Solvophobicity and amphiphilic self-assembly in neoteric and nanostructured solvents
Graphical abstract
Introduction
The self-assembly of amphiphilic solutes into micelles and bilayers, their ordering into lyotropic liquid crystals, and the formation of microemulsions have all long been understood to be inexorably linked to the hydrophobic effect [1,2]. Ray [3,4] generalized this idea to a solvophobic effect by identifying a dozen or so nonaqueous solvents that also support micelle formation. She also showed that this effect could not be correlated with bulk indicators of the polarity or the cohesive energy density of the solvent, such as dielectric constant, surface tension, or solubility parameter. Instead, like water itself, “the molecules of these solvents have two or more potential hydrogen bonding centres, and are therefore, most likely to be capable of forming three-dimensional hydrogen bonded network structures.” [4∗] While the number of molecular solvents that support surfactant self-assembly has expanded over the intervening years [5, 6, 7, 8], the essence of this conclusion did not change; a three-dimensional hydrogen-bonded network was essential. For example, Beesley et al. [9] explicitly sought out an extremely polar but non-H-bonding solvent in the form of 3-methylsydnone, but could find no evidence that it supported amphiphilic self-assembly.
The past two decades have seen the rapid emergence and growth of a new class of solvents for amphiphile self-assembly based around ionic liquids (ILs). The term “ionic liquid” is generally applied to any salt that melts below 100 °C, although salts with melting points below room temperature (RTILs) have wider applications, and consequently, are more extensively studied. The explosive growth of ILs as versatile, “designer” solvents is largely due to the ability to tune properties by mixing-and-matching cations and anions with widely different structures (see Figure 1). This, in addition to their low, sometimes negligible, vapor pressures, held out great promise as green replacements for a variety of volatile organic solvents. This is not a consideration when used to replace water as a self-assembly solvent, although resistance to evaporation and a broad temperature range of liquid stability may be considerable advantages when using ILs to formulate complex fluids.
Section snippets
Self-assembly solvents
The first IL examined as a room-temperature self-assembly solvent for surfactants was ethylammonium nitrate EAN. EAN, formed by a Brønsted-Lowry acid-base reaction between ethylamine and nitric acid, has a melting point of 12 °C, and was first reported in 1914, as a room temperature molten salt [10]. EAN exemplifies protic ionic liquids (PILs), which are salts whose cation is a weak organic acid, such as a 1°, 2° or 3° ammonium, pyrrolidinium, pyridinium, imidazolium, or analogous phosphoniums.
Amphiphile self-assembly in neoteric solvents
Amphiphiles or surfactants, comprised of one or more polar (hydrophilic) and nonpolar (hydrophobic) moieties, are generally first classified according to the type of hydrophilic “head-group”—ionic, nonionic, or zwitterionic—and hydrophobic “tail.” Nonionic surfactants are most commonly poly (oxyethylene)-n-alkyl ethers (CnEm). Systematic variation in alkyl chain length n or the number of ethoxy groups m, affects the thermodynamics of micellization—the critical micelle concentration (CMC)—as
Conclusion and outlook
Ionic liquids (deep) eutectics, and other mixed or ionic molecular hybrid solvents exhibit a staggering array of new self-assembly phenomena through the control of intermolecular forces within the solvent. Most striking is the widespread presence of amphiphilic nanostructure in the solvent itself, which can be tuned via the molecular structures of cation, anion, and molecular components, and through the intermolecular forces between them. These factors affect the solubility of various organic
Conflict of interest statement
Nothing declared.
Acknowledgements
The authors acknowledge financial support from the Australian Research Council.
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