Effects of particulate morphology, texture, and surface structures of various sorbents on bound water characteristics at low temperatures

The temperature and interfacial behaviors of water bound to various sorbents (silicas, carbons, polymers, etc.) are of interest from both theoretical and practical points of view because a certain amount of water could be always adsorbed from air and can affect the material properties, especially at low temperatures due to possible frost damage. These behaviors could be studied using low-temperature 1H NMR spectroscopy of static samples. The particulate morphology and texture of sorbents were characterized using microscopic and nitrogen adsorption methods. The study well demonstrates the influence of various factors including the morphology, texture, and surface structure of sorbents on the temperature and interfacial behaviors of bound water in the amounts smaller than the pore volume of sorbents. Upon volume infilling of pores by water, the textural and morphological effects (leading to the freezing/melting point depression) could be stronger than the effect of the surface structure (leading to the reorganization of bound water), because only one-two adsorption layers are well sensitive to the surface structure (polar or nonpolar surface functionalities). Therefore, changes in the relative amounts of unfrozen water Cuw(T)/C280K vs. temperature (at 200 K < T < 273 K) are similar for very different sorbents such as nanosilica, nano/mesoporous silica gel, and activated carbon (at close water amounts in the hydration range of h = 0.04–0.06 g/g) in contrast to that for microcrystalline cellulose. There are strong effects caused by the bound water amounts that are better observed for sorbents with a great contribution of nanopores, e.g., activated carbon AC–86 possessing very high specific surface area due to significant nanoporosity. A nonmonotonic effect of the amounts of water bound to AC–86 could be explained by nonuniform distribution of O-containing functionalities (mainly located at the edges of carbon sheets at the entrances into slitshaped hydrophobic nanopores). The clustered adsorption of water around these functionalities inhibits penetration of water into nanopores (formed by hydrophobic basal planes) especially at small amounts of water. An increase in the water content causes more intensive diffusion of the water molecules into narrower but less hydrophilic pores of AC–86 that results in enhanced freezing/melting point depression. The obtained results are of interest from a practical point of view since very different and practically important sorbents were analyzed in parallel at low temperatures upon various wetting.