Synthesis of Surface-Modified Sands with Thermoresponsive Wettability

Abstract

Engineered geomaterials are designed with functionalities on demand to address practical problems in the geoenvironment, such as filtration and/or retardation of environmentally hazardous substances. In this study, an engineered soil was introduced with controllable and reversible wettability stimulated by external temperature variation. This new geomaterial was envisioned to retard or accelerate water percolation through its mass with external stimulation when used as a barrier or a filter ex situ or in situ. The tunable wettability was implemented by grafting a thin film of thermal stimuli-responsive polymer [poly(N-isopropylacrylamide) (PNIPAAm)] on sand particles using the surface-initiated atom transfer radical polymerization (SI-ATRP) technique. The PNIPAAm has an easily accessible lower critical solution temperature (LCST), ≈32°C. The polymer-coated surface has hydrophilicity at temperatures below the LCST, and changes to hydrophobicity above the LCST. The material composition and structural conformation of synthesized polymer were examined by Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The roughness and thickness and the coverage of the polymer film on substrates were assessed by atomic force microscopy (AFM) and optical ellipsometry. Solid–liquid contact angles and water percolation behavior were examined to verify the wettability change on a surface-modified sand pack triggered by changing temperature externally. The thermal-sensitive wettability demonstration of the synthesized sand particles implies proof of concept for innovative applications of the technique in proactive hazard mitigation such as use as a tunable granular valve, barrier, intelligent fluid separator, or biofilter.