Materials with Hierarchical Porosity Enhance the Stability of Infused Ionic Liquid Films

Abstract

Defined surface functionalities can control the properties of a material. The layer-by-layer method is an experimentally simple yet very versatile method to coat a surface with nanoscale precision. The method is widely used to either control the chemical properties of the surface via the introduction of functional moieties bound to the polymer or create nanoscale surface topographies if one polymeric species is replaced by a colloidal dispersion. Such roughness can enhance the stability of a liquid film on top of the surface by capillary adhesion. Here, we investigate whether a similar effect allows an increased retention of liquid films within a porous surface and thus potentially increases the stability of ionic liquid films infused within a porous matrix in the supported ionic liquid-phase catalysis. The complex geometry of the porous material, long diffusion pathways, and small sizes of necks connecting individual pores all contribute to difficulties to reliably coat the required porous materials. We optimize the coating process to ensure uniform surface functionalization via two steps. Diffusion limitations are overcome by force-wetting the pores, which transports the functional species convectively into the materials. Electrostatic repulsion, which can limit pore accessibility, is mitigated by the addition of electrolytes to screen charges. We introduce nanoscale topography in microscale porous SiC monoliths to enhance the retention of an ionic liquid film. We use γ-Al₂O₃ to coat monoliths and test the retention of 1-butyl-2,3-dimethylimidazolium chloride under exposure to a continuous gas stream, a setup commonly used in the water-gas shift reaction. Our study showcases that a hierarchical topography can improve the stability of impregnated ionic liquid films, with a potential advantage of improved supported ionic liquid-phase catalysis.

Figure 1

LbL coating of different surfaces by SiO₂ particles. (a) Schematic representation of the LbL process that consists of immersion of a substrate (flat or porous) in the positively charged PDADMAC followed by rinsing in water to get rid of excess of polymer and then in a solution of negatively charged SiO₂ particles. A homogenous coating is obtained on flat surfaces, but in the porous material, the particles do not form a coating in the inner pores. Resulting coating on (b) a flat Si wafer and (c) a porous SiC (3 μm average pore size) monolith (i) magnification of the outer surface of the SiC monolith and (ii) magnification of the inner pore of the SiC monolith. (d) Resulting coating of SiO₂ inverse opals (350 nm average pore size). All scale bars of (b–d) equal 1 μm. Photograph of (e) SiC monolith and (f) inverse opal in a Si wafer.

Figure 2

SiC monolith with micrometer-scale porosity coated with SiO₂ particles via the LbL technique. Schematic representation of the LbL coating process in two different scenarios. The top part depicts the conventional LbL process, while the bottom part shows the modified process with an intermediate drying step in between each immersion. Panel (i) shows an inner pore immersed in PDADMAC solution, panel (ii) depicts the diffusion of SiO₂ particles inside a pore flooded from the previous coating step, panel (iii) shows the unsuccessful SiO₂ coating with the conventional coating process, and panel (iv) shows a representative SEM image of the surface of an actual pore in a SiC monolith unsuccessfully coated. The bottom row shows the modified process. Panel (v) shows a dried inner pore coated with a layer of PDADMAC, panel (vi) shows the introduction of the SiO₂ dispersion by convective wetting into a dry pore, which created a uniform concentration profile and thus achieves a homogeneous coating (vii). Panel (viii) shows a representative SEM image of a successfully coated pore.

Figure 3

Inverse opals coated with SiO₂ nanoparticles (20 nm) using the conventional LbL technique (a) and the modified LbL version with a convective flow of solutions of PDADMAC and SiO₂ colloidal particles (b).

Figure 4

Schematic illustration of the effect of the surface potential of adsorbed particles on pore accessibility. Three possible scenarios at the neck of two pores of an inverse opal are compared. (a) The surface potential of two adsorbed particles extends into the neck region and therefore restricts pore accessibility of like-charged particles. (b) With the addition of salt, the electrostatic double layer is screened, providing pore access for a like-charged particle. (c) At a high salt concentration, the charge screening becomes pronounced enough to compromise the colloidal stability, leading to adsorption of particle agglomerates, which physically block the pore.

Figure 5

(a) Zeta potential of the colloidal SiO₂ in dispersions with different concentrations of NaCl. (b) Debye length in solutions with different concentrations of NaCl.

Figure 6

Influence of salt concentration on the pore functionalization efficiency of inverse opals coated using the LbL method. (a) Representative side-view SEM images of inverse opals coated using the LbL technique with dispersions of colloidal SiO₂ with different NaCl concentrations (columns) and different immersion times (rows), scale bars: 500 nm. (b, c) Statistical analysis of the pore functionalization efficiency showing the ratio of coated pores as a function of salt concentration for immersion times of 1 h (b) and 1 day (c). Over 500 pores were analyzed for each experiment.

Figure 7

Enhanced retention of IL films by nanoscale surface roughness within a porous material. (a–c) SEM images of the inner pores of a SiC monolith coated with 15 layers of γ-Al₂O₃ nanoparticles using the LbL method with a convective flow of coating solutions. (d) Gravimetric evaluation of the retention of the IL in coated and noncoated SiC monoliths.