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Terahertz time-domain attenuated total reflection measurements of monosaccharide (glucose and fructose) and disaccharide (sucrose and trehalose) solutions from 0.146 M to 1.462 M were performed to evaluate (1) the hydration state and (2) the destructuring effect of saccharide solutes on the hydrogen bond (HB) network. Firstly, the extent of hydration water was determined by the decreased amount of bulk water with picosecond relaxation time that was replaced by that with much longer orientational relaxation time. As a result, we found glucose and trehalose exhibits stronger hydration capacity than fructose and sucrose, respectively, despite of the same number of the hydroxyl groups. For each saccharide, the hydration number tended to decrease with solute concentration. Secondly, the destructuring effect of these saccharide solutes on the HB network of the surrounding bulk water was discussed from the perspective of the fraction of non-hydrogen-bonded (NHB) water isolated from the HB network. We found the fraction of NHB water molecules that are not engaged in the HB network monotonously increased with saccharide concentration, indicating saccharide solutes promote the disruption of the water HB network. However, no noticeable differences were confirmed in the fraction of NHB water between glucose and fructose or between sucrose and trehalose. In contrast to hydration number, the number of NHB water produced by a single saccharide solute was less dependent on solute concentration, and three monosaccharide/disaccharide solutes were found to produce one/two NHB water molecules.

This study presents a methodology for an in-depth characterization of six representative commercial nanofiltration membranes. Laboratory-made polyethersulfone membranes are included for reference. Besides the physical characterization [molecular weight cut-off (MWCO), surface charge, roughness and hydrophobicity], the membranes are also studied for their chemical composition [attenuated total reflectance Fourier spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)] and porosity [positron annihilation spectroscopy (PAS)]. The chemical characterization indicates that all membranes are composed of at least two different layers. The presence of an additional third layer is proved and studied for membranes with a polyamide top layer. PAS experiments, in combination with FIB (focused ion beam) images, show that these membranes also have a thinner and a less porous skin layer (upper part of the top layer). In the skin layer, two different pore sizes are observed for all commercial membranes: a pore size of 1.25-1.55 angstroms as well as a pore size of 3.20-3.95 angstroms (both depending on the membrane type). Thus, the pore size distribution in nanofiltration membranes is bimodal, in contrast to the generally accepted log-normal distribution. Although the pore sizes are rather similar for all commercial membranes, their pore volume fraction and hence their porosity differ significantly.

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