Interaction of concentrated NaCl solutions with hydrophilic and hydrophobic fumed silicas
Authors:   Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, Gun’ko V. M. , Borysenko M.V. ,   Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, Holovan A.P. ,   Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, Krupska T. V. ,   Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, Wei Qiliang ,   Institute of Micro Nano Materials and Devices, Ningbo University of Technology, Zheng Jinju ,   Institute of Micro Nano Materials and Devices, Ningbo University of Technology, Yang Weiyou ,   Institute of Micro Nano Materials and Devices, Ningbo University of Technology, Turov V. V. ,   Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine
Published: November 2024

In porous or disperse media, the temperature and interfacial behaviors of water and solutions could be strongly affected by confined space effects (CSE). The surface nature of solids could influence the interfacial phenomena including both CSE and cryoscopic effects caused by the colligative properties of solutions. Strong changes in the characteristics of adsorption liquid layers, especially in narrow pores, are also caused by decreasing solvent activity. Therefore, it is of interest to compare the behaviors of water and NaCl solutions under CSE caused by hydrophilic and hydrophobic sorbents. Here, hydrophobic (AM–1) and hydrophilic (A–300) fumed silicas are used as representatives of disperse sorbents with different surface structure and characterized by textural porosity. This porosity is caused by voids between nonporous nanoparticles (NPNP) forming aggregates, agglomerates of aggregates, and visible particles (supra-NP structures) in the powders of low bulk density. Initial materials and related treated systems with bound water and NaCl/water were studied using nitrogen adsorption, microscopy, X–ray diffraction, infrared spectroscopy, thermogravimetry, rheometry, nuclear magnetic resonance spectroscopy, and quantum chemistry. Water bound to fumed silicas with or without NaCl could be assigned to several types: weakly (WBW, frozen at 260 K < T < 273 K) and strongly (SBW, frozen at T < 260 K) bound waters; weakly (WAW, chemical shift of dH = 0.5 – 2 ppm) and strongly (SAW, dH = 4 – 6 ppm) associated waters. WAW is not observed for A–300 systems. Additionally, in the systems with water/NaCl, there is frozen (immobile) water characterized by melting delay (T > Tm) at 273 K < T < 287 K (metastable water, MSW). The MSW appearance may be explained by release (with certain kinetic delay) of water trapped in NaCl crystallites dissolved at T > Tm = 273.15 K upon increasing amounts of liquid water with increasing temperature. The difference in the CSE in voids in hydrophobic and hydrophilic supra-NP structures onto bound water could be explained by the surface (–O)2Si(CH3)2 functionalities enhancing the clusterization of water bound to AM–1. As a whole, the difference in the surface nature of AM–1 and A–300 affects: (i) the NaCl crystallite size distributions; (ii) melting/crystallization temperatures of NaCl; (iii) viscosity and torque vs. shear rate (strain); (iv) temperature and interfacial behaviors of water alone and NaCl solutions at 215 – 287 K; and (v) effects of dispersion media influencing bound water. Obtained results are of interest not only from a theoretical point of view but also from a practical one since both silicas are used as components of composites containing water and NaCl (or other salts) in various practical applications in medicine, agriculture, etc.