Dedicated to Professor Thomas P. MurrayProposed by Singer and Nicolson in 1972, [1] the fluid-mosaic model holds that the phospholipid bilayer is a dynamic twodimensional solvent milieu. Its proper function is closely tied to its "fluidity", and that is often quantified by reference to the melting point, T m (increased fluidity corresponds to a lower T m value). The fluid-mosaic model is highly evocative of the emerging picture of nanoscale structuring in ionic liquids (ILs), [2,3] and just as the function of phospholipid bilayers is tied to the T m value, so too is the utility of ILs. Whereas the former often have low T m values despite being composed of charged species with long aliphatic appendages, the fluidity of ILs generally decreases when progressively longer aliphatic appendages are used. [4] It is a challenge to design imidazolium ILs (the most common IL class) that incorporate progressively more lipophilic structural elements while keeping their melting points below room temperature (Figure 1). [4][5][6][7][8][9] Indeed, the T m values of these ILs begin to rise dramatically once an appended Nalkyl group exceeds seven carbon atoms in length. Herein we report that by using an approach modeled on homeoviscous adaptation (HVA), [10] ILs with very long alkyl appendages and very low T m values can be prepared. This discovery may have significant implications for IL use in enzymatic catalysis, lubricants, heat-transfer fluids, and gas storage and separation, among other applications.Widely accepted as a mechanism by which the melting temperature of cell membranes is modulated, HVA is the incorporation into cell membranes of phospholipids with "kinked" tail structures.[10] It is argued that the packing efficiency of the collective membrane hydrophobic components is diminished by the presence of these phospholipids and that increased fluidity results. A comparison of the T m value of distearoylphosphatidylcholine with that of dioleylphosphatidylcholine provides a dramatic example of how much impact this seemingly trivial difference can have. The former, with its linear, saturated C 18 tails has a T m value of 58 8C; the latter, with its "kinked" C 18 tails (each of which incorporates a cis-alkenyl group), has a T m value of À22 8C. This effect is also at the heart of the T m difference between the solid triacyl glycerols called fats, and those that are liquid at room temperature known as oils. In both instances, the effect is probably entropic in nature, as in the case of anthracene ("linear", T m = 217 8C) and phenanthrene ("kinked", T m = 99 8C).[11] Accordingly, we hypothesized that ILs with long, unsaturated, aliphatic tail structures would, like the corresponding phospholipids, have significantly lower T m values than their counterparts with saturated appendages.To test the validity of our hypothesis by measuring their T m values, we prepared a series of lipid-inspired ILs in a threestep process from high-purity (99 + %) fatty-alcohol mesylates, 1-methylimidazole, NaI, and NaTf 2 N.[12] Each of the ILs ...
A series of novel lipid-inspired ionic liquids have been synthesized employing the thiol-ene "click" reaction in a single-step process. The thermal properties were determined by differential scanning calorimetry (DSC) and showed observable trends between the C16, C18, and C20 analogues. The minimum melting points for each equivalent chain length series occur at sequential odd sulfur positions, 3, 5, and 7 for the C16, C18, and C20 series, respectively. The magnitude of melting point depression relative to the saturated homologue is observed to have a strong dependence on the position of the sulfur in the side chain. Additionally, the sulfur position corresponding to the lowest melting point for a homologous series shifts further down the chain as the chain length is increased, indicating that the maximum effect takes place near the center of the ion and not the center of the thiaalkyl chain. This synthesis provides tunability and improved thermal stability for 1-methyl-3-thiaalkylimidazolium bistriflimides and insight into structure-property relationships of lipidic ionic liquids.
The thermophysical properties of three lipidic ionic liquids, 1-oleyl-3-methylimidazolium bistriflimide, 1-elaidyl-3-methylimidazolium bistriflimide, and 1-linoleyl-3-methyl-imidazolium bistriflimide are measured at 1 bar as a function of temperature over the range of 273 K to 353 K and correlated to appropriate models. Each of these compounds is a variation on 1-C18-3-methylimidazolium bistriflimide, where the C18 chain contains one or more unsaturations, incorporated into the structure to lower the melting point. Derived properties such as molar volume and volume expansivity are also calculated. The data are compared with literature values for shorter chain 1-n-alkyl-3-methylimidazolium bistriflimide salts. The longer chains impart lower densities and higher viscosities relative to their shorter chain homologues. Although the subtle structural differences between the three compounds result in significant differences in melting points, there is less of an effect on the liquid phase thermophysical properties.
Dedicated to Professor Thomas P. MurrayProposed by Singer and Nicolson in 1972, [1] the fluid-mosaic model holds that the phospholipid bilayer is a dynamic twodimensional solvent milieu. Its proper function is closely tied to its "fluidity", and that is often quantified by reference to the melting point, T m (increased fluidity corresponds to a lower T m value). The fluid-mosaic model is highly evocative of the emerging picture of nanoscale structuring in ionic liquids (ILs), [2,3] and just as the function of phospholipid bilayers is tied to the T m value, so too is the utility of ILs. Whereas the former often have low T m values despite being composed of charged species with long aliphatic appendages, the fluidity of ILs generally decreases when progressively longer aliphatic appendages are used. [4] It is a challenge to design imidazolium ILs (the most common IL class) that incorporate progressively more lipophilic structural elements while keeping their melting points below room temperature (Figure 1). [4][5][6][7][8][9] Indeed, the T m values of these ILs begin to rise dramatically once an appended Nalkyl group exceeds seven carbon atoms in length. Herein we report that by using an approach modeled on homeoviscous adaptation (HVA), [10] ILs with very long alkyl appendages and very low T m values can be prepared. This discovery may have significant implications for IL use in enzymatic catalysis, lubricants, heat-transfer fluids, and gas storage and separation, among other applications.Widely accepted as a mechanism by which the melting temperature of cell membranes is modulated, HVA is the incorporation into cell membranes of phospholipids with "kinked" tail structures.[10] It is argued that the packing efficiency of the collective membrane hydrophobic components is diminished by the presence of these phospholipids and that increased fluidity results. A comparison of the T m value of distearoylphosphatidylcholine with that of dioleylphosphatidylcholine provides a dramatic example of how much impact this seemingly trivial difference can have. The former, with its linear, saturated C 18 tails has a T m value of 58 8C; the latter, with its "kinked" C 18 tails (each of which incorporates a cis-alkenyl group), has a T m value of À22 8C. This effect is also at the heart of the T m difference between the solid triacyl glycerols called fats, and those that are liquid at room temperature known as oils. In both instances, the effect is probably entropic in nature, as in the case of anthracene ("linear", T m = 217 8C) and phenanthrene ("kinked", T m = 99 8C).[11] Accordingly, we hypothesized that ILs with long, unsaturated, aliphatic tail structures would, like the corresponding phospholipids, have significantly lower T m values than their counterparts with saturated appendages.To test the validity of our hypothesis by measuring their T m values, we prepared a series of lipid-inspired ILs in a threestep process from high-purity (99 + %) fatty-alcohol mesylates, 1-methylimidazole, NaI, and NaTf 2 N.[12] Each of the ILs ...
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