Nonionic Surfactant Behavior in Ionic Liquids

Ionic liquids are attracting increasing attention in many fields, including organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering because of their distinctive properties. Due to their high polarity and their weak coordination ability, reactions carried out in ionic liquids may lead to enhanced reaction rates and higher yields. This research was designed to investigate the nonionic surfactant performance in ionic liquids. 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) was used as the model ionic liquid and polyoxyethyleneglycol dodecyl ethers were used as nonionic surfactants. [Bmim][PF6] surface tension was observed to drop linearly with regard to the increase of the nonionic surfactant concentration until critical micelle concentration (CMC) was reached. In addition, interaction free energy between nonionic surfactant molecules immerged in ionic liquids was found to be the driving force for the aggregation potential of nonionic surfactant molecules at CMC. [DOI: 10.1380/ejssnt.2011.390]


I. INTRODUCTION
Ionic liquids consist of ions and short-lived ion pairs and the ionic bonds are usually stronger than the van der Waals forces between the molecules of ordinary liquids [1,2].Consequently, ionic liquids are often moderate to poor conductors of electricity, non-ionizing (e.g.nonpolar), highly viscous and frequently exhibit low vapor pressure and high heat capacity per unit volume [3][4][5].The miscibility of ionic liquids with water or organic solvents varies with side chain lengths on the cations and with choices of anions.In practice, they can be functionalized to act as acids, bases or ligands, and have been used as precursor salts in the preparation of stable carbenes [6][7][8][9][10][11][12].Because of their distinctive properties, ionic liquids are attracting increasing attention in many fields, including organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering.Especially, ionic liquids are good solvents for both organic and inorganic liquids.Due to their high polarity and their weak coordination ability, reactions carried out in ionic liquids may lead to enhanced reaction rates and higher yields [13][14][15][16].
Since ionic liquids have tremendous advantages for many applications, characterization of their thermodynamic properties has significant importance.The thermodynamic properties of ionic liquids depend mainly on ionic liquids' polarity and hydrogen bonding ability, which facilitates the role of their thermodynamic intermolecular interactions, ranging from weak attractions to hydrogen bonds, associations and ionic bonds [17][18][19].The ionic liquids' thermodynamic properties can help interpret different chemical and thermodynamic reactions.Specifically, ionic liquids' thermodynamic properties influence surfactant performance such as the surface tension in practical applications, which often play a central role in oil recovery, pharmaceutical industry and biotechnology [20][21][22][23].Surfactants consist of molecules containing both polar functional groups and nonpolar hydrocarbon chains, of which polar functional groups have sufficient affinity to the aqueous solution to drag nonpolar hydrocarbon chains into it.When immersed in the aqueous solution, surfactant molecules are surrounded by a distortion of the local structure of ionic liquids and hydrogen bonds between ionic liquid structures are energetically disfavored, resulting in a different organization and consequently a decreased surface tension [24,25].At the same time, hydrophobic interactions between amphiphiles of surfactant molecules may contribute to the formation of micelles when surfactant concentrations are high enough [22,26].Micelle formation becomes appreciable at a well-defined concentration known as the critical micelle concentration (CMC).
This research was designed to investigate the thermodynamic properties and nonionic surfactant performance in ionic liquids.1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF 6 ]) was used as the model ionic liquid and polyoxyethyleneglycol dodecyl ethers were used as nonionic surfactants in this research.Performance of nonionic surfactants in these ionic liquids was explored and related to the interaction free energy between surfactant molecules, which was calculated from nonionic surfactant and ionic liquid thermodynamic properties obtained from independent contact angle measurements.In addition, micellization mechanism of nonionic surfactants in ionic liquids was investigated on the basis of thermodynamic analysis of the micelle formation of a series of polyoxyethyleneglycol dodecyl ethers with varying polyoxyethylene chain length in [Bmim][PF 6 ].

A. Materials
[Bmim][PF 6 ] was purchased from IoLitec Ionic Liquids Technologies (GmbH, Germany), which are transparent viscous liquids.Nonionic surfactants used in this study consisted of an alkane chain as the hydrophobic moiety and an ethylene oxide group (C 2 H 4 O) (POE chain) as the hydrophilic moiety with varying POE chain lengths, which included pentaethylene glycol monododecyl ether (C 12 E 5 ), heptaethylene glycol monododecyl ether (C 12 E 7 ) and nonaethylene glycol monododecyl ether (C 12 E 9 ).All these nonionic surfactants were obtained from Sigma (Sigma-Aldrich Corp., St. Louis, MO) in solid form and dissolved in hexadecane before used in experiments without further purification.

B. Surface Tension Measurement
Ionic liquid surface tension in the presence of nonionic surfactants was measured using a Kruss K10 tensiometer (Kruss GmbH, Germany) with a platinum plate.The added amount of nonionic surfactants was 0.3% to 10% (weight percent) for C 12 E 5 , 0.35% to 10% (weight percent) for C 12 E 7 and 0.4% to 25% (weight percent) for C 12 E 9 .Each measurement was repeated five times and average results were reported.The temperature was held constant at 20.0 • C by circulating thermo-stated water through a jacketed vessel containing the sample.The experimental uncertainty of these surface tension measure-ments was approximately 0.1 mN/m.

C. Measurement of surface thermodynamic properties
Ionic liquid thermodynamic properties were estimated by means of contact angle measurements (Contact Angle Meter, Tantec, Schaumburg, IL) on three solid surfaces of polypropylene, polymethylmethacrylate (PMMA) and polyamide (Nylon) (Aldrich Chemical Co., Inc., Milwaukee, Wisconsin) following the method described by Grasso et al. [27].Surface thermodynamic properties of these solid surfaces were estimated using an apolar liquid, diiodomethane and two polar liquids, formamide and water in advance following the same method (Table I).After the contact angle measurements, ionic liquid thermodynamic properties are calculated according to the van Oss-Chaundhury-Good equation [28]: where θ is the measured contact angle (degree); γ LW is the Liftshitz-van der Waals component of free energy (J/m 2 ); and γ + is electron-acceptor parameter and γ − is electrondonor parameter of Lewis acid/base component of free energy (J/m 2 ).In above equation, subscript "L" denotes for ionic liquids and "S" for solid surfaces.When above equation was used for solid surface thermodynamic property characterization, subscript "L" denotes for diiodomethane, formamide or water.Total free energy can be expressed as: where γ is the total free energy (J/m 2 ).

III. RESULTS AND DISCUSSIONS
[Bmim][PF 6 ] surface tension was observed to decrease in the presence of nonionic surfactants as determined by the surfactant nature (Fig. 1).The glycol dodecyl ether surfactants used in this research were modified hydrophilic polymers of backbone (POE chains) with covalently bound hydrophobic side chains.[Bmim][PF 6 ] surface tension dropped linearly with regard to the increase of the nonionic surfactant concentration until CMC was reached, which were 1.24, 1.73 and 6.18 mM for C 12 E 5 , C 12 E 7 and C 12 E 9 , respectively.
Based on contact angle measurements, nonionic surfactants had γ LW values in the range of 20.6 mJ/m 2 to 21.3 mJ/m 2 , which increased with the increase of POE chain length (Table II).On the other hand, [Bmim][PF 6 ] had greater γ LW value than those of the nonionic surfactants.Both nonionic surfactants and [Bmim][PF 6 ] had a monopolar surface since their γ − values were at least one order of magnitude greater than γ + .It was obvious that the monopolarity was more pronounced for nonionic surfactants than [Bmim][PF 6 ].
As ionic liquid surface tension is determined by ionic liquid thermodynamic properties, ionic liquid surface tension variations in the presence of nonionic surfactants can be described by their thermodynamic properties.When nonionic surfactants are applied at low concentrations, a very compact monolayer can be formed at the interface with an interfacial volume fraction close to unity.In addition, accumulation of the nonionic surfactants at the interface follows the Frumkin adsorption isotherm [29][30][31].Mathematically, the relationship of [Bmim][PF 6 ] surface tension drop in the presence of nonionic surfactants can be related to Gibbs free energy of the interactions between nonionic surfactant molecules, which was attributed to hydrophobic interactions between amphiphiles of surfactant molecules.When surfactant concentrations are high, interactions between nonionic surfactant molecules may contribute to the formation of micelles.Micelle formation or amphiphile association is determined by the nature of both hydrophobic moieties of surfactants as well as hydrophobic moieties of the ionic liquids [22,32].Micelle formation can be evaluated in terms of CMC, which is related to the total interaction free energy between surfactant molecules when immersed in ionic liquids, ∆G TOT 131 where ∆G TOT 131 is the interaction free energy between surfactant molecules (J/m 2 ); A is the limiting area per surfactant molecule; k is the Boltzmann constant (1.38×10 −23 J/K); and T is the absolute temperature (K).∆G TOT 131 can be calculated based on the surfactant and ionic liquid surface thermodynamic properties: where ∆G LW 131 is the Lifshitz-van der Waals interaction free energy (J/m 2 ); ∆G AB 131 is the Lewis acid/base interaction free energy (J/m 2 ).As shown in Eqs. 5 and 6, ∆G TOT 131 is highly dependent upon ionic liquid thermodynamic properties.In above equations, subscript "1" denotes for nonionic surfactants and "3" denotes for ionic liquids.The limiting area per surfactant molecule, A can be estimated by: where γ is the ionic liquid surface tension when saturated with surfactants (mN/m); N is the number of ethylene oxide (PEO) segments in the surfactant tails; and a is an effective monomer size (2.1 Å).
Based on the thermodynamic properties of nonionic surfactant and ionic liquids, interaction free energy between nonionic surfactant molecules immerged in ionic liquids was calculated according to equations 4-6 (Table III  Waals interactions between nonionic surfactant molecules and ∆G AB 131 , Gibbs free energy of Lewis acid/base interactions between nonionic surfactant molecules immerged in ionic liquids were both negative, indicating that they both contributed to nonionic surfactant molecule attractions.Consequently, ∆G TOT 131 , sum of ∆G LW 131 and ∆G AB 131 , was negative, demonstrating the aggregation potential of nonionic surfactant molecules at CMC. CMC of nonionic surfactant increased from 1.24 to 6.18 mM with the increase of POE chains from 5 to 9. Increase in CMC with increasing POE chains was attributed to the increase (negative decrease) of ∆G TOT 131 (Fig. 2).In addition, [Bmim][PF 6 ] surface tension drop linearly decreased with the increase of attractive interactions (absolute vales) (Fig. 3).

As ∆G LW
131 was one order in magnitude greater than ∆G AB 131 , ∆G LW 131 was actually the driving force in determining nonionic surfactant CMC.Van der Waals forces include Keesom, Debye, and London interactions [28].Of these three, Keesom and Debye interactions are only found among molecules that have permanent dipole moments.The London interactions, however, are universal and are of preponderate importance especially in aqueous media that contain electrolytes [33,34].Therefore, the Lifshitz-van der Waals surface component, γ LW , based on which intermolecular Lifshitz-van der Waals interactions are calculated, is mainly contributed by London interactions.Owing to the induced dipole potential, Lifshitz-van der Waals interactions occur between two nonionic surfactant molecules in ionic liquids.At CMC, nonionic surfactant molecules had negative ∆G LW 131 values, indicating sur-  4).Accordingly, C 12 E 5 had smallest CMC (1.24 mM as compared to 1.73 mM and 6.18 mM).

IV. CONCLUSIONS
Thermodynamic properties of nonionic surfactants and [Bmim][PF 6 ] were measured independently.Both nonionic surfactants and [Bmim][PF 6 ] had a monopolar surface since their γ − values were at least one order of magnitude greater than γ + .Based on the thermodynamic properties of nonionic surfactant and ionic liquids, interaction free energy between nonionic surfactant molecules immerged in ionic liquids was calculated.∆G LW 131 , Gibbs free energy of Lifshitz-van der Waals interactions between nonionic surfactant molecules and ∆G AB 131 , Gibbs free energy of Lewis acid/base interactions between nonionic surfactant molecules immerged in ionic liquids were both negative, indicating that they both contributed to nonionic surfactant molecule attractions.

FIG. 2 :
FIG.2: Log(CMC) of nonionic surfactant as a function of interaction free energy between surfactant molecules.

TABLE I :
Surface thermodynamic properties of standard liquids and contact angles and thermodynamic properties of standard solids.θ DII , θ F and θ W are contact angles measured with diiodomethane, formamide and water, respectively.
FIG.4: Interaction free energy between surfactant molecules as a function of van der Waals component free energy.factantmolecules started to aggregate.C 12 E 5 had smallest ∆ Consequently, ∆G TOT 131 , sum of ∆G LW 131 and ∆G AB 131 , was negative, demonstrating the aggregation potential of nonionic surfactant molecules at CMC.As ∆G LW 131 was one order in magnitude greater than ∆G AB 131 , ∆G LW 131 was actually the driv-Chen ing force in determining nonionic surfactant CMC.In the presence of nonionic surfactants, [Bmim][PF 6 ] surface tension dropped, which linearly decreased with the increase of attractive interactions (absolute vales).