Controlled confined space effects on clustered water bound to hydrophobic nanosilica with nonpolar and polar co-adsorbates

It has been assumed that hierarchical nanoparticle/aggregate/agglomerate structure, texture, and surface characteristics of hydrophobic nanosilica AM1 stirred with a small amount of water may lead to different interfacial and temperature behaviors of bound weakly (WAW, δH = 0.5–1.5 ppm) and strongly (SAW, δH = 4–6 ppm) associated waters, depending on the amounts of weakly polar (chloroform), polar (dimethyl sulfoxide, DMSO), and ionic (trifluoroacetic acid, TFAA) co-adsorbates. Initial and hydro-compacted nanosilicas are characterized using adsorption, desorption, microscopic, spectroscopic, XRD, SAXS, and quantum chemistry methods. The 1H NMR spectroscopy (at 210–280 K) and cryoporometry (T < 273 K) results show significant re-organization of the interfacial water forming clusters and domains differently affected by chloroform, TFAA, and DMSO. Changes in the free surface energy (γS) due to interaction of unfrozen (T < 273 K) water with silica nanoparticles depend on the hydration degree and type and amounts of co-adsorbates. The γS value is maximal for SAW affected by chloroform + TFAA, and it is six times greater than that without TFAA, but a similar effect of TFAA is absent for WAW. Chloroform enhances WAW contribution, but DMSO similarly affects SAW. Thus, the use of a small amount of water bound to treated AM1 allows one to perform quantitative analysis of WAW and SAW, as well as weakly (frozen at 265–273 K) and strongly (unfrozen at T < 265 K) bound waters affected by various co-adsorbates. A fraction of water (its volume per gram of AM1 h/ρw > Vp pore volume) added to AM1 without strong stirring remains unbound due to hydrophobic properties of AM1, but another fraction of water (< Vp) is bound.