A solvent-free porous liquid comprising hollow nanorod–polymer surfactant conjugates

Abstract

Liquids having permanent porosity can offer significant processing advantages over their solid counterparts. This has recently led to tremendous activity towards the design and development of intrinsic pores in the liquid phase, predominantly for studies involving gas sequestration. We show here the development of a solvent-free mesoporous liquid material based on anisotropic “hollow-core and silica-shell” nanorods conjugated with polymer surfactant chains, which can sequester CO₂ gaseous molecules at 0 °C. Hollow silica nanorods (SiNRs) with average aspect ratios of 2.5, 8, and 11 (as obtained by transmission electron microscopy (TEM) and small angle X-ray scattering) were synthesized using a surfactant-templating methodology, and fluidity/flow processability were imparted by a three-step process involving covalent coupling of an organosilane (OS) canopy to form OS@SiNR, followed by electrostatic grafting of polymer surfactant (PS) chains to the organosilane, and subsequent removal of solvent to provide a solvent-free composite, PS-OS@SiNR. Differential scanning calorimetric and frequency sweep rheological measurements of PS-OS@SiNR indicated melting transition between 15 and 20 °C, while thermal gravimetric analysis showed ca. 20 w/w% silica content (i.e. 9.5% volume fraction of silica and containing ca. 3% volume fraction as voids). As observed using TEM, the surface modification of the nanorods resulting in the formation of PS-OS@SiNR does not lead to blockage of the hollow core. We show that whilst N₂ adsorption in the porous liquid is hindered due to the glassy polymer-surfactant layer at −196 °C, CO₂ adsorption at 0 °C showed 3.3–4.8 w/w% gas uptake. Overall we demonstrate the synthesis of an anisotropic porous liquid which not only sequesters CO₂ but also has the ability to flow like a liquid.