-assemble into mesostructured thin films. These systems are based on a solution of surfactant molecules that form micelles, which interact with the silica species. This causes aggregation and, subsequently, thin film formation. For an overview on surfactants, their properties and applications, read our explainer article.
Two possible mechanisms for the self-assembly of these films have been suggested, but neither has previously been confirmed experimentally. This study used the unique combination of SANS and SEMSANS on Larmor to study the behaviour of the micelles and growing particles in solution to determine which mechanism occurs, under these conditions.
The researchers were able to see the surfactant micelles elongate on addition of the silica species, before they aggregate to form droplets. After this droplet formation has occurred, the micelles inside the droplet order into large particles with an ordered mesostructure. The particles then migrate to the air/liquid interface, forming the thin film.
This ability to combine techniques has provided a new and detailed insight into the formation of surfactant templated mesostructured particles and thus the resulting thin films.
“The ability to measure both SEMSANS and SANS over length scales from a few nanometres to several microns simultaneously with a time resolution of several minutes is a unique capability for Larmor." Explains beamline scientist Rob Dalgleish; “The technique is enabled by technology developed by Professor Roger Pynn's group at Indiana University, Bloomington (USA) and time-of-flight neutron scattering at ISIS. With further optimisation, it is hoped that a permanent installation of the required equipment will enable routine studies of multi-length scale materials such as those often developed in additive manufacturing."
The full paper can found online at DOI: 10.1021/acsami.0c03287