Early Formation Pathways of Surfactant Micelle Directed Ultrasmall Silica Ring and Cage Structures

Abstract

By combining a surfactant, an organic pore expander, a silane, and poly­(ethylene glycol) (PEG), we have observed the formation of a previously unknown set of ultrasmall silica structures in aqueous solutions. At appropriate concentrations of reagents, ∼2 nm primary silica clusters arrange around surfactant micelles to form ultrasmall silica rings, which can further evolve into cage-like structures. With increasing concentration, these rings line up into segmented worm-like one-dimensional (1D) structures, an effect that can be dramatically enhanced by PEG addition. PEG adsorbed 1D striped cylinders further arrange into higher order assemblies in the form of two-dimensional (2D) sheets or three-dimensional (3D) helical structures. Results provide insights into synergies between deformable noncovalent organic molecule assemblies and covalent inorganic network formation as well as early transformation pathways from spherical soft materials into 1D, 2D, and 3D silica solution structures, hallmarks of mesoporous silica materials formation. The ultrasmall silica ring and cage structures may prove useful in nanomedicine and other nanotechnology based applications.

Two‐Dimensional Superstructures of Silica Cages

“…We speculate that negatively charged silica clusters adsorb at the water−oil interface attracted by positively charged CTAB surfactant head groups and further stabilized by the hydrophobic aminopropyl groups which intercalate with the CTAB layer. [ 21 ] Similar adsorption processes have recently been evidenced in TEM images of CTAB micelles decorated with individual silica clusters, [ 18 ] an effect which is promoted by the deformability of the surfactant−water interface, which is enhanced by micelle swelling with hydrophobic molecules. In turn, the surfactant molecules wrap around these clusters, increasing the degree of interaction between the organic and inorganic components.…”

“…[ 16 ] In the presence of cationic surfactants such as cetyltrimethylammonium bromide (CTAB), negatively charged clusters self‐assemble into micelle‐templated mesoporous silica, [ 1 ] with sizes controllable down to single pore nanoparticles. [ 17 ] The addition of a pore expander increases micelle size, size dispersity, and deformability, [ 9,18 ] enabling cage‐like mesoporous structures. Numerous studies have identified bulk mesoporous materials formed from such cages as basic building blocks, [ 3,8,19 ] including 5 12 , 5 12 6 2 , or 5 12 6 3 cages, where 5 x 6 y refers to a cage made of x pentagonal and y hexagonal faces.…”

Porous cage-derived nanomaterial inks for direct and internal three-dimensional printing

“…To that end, we added 3-(trimethoxysilyl)propyl methacrylate directly to the solution after cage synthesis, but with the structure-directing surfactant micelles still present. Previous studies suggested that as part of their formation mechanism, the cage vertices and struts deform the micelle surface, with positively charged surfactant molecules wrapping the negatively charged inner silica cage surface 22 . As illustrated in the inset of Fig.…”

Mesoporous Nanoarchitectures for Electrochemical Energy Conversion and Storage

“…[ 68 ] The sol of as‐synthesized methyl‐group‐modified silica‐coated block copolymer micelles could be stable for at least 6 months, indicating that such a modification process is effective to prevent agglomeration. Similarly, ultrasmall silica nanocages [ 69 ] (Figure 4b) and nanorings [ 70 ] (Figure 4c) can also be prepared through the arrangement of primary silica clusters on the surface of surfactant micelle templates and a follow‐up silica‐surface PEGylation. These ultra​small single‐micelle nanounits could provide a better understanding of the formation process of mesoporous nanoparticles.…”

“…c) Reproduced with permission. [ 70 ] Copyright 2018, American Chemical Society. d) Reproduced with permission.…”