Surroundings effects on the interfacial and temperature behaviors of NaOH/water bound to hydrophilic and hydrophobic nanosilicas

Abstract

Hypothesis: It has been assumed that the temperature and interfacial behaviors of concentrated alkali solutions under confined space effects may depend on adsorbent surface structure, hydrophilicity/hydrophobicity, porosity of solids, and dispersion media properties causing kosmotropic or chaotropic effects onto hydrogen bond network (HBN) in bound water and NaOH solution.

Experiments: To analyze these effects, systems with NaOH/water (0.1 g/g/0.1 g/g) deposited onto compacted hydrophilic (A-300) and hydrophobic (AM1) nanosilicas were studied using ¹H NMR spectroscopy (215-287 K). The materials were characterized using several experimental and theoretical methods.

Findings: It has been shown that bound water and water/NaOH represent various clusters and domains whose characteristics depend strongly on nanosilica hydrophilicity/hydrophobicity, dispersion media (air, CDCl₃, DMSO, CDCl₃/DMSO), subsequent or simultaneous deposition of NaOH and water, and temperature. Water amount (0.1 g/g) was selected too small to completely dissolve NaOH (0.1 g/g) under confined space effects and low temperatures. Chaotropic hydrophobic AM1 and CDCl₃ enhance water clusterization and HBN disorder (weakly associated water, WAW appears) in contrast to kosmotropic hydrophilic A-300, NaOH, and DMSO reducing the clusterization and HBN disorder in bound water (WAW disappears). Several aspects related to the interfacial and temperature behaviors of water and co-adsorbates bound to the nanosilicas were elucidated.

Keywords: Concentrated NaOH solution; Confined space effects; Hydrophilic nanosilica; Hydrophobic nanosilica; Interfacial solution behavior; Interfacial water; Sodium hydroxide; Temperature effects.