College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou215123, China.
Analysis and Testing Center, Soochow University, Suzhou215123, China.
J Phys Chem B. 2023 Feb 2;127(4):1013-1021. doi: 10.1021/acs.jpcb.2c07927. Epub 2023 Jan 19.
Reline, which is composed of choline chloride (ChCl) and urea, is the first and most widely used deep eutectic solvent (DES) described by Abbot and co-workers. Due to the hygroscopic feature, traces of water are unavoidable, which significantly affect the physicochemical properties of reline. At present, the local structure of molecules and the impact from the presence of water are still the most significant questions in this field. Herein, reline and six aqueous dilutions with a controlled amount of water (from 3.2 to 50.0 wt %) were studied mainly by using a combination of nuclear magnetic resonance (NMR) techniques. According to 1D Cl NMR, 1D N NMR, and 2D H-Cl heteronuclear Overhauser effect spectroscopy, we probed the interactions of urea···Cl and Ch···Cl in pure reline, which gradually dissociated in the presence of water. Moreover, it was revealed that the dissociation rate altered when the water content reached 9.0 wt %, which is ascribed to the higher preference of hydration for Cl ion compared to other species in the system. Furthermore, selected cross peaks in H-H correlation spectroscopy spectra were analyzed. Accordingly, an enhanced correlation was observed for urea···Ch at a lower water fraction within 9.0 wt %. When the water content increased to 24.9 wt %, the water solvation of Ch and urea was also observed in COSY spectra. The interaction of HO···Ch got continuously stronger when the water content increased from 24.9 to 50.0 wt %, while HO···urea got enhanced when the water content reached 33.3 wt % and then diminished gradually from 33.3 to 50.0 wt %. H-H nuclear Overhauser effect spectroscopy and H-H rotating frame Overhauser effect spectroscopy experiments were also conducted for dynamics investigation. The τ value for the species in 9.0 wt % aqueous reline is very close to τ of 0.44 ns. For pure reline and the aqueous reline with a water fraction of less than 9.0 wt %, the τ value of the species is longer than 0.44 ns, while for the sample with water of 24.9 wt %, the τ value is much shorter than 0.44 ns. Based on our NMR study, we revealed that with the water amount increasing from 0 to 50.0 wt %, the species involved in the system behaved as the large molecules or molecules in viscous liquids transiting to the medium-sized molecules in nonviscous liquids and finally to small molecules in nonviscous liquids.
润唇膏由 Abbott 等人首次描述，是由胆碱氯化物（ChCl）和尿素组成的第一种也是最广泛使用的深共晶溶剂（DES）。由于润唇膏具有吸湿性，因此不可避免地会有微量的水分存在，这会显著影响润唇膏的物理化学性质。目前，分子的局部结构以及水的存在的影响仍然是该领域的最关键问题。在这里，我们主要通过结合使用核磁共振（NMR）技术来研究润唇膏和六种具有一定含水量（从 3.2 到 50.0wt%）的水稀释液。根据 1D Cl NMR、1D N NMR 和 2D H-Cl 异核 Overhauser 效应光谱，我们探测了纯润唇膏中尿素···Cl 和 Ch···Cl 的相互作用，它们在水的存在下逐渐解离。此外，当含水量达到 9.0wt%时，发现解离速率发生了变化，这归因于系统中 Cl 离子比其他物种更优先水合。此外，还分析了 H-H 相关光谱谱中的选定交叉峰。相应地，在较低的水分数（9.0wt%）内观察到尿素···Ch 的相关性增强。当含水量增加到 24.9wt%时，在 COSY 光谱中也观察到了 Ch 和尿素的水合作用。当含水量从 24.9 增加到 50.0wt%时，HO···Ch 的相互作用不断增强，而当含水量达到 33.3wt%时，HO···urea 的相互作用增强，然后从 33.3 到 50.0wt%逐渐减弱。还进行了 H-H 核 Overhauser 效应光谱和 H-H 旋转框架 Overhauser 效应光谱实验以进行动力学研究。9.0wt%水合润唇膏中物质的τ值非常接近 0.44ns。对于纯润唇膏和含水量小于 9.0wt%的水合润唇膏，物质的τ值大于 0.44ns，而对于含水量为 24.9wt%的样品，τ值远小于 0.44ns。基于我们的 NMR 研究，我们揭示了随着水量从 0 增加到 50.0wt%，系统中涉及的物质表现为从大分子量或粘性液体中的分子过渡到非粘性液体中的中等分子量分子，最后过渡到非粘性液体中的小分子。
