(232k) Self-Assembly of Anionic Surfactants in Aprotic Imidazolium Ionic Liquids

B. Kusiak,^a P. Bernatowicz,^a P. Korczyk,^bJ. Gregorowicz^a

^aInstitute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

^bInstitute of Fundamental Technological Research, Polish Academy of Science, Pawninskiego 5B, 02-106 Warsaw, Poland

There is a very limited number of known solvents possessing capability to support self-assembly of surfactants. Fortunately, quite a few ionic liquids possess this ability. Moreover, apart from the protic imidazolium ionic liquids, some aprotic imidazolium ionic liquids were found to be capable of supporting amphiphile self-assembly. Investigation of amphiphile behavior in ILs is important for better understanding of the effect of solvents on self-assembly and is also necessary for expanding applications of these nonaqueous solvents.

Ionic liquids (ILs) provide an attractive medium for a preparation of nanocomposites in particular cellulose/carbon nanotubes (CNTs) composites (patent number P-392221 and GB2483158). The limiting step for the process of the composite preparation with an ionic liquid as a solvent is dispersion of CNTs (patent application P-395835). The most common approach to circumventing this problem is to disperse the CNTs in the solvent containing some amount of an anionic surfactant. The disadvantages of this method is formation of the surfactant micelles. That is why in this work we concentrate our attention on ILs which dissolve cellulose, like AMIMCl, EMIMDEP or BMIMCl.

Aggregation behavior of anionic surfactant sodium dodecyl sulfate (SDS) and sodium n-octadecylsulfate (SOS) in 1-allyl-3-methylimidazolium chloride (AMIMCl), 1-butyl-3-methylimidazolium chloride (BMIMCl), 1-hexyl-3-methylimidazolium chloride (HMIMCl) and 1-ethyl-3-methylimidazolium diethylphosphate (EMIMDEP) was investigated by the surface tension measurement. Additionally NMR spectroscopy was used to analyze micelle formation in the SDS/AMIMCl and SDS/EMIMDEP systems.

It was established that SDS and SOS formed micelles in AMIMCl and BMIMCl, while no aggregation of the surfactant molecules occurred in EMIMDEP. The cmcs estimated from surface tension were 54 mM, 53 mM, 87 mM and 72.5 mM for SDS/AMIMCl, SDS/BMIMCl, SOS/AMIMCl and SOS/BMIMCl, respectively. The free energy of micellization is negative: -28.3 kJ/mol for SDS/AMIMCl, -25.8 kJ/mol for SDS/BMIMCl, -31.8 kJ/mol for SOS/AMIMCl and -30.0 kJ/mol for SOS/BMIMCl. Therefore the formation of micelles becomes spontaneous in this systems. The critical micelle concentration of SDS and SOS in AMIMCl and BMIMCl are higher than in aqueous solution mostly due to higher solubility of the surfactants in ILs, which leads to weaker solvophobic interactions, and also increased viscosity of the system, limiting diffusion of the molecules.

The impact of water present in ILs on self-assembly of surfactants was also investigated. The results of the ¹H NMR measurements revealed significant differences in interactions of water with AMIMCl and EMIMDEP. Water interactions with EMIMDEP is strong enough to cause even distribution of water molecules in the system. In AMIMCl mutual interactions of water molecules was preferred and as a result water domains within the IL were formed.

Micelle formation is an indication that also liquid crystalline phases are formed in a system. The differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarized optical microscopy (POM) were used to study formation of liquid crystalline phases in the SDS/AMIMCl, SDS/BMIMCl, SDS/HMIMCl, SOS/AMIMCl, SOS/BMIMCl and SOS/HMIMCl systems. In all investigated surfactant/ionic liquid systems transitions from a lamellar to isotropic phase were observed. In three systems (SOS/AMIMCl, SOS/BMIMCl and SOS/HMIMCl) transitions from a solid phase to liquid crystalline lamellar phase were also observed. The solid phase was formed by pure surfactants. The POM observations showed that in the systems the phase transitions from the lamellar phases to the isotropic phases are reversible. This is confirmed by DSC, where only small temperature hysteresis upon consecutive heating and cooling runs was observed.