Zwitterionic and mesoionic liquids: Molecular aggregation in 3-methylsydnone

Abstract

Ionic liquids are green solvents with interesting properties: displaying low melting points and high boiling points. They offer a new approach applicable in many instances. Nevertheless, the presence of free ions can be a matter in some cases, e.g. for the study of nucleophilic reactions, in electrochemistry, and in each situation where there is a competition between counter ions, as in micellization of ionic surfactants. Neutral compounds having formal unit electrical charges of opposite sign, and the same physical properties than ionic liquids would be a nice alternative to these latter solvents. There are two classes of chemical compounds having these characteristics: zwitterionic liquids (with no uncharged canonical representation) and mesoionic liquids (in which the negative and the positive charges are delocalized). In that last class we have chosen to work with 3-methylsydnone in order to examine, in this aprotic solvent, if it was possible to observe aggregation of surfactants in the same manner as in water. With all kinds of surfactants studied (ionic, zwitterionic and mesoionic) we have been able to demonstrate the formation of direct micelles: hydrogen bonding is thus not mandatory for molecular aggregation. Comparison of the behavior in water and in formamide showed that solvophobic interactions were qualitatively comparable but with a lower intensity.

Solvent nanostructure, the solvophobic effect and amphiphile self-assembly in ionic liquids

“…There have been a number of non-aqueous molecular polar solvents which have been reported as amphiphile selfassembly solvents. These include the small glycol solvents of glycerol, [1][2][3][4] ethylene glycol, 1,5,6 1,3-propanediol, 2 1,2-propanediol, 2 1,4-butanediol, 2 1,3-butanediol, 2 small amides consisting of formamide, 1-4,6-15 N-methylformamide, 4,15 N-methylacetamide, 7 N,N-dimethylformamide, 2,4,6-8,15 N-ethylformamide, 15 N-pentylformamide, 15 N-(2-hydroxyethyl)acetamide, 15 N-tetrabutylformamide, 15 N,N-diethylacetamide, 15 and other solvents including hydrazine, 7,8 N-methylsydnone, 16,17 dimethyl sulfoxide, 6,7 formic acid, 2 2-aminoethanol, 2 1-amino-2-propanol, 2 2-mercaptoethanol, 2 ethylene diamine 2 and acetonitrile. 6 In addition to the molecular solvents it has been well established that many ionic liquids and molten salts share this ability.…”

“…However, since then 3-methylsydnone has been shown to have self-assembly ability and hence a network of hydrogen bonding is not a requisite property. 16,17 Cassel et al reported on the self-assembly in the aprotic 3-methylsydnone and commented that the discrepancy between studies is due to the Krafft temperature being solvent and surfactant dependent, and hence higher temperatures were required to observe amphiphile self-assembly in 3-methylsydnone. 17 It has been suggested that in aprotic ionic liquids there is an ionic network, instead of a hydrogen bonded network, and thus they are still structured around surfactant alkyl chains.…”

“…16,17 Cassel et al reported on the self-assembly in the aprotic 3-methylsydnone and commented that the discrepancy between studies is due to the Krafft temperature being solvent and surfactant dependent, and hence higher temperatures were required to observe amphiphile self-assembly in 3-methylsydnone. 17 It has been suggested that in aprotic ionic liquids there is an ionic network, instead of a hydrogen bonded network, and thus they are still structured around surfactant alkyl chains. 157 There are a significant number of aprotic polar solvents which are known self-assembly media, as listed in Table 1, in addition to the aprotic ionic liquids.…”

“…Prior to 2006 we estimate there to have been 17 known solvents with this ability, of which all but two were molecular solvents. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] However, this number has been vastly increased since 2006 due mainly to the identification of the widespread ability of ionic liquids (ILs) to be self-assembly media for amphiphiles. Ionic liquids consist of ions and are liquid below 100 1C.…”

Linking molecular/ion structure, solvent mesostructure, the solvophobic effect and the ability of amphiphiles to self-assemble in non-aqueous liquids

“…31 The requirement for solvent hydrogen bonding has been challenged more recently through reports of amphiphile self-assembly in for example 3-methylsydnone. 26,32,33 In amphiphile-solvent systems, lyotropic mesophases can include lamellar, and normal and inverse micellar, hexagonal, and discrete and bicontinuous cubic structures. Lamellar, hexagonal and cubic mesophases are ordered with one-, two-and three-dimensional periodicity, respectively.…”