For 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides and their cationic C2-methylated isomers, that is, [C(n)MIm]NTf2 and [C(n-1)DMIm]NTf2 (n = 3-8), the latter always has much higher viscosities (∼1.66 times at 25 °C) than the former. This finding is unexpected and contrary to the loss of the predominant hydrogen bonding between the cation and anion, caused by the C2-methylation in the imidazolium ionic liquids. In the present study, we propose a free volume model to explain this mysterious phenomenon. For the isomeric pairs, [C(n-1)DMIm]NTf2 was found to possess less unoccupied (free) volume than [C(n)MIm]NTf2 by ∼4.5 mL mol(-1) determined by both Fürth's surface tension and molar volume methods, and the free volume difference can be quantitatively correlated with the C2-methylation effect on viscosity. Generally, the less free volume in [C(n-1)DMIm]NTf2 relative to [C(n)MIm]NTf2 reduces the number of the hole carriers for molecular transport with a resulting increase in viscosity. Besides, the free volume model can also be applied to account for the C2-methylation effect on other properties, such as conductivity, surface tension, density, and refractive index.
The Hildebrand solubility parameters of amidium ionic liquids were investigated using the approach of intrinsic viscosity. It was discovered that the nature of the anion influences the Hildebrand solubility parameters by which distillable trifluoromethanesulfonate-(TfO-) based ionic liquids present higher Hildebrand solubility parameters than bis-(trifluoromethylsulfonyl)imide-(NTf 2 -) based ionic liquids. In this work, we also studied the effect of dissolution temperature on the Hildebrand solubility parameters of ionic liquids. An increase in temperature from 25 to 60 °C results in a decrease in the Hildebrand solubility parameters of ionic liquids. Moreover, the cohesive energy density, molar internal energy, and enthalpy of dissolution were evaluated. The molar internal energy and enthalpy of dissolution are almost constant or show a slight decrease with increasing dissolution temperature for O-methylated TfO-based ionic liquids.
In the present study, a new series of ionic liquids (ILs) derived from low-cost amides and lactams (cyclic amides), such as N,N-dimethylformamide (DMF), were synthesized and characterized. Unlike other nucleophiles like amines, the alkylation reaction of the amides with alkyl triflates to form cationic species takes place at the carbonyl oxygen atom, instead of the nitrogen atom. For these O-alkylated amidium ILs, the basic physicochemical properties, such as melting point, glass transition temperature, plastic crystal phase transition, thermal stability, density, surface tension, viscosity, ionic conductivity and electrochemical window, were investigated and studied. Generally, these ILs are distinguished by low viscosity and high conductivity, in particular the DMF-derived ILs with viscosity as low as 21.6 cP and conductivity up to 15.45 mS cm–1 at 25 °C. This result should be attributed to the cationic DMF structures: planar geometry, low symmetry, C2-proton and ether moiety, resulting in much lower viscosity and higher conductivity than the best-known imidazolium ILs. Meanwhile, these amidium ILs also possess wide electrochemical windows (∼4.5 V) comparable to imidazolium ILs, implying their potential in electrochemical applications. Furthermore, several of the amidium ILs can form plastic crystal with a maximum enthalpy gain of −35.7 J g–1 at the temperature range of −10–90 °C.
An extracellular matrix-like poly (ε-caprolactone) (PCL)/gelatin core-shell nanofibrous scaffold with high hydrophilicity was developed by coaxial-elecrospinning. However, the behavior of vascular endothelial cells (ECs) on the modified scaffold remains limited. In this study, human umbilical vein ECs (HUVECs) were seeded onto PCL scaffolds with or without gelatin. Morphological changes of HUVECs were observed under confocal laser scanning microscopy (LSCM). HUVECs’ adhesion, proliferation and apoptosis were detected by MTT assay and flow cytometry (FCM). Our results showed that HUVECs on PCL/gelatin scaffolds with identical polygonal and cobblestone-like characteristics reached confluence after 7 days. Modification of PCL nanofibers significantly promoted the attachment of HUVECs onto scaffolds within 1 hour. Compared to pristine PCL, a two-fold increase in proliferation of HUVECs was also observed after 7 days, whereas the apoptosis of HUVECs was obviously reduced by 40% on the modified scaffolds. In summary, these results indicated modified PCL/gelatin scaffold developed by coaxial-elecrospinning can increase the adhesion, proliferation, and suppress apoptosis of HUVECs, suggesting it has a great potential and promising vascular graft in tissue engineering applications.
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