hao et al., 2007

Science 12 (2007) 129–137www.elsevier.com/locate/cocis

Current Opinion in Colloid & Interface

Self-assembled structures and chemical reactionsin room-temperature ionic liquids

Jingcheng Hao a,⁎, Thomas Zemb b,⁎

a Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, P. R. Chinab Service de Chimie Moléculaire, C.E.A./Saclay Bldg.125, F91191 Gif-sur-Yvette Cedex, France

Available online 18 January 2007

Abstract

Amphiphilic association in room-temperature ionic liquids (RT-ILs) — a “green” solvent shows analogies as well as clear differences fromself-assembly in water. In this review, we summarize the known features of amphiphilic association structures in the form of micelles,microemulsions, vesicles and lyotropic liquid-crystalline phases in ionic liquids. Most of the methods making use of association to controlreactivity could be developed also in RT-ILs and we give a few recently published examples of this strategy.© 2007 Published by Elsevier Ltd.

Keywords: Self-assembly; Micelles; Vesicles; Emulsions; Room-temperature ionic liquids; Nanoparticles; Interfacial reactivity

Room-temperature ionic liquids (RT-ILs) which are made ofweakly bound anions and usually large organic cations are a specialclass of salts having a melting point below room temperature [1•].These fluids are currently being used as a “green solvent”media inwhich chemical reactions or many chemical process industries canbe performed. Their low vapor pressures are linked to intrinsicproperties of organic ions in strong electrostatic coupling such as inthe case of perfluoro-type anions. RT-ILs are functional isotropicliquids exhibiting extremely high electrical conductivities, non-flammability and have thermally high stability. Chemical reactionsin RT-IL′s, such as organic synthesis, chemical separations, andtransition-metal-mediated catalysis have been reviewed [1•]. Wesummarize in this short review the recent results on the formation ofself-assembled structures in room-temperature ionic liquids. Recentdescriptions of chemical reactions making use of self-assembly forthe preparation of nanoparticles inRT-ILs are also described briefly.

Amphiphilic molecular self-assemblies are known in theform of micelles, microemulsions and lyotropic liquid crystals.An ubiquitous sub-class of self-assembly includes smecticphases, alias lamellar phases, alias multilayered vesicles ifexcess water is present [2]. The first documented example

⁎ Corresponding authors. Hao is to be contacted at Tel.: +86 531 88366074;fax: +86 531 88564750. Zemb, Tel.: +33 1 69086328; fax: +33 1 69086640.

E-mail addresses: [email protected] (J. Hao), [email protected](T. Zemb).

1359-0294/$ - see front matter © 2007 Published by Elsevier Ltd.doi:10.1016/j.cocis.2006.11.004

concerns self-assembly in ethylammonium nitrate (EAN). Theexistence of micelles formed by surfactants [3,4] as well aslamellar liquid crystals formed by lipids was documented, aswill be further discussed below [5–7].

Unlike the most other RT-ILs, EAN at room temperature issometimes supposed to form a three-dimensional hydrogen-bond network, which was sometimes postulated to be anessential feature in supporting self-assembly [8], as well as notbe essential, following Charles Tanford in his classical textbook“The hydrophobic effect”.

The chemical structures of RT-ILs referred to here are shownbelow.

1) Ethylammonium nitrate (EAN):

2) 1-butyl-3-methylimidazolium chloride ([bmim][Cl]):

130 J. Hao, T. Zemb / Current Opinion in Colloid & Interface Science 12 (2007) 129–137

3) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]):

4) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]):

1. Self-assembly of surfactants into micelles in RT-ILs

Surfactant micelles are classically obtained in water and themechanism of formation has been dubbed the “hydrophobic”effect, surfactants apolar parts are rejected from water whilepolar head-groups “bind weakly” to water. The theory of self-assembly at that level explains the high enthalpy gain linked toan entropy loss during micelle formation. The fundamentalproperty is the link between interfacial curvature and thechemical potential of molecules self-assembling in a “solvent”.At more microscopic level, the chemical potential of anysurfactant in a self-assembly is a simple function of the interfacecurvature, since the lateral packing of molecules excluded fromsolvent with a given shape: the packing parameter p which isthe ratio of the equilibrium area per molecule divided by themolecular volume to average length at statistical equilibriumratio of the molecule [9••]. Phase diagrams includingconcentrated microemulsions or liquid crystals can only beunderstood if spontaneous packing parameter and effectivepacking parameter are considered as separate properties (oneintrinsic, and the other one composition dependant) in theformation of any self-assembled structure [10]. The physicalorigin of the sequence of phases observed is due to thefrustration of the packing constraint due to composition and thespontaneous curvature of a film made by surfactants. Hence,phase sequences should be independent from solvent used, andsome features of lyotropic liquid crystals should also observedin RT-ILs. Indeed, the free energy of formation of any self-assembled microstructure is given by the integral over thesurface of the square of the difference between the effective andspontaneous packing parameters:

G ¼ 1=2k⁎ðp� p0Þ2

Where k⁎ is the bending rigidity of the surfactant film andreflects the cost in free energy to frustrate spontaneous shapesinduced by lateral packing of the molecules considered [11•].Reduced forms and approximations of this fundamental linkbetween packing energy and curvature, with principal andGaussian curvature as variables instead of the packingparameter as the one given by Helfrich of this expressioncentral to self-assembly have become popular [12].

The chemical potential can be accessed via the concentrationof monomers, in most conditions, close to the so-called criticalmicellar concentration, i.e. the highest concentration possiblebefore any aggregate forms.

First observations on self-assembly of surfactants intomicelles in RT-ILs have been reported over 20 years ago[3,4]. Up to the present, the micellar solutions of differentsurfactants in the most ionic liquids of current interest such as 1-butyl-3-methyl imidazolium chloride and hexafluorophosphatewere explored at different concentrations and at a range oftemperatures [13••].

Dissolution of different surfactants in RT-ILs decreases thesurface tension in a manner analogous to aqueous solutions. Theslope of the surface tension curve allows access to the area permolecule at the interface using the Szyszowski–Langmuir (aliasmodified Gibbs) equation. The value at the break-point at thecmc gives access to the free energy of formation of a micelle. Asmooth behavior instead of a discontinuity of the derivative isan indication of pre-micellisation, i.e. formation of micelleswith a low number of monomers [14].

A recent systematic study compares micelles formed bynonionic surfactants in water, formamide and RT-ILs. Aggre-gation numbers in RT-ILs are smaller than those in water, anobservation always linked to higher values of cmcs in RT-ILsthan those in water. Surprisingly, the area per molecule is alsosmaller in RTIL than that in water: there is no lateral strongrepulsion between head-groups in ionic liquids [15•].

Surface tensions vs. molar concentrations of two surfactants,anionic sodium dodecyl sulfate (SDS) and nonionic Brij 35(polyoxyethlene-23-lauryl ether), were carried out. The surfacetension depression behavior indicates that the possibility ofnormal micelles in the two kinds of RT-ILs, [bmim][Cl] and[bmim][PF6], which is the first evidence in favor of surfactantself-assembled micelles in imidazolium ionic liquids. The resultsclearly show that the initial surface tensions of the neat ILs aresomewhat lower than those in water, for water γw=72.6 mN.m−1

but for [bmim][Cl] and [bmim][PF6] γILs=∼48mN.m−1 at roomtemperature. The final surface tensions of different surfactants inILs and water could be comparable. The relative lowering ofsurface tensions in water is greater solely due to the initial highersurface tension of water.

The critical micellar concentrations of surfactants in ILs arehigher than those of the same ones in water, the typical values ofsodium dodecyl sulfate (SDS) in water and in [bmim][Cl] are:cmcw =∼ 8.0 and cmc[bmim][Cl] = (48 ± 4.4) mmol.L− 1,respectively.

The presence of “solvatophobic” interactions between RT-ILsand the hydrocarbon portion of surfactants have been checked in[bmim][Cl] and [bmim][PF6] using a linear free energyrelationship through inverse gas chromatography. These solva-topobic interactions could be used to explain the formation ofother aggregates such as uni- and multilamellar vesicles [16•].

2. Self-assembly of surfactants into vesicle phases in RT-ILs

The evidence of in favor of surfactant vesicle-phase in RT-ILs was indeed first reported for diakyldimethylammonium

Fig. 2. Small-angle X-ray scattering curves of 100 mM C14DMAO/11 mM Zn(OOCCH2C6F13)2 mixture and 11 mM Zn(OOCCH2C6F13)2 in ionic liquid,[bmim][PF6]. The scattering peaks for both systems can be indexed as a bilayerlamellar structure but no scattering peaks for the ionic liquids have beenobserved. (Reproduced from Ref. [17•], with permission).

131J. Hao, T. Zemb / Current Opinion in Colloid & Interface Science 12 (2007) 129–137

bromide in an ether-containing IL system [16•]. Ether linkagewith the general ionic liquids increases the net polarity of ionicliquids and may promote the case for purely “ionophobic”surfactant self-assembly. In the absence of additional use ofother morphological observation technique such as X-rayscattering measurements or freeze-fracture transmission elec-tron microscopy (FF-TEM), the presence of vesicles can beascertained from dark field optical microscopy images forvesicle membranes in ionic liquids. The aggregate size and theinterlamellar spacing between the bilayers were not yet reportedin the binary cationic surfactant/ionic liquid systems. There is arecent clear evidence for the self-assembled vesicles, structur-ally equivalent to some hydrotropes, from a Zn2+-fluoroussurfactant or in the mixture of Zn2+-fluorous surfactant/zwitterionic surfactant in room-temperature ionic liquids[17•]. The size and interlamellar spacing between the bilayersarranged in vesicles in RT-ILs were obtained using FF-TEMobservations and small-angle X-ray scattering (SAXS) mea-surements. In comparison with the samples of birefringent Lα-phase prepared by Zn(OOCH2C6F13)2 alone or the mixtures ofZn(OOCH2C6F13)2/the zwitterionic surfactant tetradecyldi-methylamine oxide (C14DMAO) in aqueous solutions, wefound that the samples of Zn(OOCH2C6F13)2 alone or themixtures of Zn(OOCH2C6F13)2/C14DMAO in two common RT-ILs ([bmim][PF6] and [bmim][BF4]) are translucid and presentpuce-color, which may indicate some partial complexation ofZinc cation with the organic anion in the solvent [18]. Thephysical origin of the color is however still unclear.

Typical vesicle structures examined by negative-stainingTEM and FF-TEM observations are shown in Fig. 1, whichshow distinctively the spherical aggregates with the diametersof about 20 to 150 nm. Multilamellar vesicles and clusters ofnanospheres are also visible in the FF-TEM and negative-staining TEM images, respectively. As for other chargedsurfactants in the presence of excess ions, these structures arelocated in a region were tie lines between excess solvent and alamellar phase [19].

The lamellar microstructure has also been evidenced bySAXS experiments, as shown in Fig. 2. The peaks are sharp,demonstrating the high rigidity of the bilayers [19] formed with

Fig. 1. Typically negative staining-TEM and FF-TEM image of self-assembled surfaC14DMAO and Zn(OCCH2C6F13)2 in [bmim][PF6] (b), phosphotungstic acid was usZn(OCCH2C6F13)2 in [bmim][PF6] (c). (Reproduced from Ref. [17•], with permissi

a typical interbilayer repeat distance of 6 nm. Spacing whenexcess solvent is present implies that a repulsive mechanismexists between bilayers in RT-IL′s, probably in the general“head-group solvation mechanism” as predicted by Marcelja intaking into account explicit ion-ion potentials in the solvent[20].

3. Self-assembly of surfactants intomicroemulsions inRT-ILs

Microemulsions are thermodynamically stable fluids con-taining at least two immiscible solvents and one surface-activemolecule, presenting no long range order. Two different types ofmicroemulsions: “fluid” microemulsions which can be seen asrandom dispersions of water in oil as well as “rigid”microemulsions which are present in connected tubules orlocally planar, i.e. molten liquid crystals have very differentbehavior, for example as far as electrical conductivity isconcerned [9••, 10, 11•]. Recently, ionic liquids-in-oil micro-emulsions formed in mixtures of a nonionic surfactant,octylphenol ethoxylate (alias Triton X-100, T-X100)/[bmim][BF4])/cyclohexane have been reported. Light scattering [21•]

ctant bilayer vesicles of Zn(OCCH2C6F13)2 in ionic liquid, [bmim][BF4] (a) anded as the negatively charged dye. Freeze-fracture TEM image of C14DMAO andon).


and contrast variation small-angle neutron scattering (SANS)[22••] were, respectively, used to demonstrate the formation ofsurfactant-stabilized dispersed nanodroplets with IL cores. Allthree possible topologies, i.e., oil-in-IL (oil is as dispersed-phase in ILs), IL-in-oil (IL is as dispersed-phase in oil), andbicontinuous region were characterized by conductivity mea-surements. FF-TEM images of microemulsion samples withdifferent IL to TX-100 molar ratios, are shown in Fig. 3. Thediameters of the droplets vary from 15 to 80 nm, depending onthe molar ratios of IL to TX-100.

Dynamic light scattering measurements were also used inthis work to characterize the size and the size distribution of thedroplets in microemulsions. In the same ternary system, SANSexperiments on IL-in-oil ([bmim][BF4]-in-cyclohexane) micro-emulsions were carried out [22••], which demonstrated theformation of surfactant-stabilized dispersed nanodroplets withIL cores. SANS curves for 4 single-phase solutions wereobtained, which were shown in Fig. 4. For the molar ratio R=[IL]/[TX-100]=0 the sample represents scattering from IL-freereversed micelles of TX-100. With added IL, there is regularincrease in SANS intensity at a constant level of TX-100 suchas at φTX-100=0.41. The SANS data in Fig. 4 had been treated

Fig. 3. FF-TEM images of microemulsion samples of [bmim][BF4]/TX-100/cyclohe(below, left), and 1.5 (below, right), respectively. The weight fraction of TX-100 is

in terms of an ellipsoid form factor, showing a regular increasein droplet volume as micelles are progressively swollen withadded IL, behavior consistent with “classic” water-in-oilmicroemulsions. The estimated corresponding calculatedsurfactant molecular areas at the IL-solvent interface arebetween 0.64 and 0.95 nm2 on the basis of the fitted particledimensions and intensity scale factors. These surfactantmolecular areas are similar to those found in typical W/Omicroemulsions, suggesting that the TX-100 monolayercoverage of IL-in-oil microemulsions is physically reasonable.An important difference is that instead the linear dependence ofmicelle radius with surface to volume ratio, in RTIL, we have alinear relation between surface to volume ratio and the volumeof the dispersed micelles, which implies a non-isometric andnon-trivial swelling behavior, different from the classical waterin oil micelles.

4. Self-assembly of surfactants into lyotropic liquid crystals(LLCs) in RT-ILs

More fully solvophobic self-assembly by amphiphilic mole-cules (conventional cationic, anionic, nonionic, and zwitterionic

xane with the molar ratios (R) of IL/TX-100, 0.2 (top, left), 0.5 (top, right), 1.00.45. (Reproduced from Ref. [21•], with permission).

Fig. 4. SANS curves from single-phase ionic liquid-in-oil microemulsions at55 °C. The molar ratio R=[IL]/[TX-100]=0 (R ), 0.2 (▪), 0.5 (q), and 1.0(•) at φTX-100=0.41. The fits shown as lines are to a form factor forhomogeneous ellipsoids. Inset shows the micelle swelling behavior in terms ofthe ellipsoid volume V as a function of added IL. (Reproduced from Ref. [22•],with permission).


surfactants or amphiphilic block copolymers) in room-tempera-ture ionic liquids into lyotropic liquid-crystal phases have rarelybeen reported although this kind of liquid-crystal phase ofdistearoylphosphotidylcholine in EANwas explored 20 years ago[5]. Recently two reports including our results on the self-assembly of amphiphilic triblock copolymer into hexagonal (H1)and lamellar (Lα) phases in [bmim][PF6] [23•] and nonionicsurfactants (polyoxyethylene, CnEm) in EAN into H1, Lα-phases,and a bicontinuous cubic phase [24•] have appeared.

The typical SAXS determination of LLCs (H1-and Lα-phase) of amphiphilic, Pluronic P123 (EO20PO70EO20) triblockcopolymer is shown in Fig. 5 at T=25 °C [23•]. For anisotropicregion at P123 triblock copolymer at lower concentration of 45%(wt.%), Fig. 5A shows the scattering peaks having the relativepositions of 1:3½:2, characterizing a H1-phase. At higher P123concentration, the scattering peak position emerges in the ratio1:2:3, indicating a Lα-phase. Comparing the extracted structure

Fig. 5. SAXS curves of anisotropic phases of P123 in water (a) and [bmim][PF6] (b)lamellar phase. 1, 2 and 3 represent 1st, 2nd and 3rd peaks, respectively. (Reproduc

parameters of LLCs formed in [bmim][PF6] with those inaqueous systems, it is clear that the lattice spacing aresystematically smaller than those of aqueous systems, whichdemonstrates that repulsive interaction between bilayers areweaker in RT-IL′s than in water [23•]. Increasing temperatureon anisotropic samples of H1 and Lα-phases induces the firstpeak position to a smaller qmax value, i.e. swelling, which is theexpected result if Helfrich-type of interactions due to fluctua-tions are present. However, a phase transition from hexagonal tolamellar in the 45% (wt.%) P123-water system was observed atT=65 °C: like in water, the equilibrium area per head-group ofthe P123 shrinks with temperature.

A pioneering work has been recently published by Araos andWarr [24•]. These authors reported the self-assembly ofnonionic surfactants (CnEm) into lyotropic liquid crystals inEAN. Here, the strong exclusion of large hydrophobic tails fromEAN is used as a driving force for aggregation. The authorscould map out the lyotropic liquid-crystalline phases con-structed by a number of polyoxyethylene nonionic surfactants(CnEm) in EAN. The large domain of existence of liquid crystalsis attributed in this case to the H-bonding in the solvent. Twotypical phase diagrams of the binary C16E6/EAN and C18E6/EAN systems are shown in Fig. 6.

The micelles formed in the diluted region of the phasediagram present the same sphere to rod transition in EAN thanin water [25]. Since van der waals attraction is much moreeffective between parallel cylinders, a cloud point is alsoobserved in room temperature, and coincides with the sphere torod transition associated to a temperature-induced decrease ofthe area per head-group.

As expected from packing constraints and the correspondingtheory [9••], the authors found similar phase sequence in eachbinary mixture: alkyl and ethoxy chain lengths affect greatly thesequence of phases formed in each binary mixture followedexpectations for aqueous systems. For the octadecyl chains,only lamellar phases for the surfactants with the smallestsolvophilic ethoxy chains and hence the lowest spontaneouscurvature (highest surfactant-packing parameter) C18E2 andC18E4 were observed. Increasing ethoxy chain length to C18E6

produces H1 and V1 phases, but these all melt at lower

at 25oC. P123 concentrations: A) 45% (wt.%), hexagonal phase; B) 75% (wt.%),ed from Ref. [23•], with permission).

Fig. 7. SEM images of hollow TiO2 microspheres prepared in RT-ILs.(Reproduced from Ref. [27], with permission).

Fig. 6. Phase diagrams of the binary CnEm (n=16 and 18, m=6)/EAN systems, showing a large single-phase isotropic (L1) region, together with discrete cubic (I1, inC16E6/EAN system), hexagonal (H1), bicontinuous cubic (V1), and lamellar phases (Lα). Left phase diagram is C16E6/EAN system and right C18E6/EAN one. Dashedlines indicate approximate phase boundaries determined for the smaller lyotropic phases, and horizontal hatching denotes tie lines for two phase coexistence.(reproduced from Ref. [24•], with permission).


temperatures than Lα, suggesting that low curvature is stillpreferred. However, upon increasing ethoxy chain length toC18E8, the lamellar phase melts at lower temperature than thehexagonal phase.

The authors have shown that similarity in binary phasediagrams between surfactant and ionic liquid as polar solventallows to formulate ternary microemulsions containing a room-temperature ionic liquid as the polar phase. In ternary systemscontaining linear nonionic surfactants, ethylammonium nitrateand water, the spontaneous curvature is also tuned by curvature.Efficiency of the surfactant is lower and the rigidity of thewater/oil interface is higher [26]. Thus, the microemulsionregion competes with a large domain of lamellar phase and themicroemulsion microstructure is not the fluctuation-dominatedrandom bicontinuous structure observed with water, but thelocally lamellar microstructure. This microstructure as randomopen and connected lamellae can be detected by the peculiarswelling behavior observed when the water/oil volume ratio isvaried [27,28].

5. Preparation of nano- and micro-particles in RT-ILs

In most cases, in aqueous colloidal template systems, aparadoxal behavior is observed: the “templating”, i.e. conser-vation of the shape of the surfactant template as also the finalshape of the nanoparticle produced is not relevant. This conser-vation of shape is relevant when high self-assembling polymersinstead of surfactants are used as templates [29]. The onlytheory with predictive power in this matter relies in the obser-vation that number of nanoparticles produced is always muchlower than the number of initially present globular or cylin-drical giant micelles/microemulsions/giant micelles. Hence, themicelles act as an initial reservoir and the surfactant “poison”the surface, thus limiting growth of the nanoparticles even invery reactive systems [29]. The final nanoparticle shape andsize are not related to those of the self-assembled structures inaqueous solutions, but controlled by exchange between micellesas reservoirs and poisoning of faces [30]. Since in RT-IL, wehave seen that there is less “incompatibility” between thehydrophobic chains, the exchange process will be more efficient

in RT-IL and the poisoning of surface less efficient, two generaleffects which can be used to use organo-metallic precursors oflower reactivity and/or to obtain larger particles.

Using this strategy, i.e. using ionic liquids as solvents for thesynthesis of inorganic materials has only been reported to theacid-catalyzed sol–gel synthesis of silica aerogels and formationof palladium or iridium nanoparticles [31••]. In this case hollowmetal-oxide microspheres were obtained: the shape of micron-sized particles obtained in RTIL are similar to the shape obtainedin emulsion or atomization. Metal alkoxides used, titanium tetra-butoxide [Ti(OBu)4], are first dissolved in anhydrous toluene. Thesolution of Ti(OBu)4 in toluene is then injected into a RT-ILsolvent, [bmim][PF6], under vigorous stirring, like in emulsionpolymerisation. The resulting titania microspheres are shown inFig. 7. As in emulsion polymerisation, the microspheres averagesize is controllable via the stirring rate and the chemical reactiontemperature. The authors also pointed that this simple method iswidely applicable to the reactive metal alkoxide for producing themetal-oxide microspheres.

Another use of the reservoir effect using self-assembly inRTIL and low reactivity is the preparation of ZnS precipitateswere prepared by vesicle-phase as reaction media in [bmim][PF6] [17

•]. The organo-zinc precursor Zn(OOCCH2C6F13)2 isused to synthesize nanometer scale semiconductor ZnS particleswith H2S gas as source of sulphur. The “template” C14DMAObilayer vesicle solutions are prepared in ionic liquids. After H2S

Fig. 9. Partition coefficient D observed for a lanthanide comparing standardwater-dodecane equilibrium of tributyl phosphate and by using a room-temperature ionic liquid instead of the “oil” phase. Above 2 M nitric acid in thewater phase, water is co-extracted and nucleate reverse micelles as well in oilthan in ionic liquid.

Fig. 8. TEM images showing the spherical and regularly hexahedral re-united ZnS masses obtained from Zn(OOCCH2C6F13)2 and C14DMAO bilayer vesicle ionicliquid solutions via injecting H2S. Some regularly hexahedral re-united masses were visible. (Reproduced from Ref. [17•], with permission).


gas was injected into Zn(OOCCH2C6F13)2 and C14DMAObilayer vesicle-phase (24 h), the appearance of the sample haschanged to be more transparent puce-colored visco-elasticsolution and stable. In Fig. 8, one can see that the spherical andhexahedral primary structures of the ZnS crystallites. Here also,neither the size nor the shape of original vesicles had beenpreserved via an hypothetic “templating” effect.

6. Open questions under investigation

It is clear that “solvophobic” effect is smaller in RTIL thanthat in water. Therefore, concentrations must be higher or the“amphiphilic” molecules used of higher molar mass. Pluronicsor any other co-polymers can be used in RT-ILs as they are“macro-surfactants.” The weaker rejection of the surfactantfrom the bulk solvent gives advantages when the function as areservoir is needed when growing nanoparticles in RT-ILs.

Unknown is yet the equation of state of a lamellar phase, i.e.the colloidal interactions involved: this would require the pos-sibility of determining osmotic pressure via a semi-permeablemembrane. Also the value of the bending energy of a surfactantfilm is not known: some aspects show the surfactant film is stiffand could be in a “gel” state (i.e. the shape of the X-ray peak)when other observations show theymay be fluid (i. e. the radius ofdroplets formed is indifferent, i.e. no electrical anti-percolationnor emulsification failure was reported.

The most interesting effect is still to be explored: in principlesome RT-ILs can solubilize some water. Presence of waterwould greatly enhance exclusion of hydrophobic parts of thesurfactant molecule from the solvent (as suggested by H.-F.Eicke more than twenty-five years ago [32••]). Thus,aggregation, spontaneous liquid-crystal formation, low energyemulsification due to curvature inversion as well as encapsu-lation could be driven by adding only small quantities of waterto the structured RT-IL sample. Once these effects are sortedout, the synthesis methods used to obtain micron-sized spheres,similar to latex preparation using polymerisation in emulsions,will be under control and the effect of parameters such ascontact between immiscible liquids, one of them being amicroemulsion understood. Recipes such as one-step prepara-tion of hollow microspheres, now reported without knowledgeof the mechanism of formation, will become secure route fornanomaterial synthesis.

Very recently, a new class of surfactant-like ionic liquids hasbeen designed and synthesized: they combine a classical ionicliquid with a long hydrocarbon chain, typically a chain up to 16carbon atoms. This new class of ionic liquid is miscible andforms aggregates in mixtures with another ionic liquid (EAN).Critical aggregate concentrations and estimations of object sizeshave been given and compared to aqueous systems [33•]. Themixtures investigated are stable up to more than 200 °C and canprobably be used to extend the limited temperature range ofwater-based colloids.

Finally, it was realized recently that any efficient liquid–liquid extraction of chaotropic salt, such as uranium nitrate innuclear waste relies on the presence of reverse micelles in the“solvent phase” [34]. Very recent discovery is that efficientextraction of lanthanide ions can be obtained by using a room-temperature ionic liquid and a classical “complexant” shirtchain molecule. The most used “extractant”molecule is tributyl-phosphate, which forms reverse micelles in water. Comparedefficiency of extraction using a standard hydrocarbon solventand a room-temperature ionic liquid is shown in Fig. 9. Theefficiency of extraction observed in the presence of nitric acid


makes very likely that extracting molecules also aggregate asreverse micelles in ionic liquids [35]. This observation may becrucial to design completely new liquid–liquid extractionprocesses.

References and recommended readings

[1]•

Welton T. Room-temperature ionic liquids: solvents for synthesis andcatalysis. Chem Rev 2005;9:2071–84;Handy ST. Current organic chemistry 2005;9:959–88. In the first review,the related preparation, handling, solvent properties of room temperatureionic liquids, chemical reactions including organic reactions andtransition-metal-mediated catalysis, etc. are summarized. In the secondreview physical chemistry and solvent properties of RTIL's are criticallyreviewed.

[2] Dubois M, Zemb Th. Phase behavior and scattering of double-chainsurfactants in diluted aqueous solutions. Langmuir 1991;7:1352–60;Dubois M, Zemb Th. Swelling limits for bilayer microstructures: theimplosion of lamellar structure versus disordered lamellae. Curr OpinColloid Interface Sci 2000;5:27–37.

[3] Evans DF, Yamauchi A, Roman R, Casassa EZ. Micelle formation inethylammonium nitrate, a low-melting fused salt. J Colloid Interface Sci1982;88:89–96.

[4] Evans DF, Yamauchi A, Wei G, Bloomfield VA. Micelle size inethylammonium nitrate as determined by classical and quasi-elastic lightscattering. J Phys Chem 1983;87:3537–41.

[5] Evans DF, Kaler EW, Benton WJ. Liquid crystals in a fused salt: .beta.,.gamma. -distearoylphosphatidylcholine in N-ethylammonium nitrate. JPhys Chem 1983;87:533–5.

[6] Tamura-Lis W, Lis LJ, Quinn PJ. Structures and mechanisms of lipid phasetransitions in nonaqueous media: dipalmitoylphosphatidylcholine in fusedsalt. J Phys Chem 1987;91:4625–7.

[7] Tamura-Lis W, Lis LJ, Quinn PJ. Structures and mechanisms of lipid phasetransitions in nonaqueous media Dipalmitoylphosphatidylethanolamine infused salt. Biophys J 1988;53:489–92.

[8] Evans DF, Chen SH, Schryver GW, Arnett EM. Thermodynamics ofsolution of nonpolar gases in a fused salt Hydrophobic bonding behavior ina nonaqueous system. J Am Chem Soc 1981;103:481–2.

[9]••

Hyde S, Andersson S, Larsson K, Blum Z, Landh R, Lidin S, et al, editors.The language of shape: the role of curvature in condensed matter: physics.Elsevier, Oxford: Chemistry and Biology; 1997. The classical text aboutself-assembly and packing constraints.

[10] Chevalier Y, Zemb Th. The structure of micelles and microemulsions. RepProg Phys 1990;53:279–371;Zemb Th. The DOC model of microemulsions: microstructure, scattering,conductivity and phase limits imposed by sterical constraints. Colloid SurfA, Physicochem Eng Asp 1997;129:130:435–54.

[11]•

Fogden A, Hyde ST, Lundberg G. Bending energy of surfactant films.J Chem Soc Faraday Trans. 1991;87:949–55. The phenomenologicaltheory of the bending energy in a surfactant membrane is analyzed andextended to account for the case of spontaneously curved membranes inthis paper.

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[13]••

Anderson JL, Pino V, Hagberg EC, Sheares VV, Armstrong DW.Surfactant solvent effects and micelle formation in ionic liquids. ChemCommun 2003:2444–24445. The formation of micelles of differentsurfactants in 1-butyl-3-methyl imidazolium chloride and hexafluoropho-sphate, are described.

[14] See standard textbooks such as D. F. Evans et H Wennerstroem, “Thecolloidal domain” or Lindman B, Wennerstroem H: Micelles. Amphi-phile aggregation in aqueous solution. Topics in current chemistry1980;87:1–87.

•Of special interest.

••Of outstanding interest.

[15]•

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Nakashima T, Kimizuka N. Vesicle in salt: formation of bilayermembranes from diakyldimethylammonium bromides in ether-containingionic liquids. Chem Lett 2002:1018–9.

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Hao J, Song A, Wang J, Chen X, Zhuang W, Shi F, Zhou F, et al. Self-assembled structure in room-temperature ionic liquids. Chem Eur J2005;11:3936–40.

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[19] Brotons G, Dubois M, Belloni L, Grillo I, Narayanan T, Zemb T. The roleof counterions on the elasticity of highly charged lamellar phases: a small-angle x-ray and neutron-scattering determination. J Chem Phys2005;123:24704.

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[22]••

Eastoe J, Gold S, Rogers SE, Paul A, Welton T, Heenan RK, et al. Ionicliquid-in-oil microemulsions. J Am Chem Soc 2005;127:7302–3.SANSexperiments were carried out for determining the ionic liquid-in-oilmicroemulsions, which clearly demonstrated the formation of surfactant-stabilized dispersed nanodroplets with IL cores.

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Wang L, Chen X, Chai Y, Hao J, Sui Z, Zhuang W, et al. Lyotropic liquidcrystalline phases formed in an ionic liquid. Chem Commun 2004: 2840–1.Lyotropic liquid crystalline phases including a hexagonal phase (H1) and alamellar phase (Lα) of an amphiphilic block copolymer in an ionic liquid,[bmim][PF6], were characterized by SAXS measurements. The phasetransition with comparison of component and temperature was determined.

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Araos MU, Warr GG. Self-assembly of nonionic surfactants into lyotropicliquid crystals in ethylammonium nitrate, a room-temperature ionic liquid.J Phys Chem B 2005;109:14275–7. Lyotropic liquid crystalline phasesincluding H1, Lα, and a bicontinuous cubic phase (V1) of polyoxyethylenenonionic surfactants in a RT-IL, ethylammonium nitrate (EAN) wereobserved and phase diagrams of binary CnEm (n=16 and 18, m=6)/EANsystems were determined using the serial dilution technique. This is thefirst observation of phase behavior for differently lyotropic liquidcrystalline phases in ionic liquids.

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Nakashima T, Kimizuka N. Interfacial synthesis of hollow TiO2 micro-spheres in ionic liquids. J Am Chem Soc 2003;125: 6386–7. A single-stepsynthesis of hollow titania microspheres in ionic liquids by adding theanhydrous toluene solution containing Ti(OBu)4 was described, whichshould be widely applied to the design of smart organic/inorganic hybridnanometer materials.

[32]••

Eicke HF, Christen H. Nucleation process of micelle formation in apolarsolvent. J Colloid Interface Sci. 1974;48:281–90;


Eicke HF, Shepherd JCW, Steinemann A. Exchange of solubilized waterand aqueous electrolyte solutions between micelles in apolar media.J Colloid Interface Sci 1996;56:168–76;Eicke HF. Surfactants in nonpolar solvents Aggregation and micellization.Top Curr Chem 1980;87:85–145. In his now classical work, Eicke hasdemonstrated that aggregation of any surfactant in oil critically depends on thepresence or absence of low amounts of water. From this very general work, aneffect of residual water on the self-assembly in RT-ILs is expected.

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Thomaier S, Kun W. Aggregates in mixtures of ionic liquids. J Mol Liq2007;130(1–3):104–7.

[34] Nave S, Mandin C, Martinet L, Berthon L, Testard F, Madic C., et al.Supramolecular organisation of tri-n-butyl phosphate in organic diluent onapproaching third phase transition. Phys Chem Chem Phys 2004;6(4):799–808. This paper shows on the example of tributyl phosphatethat aggregation in the form of reverse micelles able to receive ionsextracted from aqueous medium above a certain concentration bothenhances liquid–liquid extraction efficiency as well as formation ofWInsor III esquilibria observed in quaternary oil–water-extractantand salt systems.

[35] Billard I. (CNRS Strasbourg, France), private communication.

hao et al., 2007

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