Percolation Diffusion into Self-Assembled Mesoporous Silica Microfibres

Abstract

Percolation diffusion into long (11.5 cm) self-assembled, ordered mesoporous microfibres is studied using optical transmission and laser ablation inductive coupled mass spectrometry (LA-ICP-MS). Optical transmission based diffusion studies reveal rapid penetration (<5 s, D > 80 μm²∙s^-¹) of Rhodamine B with very little percolation of larger molecules such as zinc tetraphenylporphyrin (ZnTPP) observed under similar loading conditions. The failure of ZnTPP to enter the microfibre was confirmed, in higher resolution, using LA-ICP-MS. In the latter case, LA-ICP-MS was used to determine the diffusion of zinc acetate dihydrate, D~3 × 10^-4 nm²∙s^-1. The large differences between the molecules are accounted for by proposing ordered solvent and structure assisted accelerated diffusion of the Rhodamine B based on its hydrophilicity relative to the zinc compounds. The broader implications and applications for filtration, molecular sieves and a range of devices and uses are described.

Keywords: colloids; filters; laser ablation inductive coupled mass spectroscopy; mesoporous; microfibres; microfluidics; microwires; molecular sieves; nano-composites; nanoparticles; nanopores; self-assembly; sensors; super diffusion.

Figure 1

An illustration of hcp packing and the two types of interstitial regions. As an approximation a round sphere that can fit into the pores is often assumed to determine the size of a molecule that can percolate or diffuse into the structure. This does not, however, take into account irregular shapes and quantities of larger molecules, more typical of real situations. The site size distribution shown reflects that calculated from the bulk of the size distribution of the nanoparticles used in this work, measured by dynamic light scattering (DLS).

Figure 2

The formulas, schematic and space filling structures, as well as dimensions of each of the three molecules used in this work. The Cl⁻ anion of Rhodamine B is free in solution and may be displaced by the negatively charged water at a silica-water interface.

Figure 3

Surface characterisation of a self-assembled silica microwire: (a) optical micrograph of long wires produced using gravity assisted deposition by evaporative self-assembly; (b,c) SEM local images of the wire surface reproduced from earlier work (Naqshbandi et al. [1]) to illustrate the presence of a finite thickness layer, ~(250–500) nm, (a) and (b) on top of an inner core. An examination of the surface layer (c,d) reveals what appears to be a mix of fcc and hcp packing in places; (e) shows an AFM analysis of the >11 cm wires used in this work with similar hcp packing to that of previous work [3].

Figure 4

Schematic of the optical transmission measurement through the microfibre with a drop of Rhodamine B containing water. The coupling area between standard fibre and slab microfiber is zoomed in for clarity.

Figure 5

Optical transmission of 632 nm through a self-assembled wire as a function of time during percolation of Rhodamine B using the setup illustrated in Figure 4. Inset shows a close-up of the initial attenuation the signal has been normalized to the initial signal level measured.

Figure 6

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) of self-assembled wires doped with ZnTPP and zinc acetate solutions (c₀ = 100 mM). (a) Signal intensity as a function of each ablation over time for both molecules and (b) Diffusion analysis (natural log of signal intensity vs. area) of the hydrated Zn²⁺ from the zinc acetate solution.