Saturation transfer difference (STD) NMR spectroscopy is a well-known ligand-based solution NMR technique used extensively for ligand epitope mapping, the identification of the nature of ligand binding sites, and the determination of ligand binding affinity. Recently, we have shown that STD NMR can be also applied to monitor changes in bound water during gelation of particulate dispersions. However, this technique is strongly dependent on gelator and solvent concentrations and does not report on the degree of organisation of the solvent within the particle network. This obscures the detailed understanding of the role of the solvent on gelation and precludes the comparison of solvation properties between dispersions prepared under different experimental conditions. In this work we report a novel STD NMR method to characterise the degree of solvent structuration in carbohydrate-based particulate dispersions by demonstrating for the first time that, for solvents interacting with large particles, the spin diffusion transfer build-up curves can be modelled by the general one-dimensional diffusion equation. Our novel approach, called Spin Diffusion Transfer Difference (SDTD) NMR, is independent of the gelator and solvent concentrations, allowing 2 to monitor and compare the degree of solvent structuration in different gel networks. In addition, the simulation of SDTD build-up curves report on minimum distances (r) and spin diffusion rates (D) at the particle-solvent interface. As a case study, we have characterised the degree of structuration of water and low molecular weight alcohols during the alcohol-induced gelation of TEMPO-oxidised cellulose hydrogels by SDTD NMR, demonstrating the key role of water structuration on gel properties. SDTD NMR is a fast, robust and easy-to-implement solution NMR protocol that overcomes some of the limitations of the classical STD NMR approach when applied to the study of solvation. This technique can be readily extended to characterise the solvent(s) organisation in any type of particulate gels. Hence, the SDTD NMR method provides key insights on the role of water in the mechanism of gelation and the macroscopic properties of particulate gels, of fundamental importance for the design of soft matter materials with tuneable properties.
The titratable acidity, alkalinity and carboxylate content are fundamental properties required for the understanding of aqueous chemical systems. Here, we present a set of new methods that allow these properties to be determined directly by 1 H NMR without the labor, cost and sample quantity associated with running separate potentiometric or conductometric titrations. Our methods require only the measurement of the pH sensitive 1 H chemical shifts of indicator molecules and do not require the tedious titration of reagents into a sample. To determine the titratable acidity, an excess of 2-methylimidazole (2MI) is added to a sample and the quantity of protons absorbed by 2MI determined from its 1 H chemical shifts. The titratable alkalinity of a sample can be similarly determined using acetic acid. To determine the concentration of deprotonated carboxylates, a sample is acidified with HCl and the quantity of H + absorbed determined from the 1 H chemical shift of methylphosphonic acid. We validate our methods by demonstrating the measurement of the acidity of fruit-flavored drinks, the alkalinity of tap water and the carboxylate content of nanocellulose dispersions.
The binding of calcium and magnesium ions (M 2+ ) by polymers and other macromolecules in aqueous solution is ubiquitous across chemistry and biology. At present, it is difficult to assess the binding affinity of macromolecules for M 2+ without recourse to potentiometric titrations and/or isothermal titration calorimetry. Both of these techniques require specialized equipment, and the measurements can be difficult to perform and interpret. Here, we present a new method based on 1 H NMR chemical shift imaging (CSI) that enables the binding affinity of polymers to be assessed in a single experiment on standard high-field NMR equipment. In our method, M 2+ acetate salt is weighed into a standard 5 mm NMR tube and a solution of polymer layered on top. Dissolution and diffusion of the salt carry the M 2+ and acetate ions up through the solution. The concentrations of acetate, [Ac], and free (unbound) M 2+ , [M 2+ ] f , are measured at different positions along the sample by CSI. Binding of M 2+ to the polymer reduces [M 2+ ] f and hinders the upward diffusion of M 2+ . A discrepancy is thus observed between [Ac] and [M 2+ ] f from which the binding affinity of the polymer can be assessed. For systems which form insoluble complexes with M 2+ , such as sodium polyacrylate or carboxylate-functionalized nanocellulose (CNC), we can determine the concentration of M 2+ at which the polymer will precipitate. We can also predict [M 2+ ] f when a solution of polymer is mixed homogeneously with M 2+ salt. We assess the binding properties of sodium polyacrylate, alginate, polystyrene sulfonate, CNC, polyethyleneimine, ethylenediamenetetraacetic acid, and maleate.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.