离子液体表面活性剂的有关内容

Journal of Colloid and Interface Science326(2008)
483–489
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science
/locate/jcis
Cloud point phenomena for POE-type nonionic surfactants in a model room temperature ionic liquid
Tohru Inoue∗,Takeshi Misono
Department of Chemistry,Faculty of Science,Fukuoka University,Nanakuma,Jonan-ku,Fukuoka814-0180,Japan
a r t i c l e i n f o a
b s t r a
c t
Article history:
Received14March2008 Accepted10April2008 Available online26April2008
Keywords:
Polyethylene glycol alkyl ether Nonionic surfactant
Cloud point
Room temperature ionic liquid Solvophilicity
Solvophobicity The cloud point phenomenon has been investigated for the solutions of polyoxyethylene(POE)-type nonionic surfactants(C12E5,C12E6,C12E7,C10E6,and C14E6)in1-butyl-3-methylimidazolium tetrafluoroborate(bmimBF4),a typical room temperature ionic liquid(RTIL).The cloud point,T c,increases with the elongation of the POE chain,while decreases with the increase in the hydrocarbon chain length.This demonstrates that the solvophilicity/solvophobicity of the surfactants in RTIL comes from POE chain/hydrocarbon chain.When compared with an aqueous system,the chain length dependence of T c is larger for the RTIL system regarding both POE and hydrocarbon chains;in particular,hydrocarbon chain length affects T c much more strongly in the RTIL system than in equivalent aqueous systems.In a similar fashion to the much-studied aqueous systems,the micellar growth is also observed in this RTIL solvent as the temperature approaches T c.The cloud point curves have been analyzed using a Flory–Huggins-type model based on phase separation in polymer solutions.
©2008Elsevier Inc.All rights reserved.
1.Introduction
Room temperature ionic liquids(RTILs)are a class of organic electrolytes which are in molten state around room tempera-ture[1].Owing to their unique chemical and physical properties, RTILs have currently attracted much interest for applications as novel solvents in organic synthesis[1],catalysis[1,2],electrochem-istry[3],and liquid/liquid extraction[4–6].Particularly,they have an advantage as an environmentally benign solvent,i.e.,“green sol-vent,”since their nonvolatile nature can prevent the environmental pollution.Another advantage of RTILs is that their physical and chemical properties can be readily adjusted by suitable selection of cation,anion,and substituent.
It is well known that surfactant molecules undergo self-association in aqueous solution to form micelles.The surfactant micelles can solubilize substances being essentially insoluble in water.If surfactant molecules form aggregates like micelles in RTILs,inherently insoluble solutes would be dissolved in the medium with the aid of solubilizing power of micelles,and hence, the applicationfield of RTILs as a solvent would be extended.From this point of view,the surfactant solution in RTILs has currently become a research subject of considerable interest.
1-Alkyl-3-methylimidazolium salts are typical RTIL,and sev-eral reports have been published regarding the surfactant self-assemblies in imidazolium based RTILs.For instance,micelle for-
*Corresponding author.
E-mail address:inouetr@fukuoka-u.ac.jp(T.Inoue).mation of nonionic Brij and anionic SDS in1-butyl-3-methylimid-azolium chloride(bmimCl)and hexafluorophosphate(bmimPF6) has been reported by Anderson et al.[7].Fletcher and Pandey have studied the aggregation behavior of anionic SDS,cationic CTAB,and nonionic Brij,Triton X-100,and Tween20in1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide(emimTf2N) using solvatochromic probe technique[8].The micelle formation behavior of several polyoxyethylene(POE)-type nonionic surfac-tants in1-butyl-3-methylimidazolium tetrafluoroborate(bmimBF4), bmimPF6,and bmimTf2N has been studied by Patrascu et al.[9], where it was demonstrated that the critical micelle concentrations are two to four orders of magnitude higher than those in aqueous system.Temperature dependent self-assembly of Brij surfactant in bmimBF4has also been reported by Tang et al.[10].Although sev-eral reports are seen in literature as mentioned above,the study concerning the micelle formation in surfactant/RTIL system is still rather few.
It is well known that the aqueous solution of polyoxyethy-lene(POE)-type nonionic surfactants exhibits a cloud point phe-nomenon in addition to the micelle formation[11].When tem-perature of an aqueous solution of this type of surfactants is in-creased,a phase separation takes place at a certain temperature from homogeneous solution to two-liquid phases,one is dilute so-lution and the other is concentrated solution of the surfactant, and the appearance of the solution becomes highly turbid.This phenomenon is attributed to the reduction of hydrophilicity of POE chain associated with the temperature rise.Regarding the cloud point phenomenon in surfactant/RTIL system,no systematic
0021-9797/$–see front matter©2008Elsevier Inc.All rights reserved. doi:10.1016/j.jcis.2008.04.047
484T.Inoue,T.Misono/Journal of Colloid and Interface Science326(2008)483–489
study has been reported,although the cloud point temperatures have been reported for several polyethylene glycol alkyl ethers in bmimBF4solution at a single concentration around35%(sup-porting information in Ref.[9]).Elucidation of cloud point phe-nomenon as well as micelle formation in RTIL system must be helpful to understand the behavior of amphiphilic compounds in ionic liquids.In the present work,we investigated cloud points of POE-type nonionic surfactants in bmimBF4.
2.Experimental
2.1.Materials
Polyethylene glycol alkyl ether(C n E m)investigated in this work are C12E5,C12E6,C12E7,C10E6,and C14E6.The surfactant samples with homogeneous chain length distribution were obtained from Nikko Chemicals(Tokyo,Japan),and used as received.1-Butyl-3-methyl imidazolium tetrafluoroborate(bmimBF4)with purity of 98%was purchased from Wako Pure Chemicals(Tokyo,Japan),and used without further purification.
2.2.Sample preparation
The ionic liquid was kept at80◦C under vacuum for longer than 4h before use in order to remove water contaminated from mois-ture.The mixtures of the surfactant and bmimBF4with various compositions were prepared by weighing the two samples into a glass cuvette followed by mixing with shaking until a clear solu-tion was obtained.
2.3.Differential scanning calorimetry(DSC)
The DSC measurements were carried out for the mixtures of C12E7and bmimBF4with different compositions using a Seiko Denshi Model Exstar6000(Tokyo,Japan).The mixture was weighed into an aluminum sample pan and the pan was sealed tightly.The total amounts of the mixture were in the range10–20mg.The cooling–heating cycle was repeated twice with the scanning rate of1◦C/min for heating run and3◦C/min for cool-ing run.A good reproducibility was obtained for the repeated runs.
2.4.Turbidity
The turbidity change of C12E6solution in bmimBF4associated with the temperature increase was measured by monitoring the scattered-light intensity from the solution using a JASCO FP550A spectrofluorometer(Tokyo,Japan)in the90◦light-scattering mode at400nm.The temperature of the sample solution was raised by the use of a Peltier-type electronic temperature controlling system attached to the spectrofluorometer,during which the sample was stirred continuously by a Teflon-coated magnet.The temperature of the sample was monitored using a platinum resistance ther-mometer,the tip of which was inserted into the sample.The pho-tometer output was recorded on an X–Y recorder together with the temperature signal.The temperature scans were made at the rate of2◦C/min.
2.5.Viscosity
Viscosity of bmimBF4and C12E6solution in bmimBF4was mea-sured at different temperatures by the use of a Brookfield Vis-cometer Model DV-II+(Stoughton,MA,USA)with cone spindle. The apparatus was calibrated using a silicone standard sample with known viscosity.Temperature was controlled by circulating thermostated water through a water jacked attached to a sam-ple container using a Neslab refrigerated circulation bath RTE-111 (Portsmouth,NH,USA).The viscosity reported below is the average of the values obtained with different shear rates in a range where the viscosity is independent of the shear rate.
2.6.Dynamic light-scattering(DLS)
The size of micelles formed in the surfactant solution was mea-sured by means of DLS method using a NICOMP Submicron Particle Sizer Model370(Santa Barbara,CA,USA)with an argon ion laser (λ=488.0nm)of a maximum power of75mW.The data analysis for DLS measurements was carried out using the software supplied by the manufacturer,which utilizes the cumulants analysis.Mea-surements were made for the solution of C12E6in bmimBF4as a function of temperature.The temperature of the sample was con-trolled by a Peltier-type electronic temperature controlling system attached to the apparatus.The values of viscosity and refractive in-dex of the solution are necessary to derive the particle size from the DLS measurements.The viscosity of C12E6solution in bmimBF4 was determined as a function of temperature,as mentioned above. The refractive index of the solution is not available,and hence,that of water was substituted.
2.7.Determination of cloud point temperatures
Cloud point temperatures,T c,were determined by either vi-sual observation or optical microscopic observation depending on the temperature range.The glass cuvette containing sample solu-tion was immersed into the water poured in a glass water jacket, the temperature of which was changed by circulating thermostated water through the water jacket using a Neslab refrigerated circula-tion bath RTE-111(Portsmouth,NH,USA).The sample was heated at the rate of about0.5◦C/min after the temperature reached a few degrees below the preestimated cloud point temperature.The appearance of the sample solution was observed visually during the heating process,and the temperature at which the solution became turbid was taken as T c.After the temperature exceeds the cloud point,the sample was cooled below T c,and then it was heated again to check the reproducibility of the measurements. This method was adopted for the cases that the cloud point is lower than about60◦C.For higher T c,optical microscopic obser-vation was made using an Olympus Model BHSP polarized optical microscope(Tokyo,Japan)equipped with a Linkam Model THMS 600temperature controlling stage(London,UK)under parallel po-larizer.Both visual and microscopic observations were repeated three times for a given sample,and the good reproducibility was obtained for T c values.The polarized optical microscope(POM) was also utilized to observe the solid-to-liquid transition of C12E7 in the ionic liquid under crossed polarizer.
3.Results and discussion
3.1.DSC results
DSC measurements were carried out to investigate the phase behavior of C12E7/bmimBF4mixture over a wide temperature range.Fig.1shows DSC curves obtained for the solutions of25 and70%C12E7.In both cases,a single endothermic peak appears around22◦C.The POM observation demonstrated the existence of solid fragments in a darkfield below the temperature of these endothermic peaks.This solid must be composed of C12E7,since bmimBF4does not solidify[12].Thus,the POM observation shows that a solid of C12E7and a liquid of bmimBF4coexist at the tem-perature below about22◦C,and hence,the heat effect in the present DSC curve is attributed to the melting of C12E7solid.
In addition,it was found that the melting temperature is almost independent of the composition of the mixture over the whole composition range.This indicates that the solid is pure C12E7;i.e.,
T.Inoue,T.Misono/Journal of Colloid and Interface Science326(2008)483–489
485
Fig.1.DSC curves obtained for C12E7solution in bmimBF4.Optical microscopic pho-tographs are for70%C12E7solution.
no solid solution is formed between C12E7and bmimBF4.Then,the heat absorbed during the heating process of the mixture, H,may be regarded as a sum of the heat of fusion of C12E7, H f,and the heat of mixing, H mix,which is required for the surfactant to mix with bmimBF4and form a solution.Thus, H in J/g is expressed by
H=x H f+ H mix,(1)
where x represents the weight fraction of C12E7.When we assume the C12E7/bmimBF4solution as a regular solution, H mix is given by[13]
H mix=x(1−x)w,(2)
where w is the interaction energy between C12E7and bmimBF4. Thus,we can obtain the following expression for H as a function of x.
H=−wx2+( H f+w)x.(3) According to Eq.(3), H becomes a quadratic function of x with zero intercept of H axis.The values of H obtained from DSC experiments are plotted in Fig.2as a function of x.Equation(3) wasfitted to these data points by the use of a least squares method.The solid line drawn in Fig.2is a best-fit curve thus obtained.The value of w was estimated from this curve-fitting procedure to be−17.4kJ/mol.
Negative sign of w means that the interaction of C12E7with bmimBF4in a solution is energetically favorable compared to the like-pair interactions.It may be reasonable to assume that the in-teraction is between POE chain of the surfactant and bmimBF4, because C12E7molecules are mostly in a micellar form[9]and the hydrocarbon chain must be buried in the interior of the micelle to avoid the contact with bmimBF4.It has been suggested that bmim cation has hydrogen-bond donating ability through H atoms of im-idazolium ring[14,15],and this has been supported by molecular dynamics(MD)simulation[16].Thus,the interaction energy,w, may be regarded as the energy related to the hydrogen-bond for-mation between bmim cation and oxygen atoms in POE chain.
3.2.Cloud points of C n E m in bmimBF4
Fig.1includes optical microscopic photographs taken for70% C12E7solution in bmimBF4at elevated temperature.When tem-perature of the solution is increased,small droplets appear in the solution at84◦C,which grow in size with further increase in tem-perature.This demonstrates that a phase separation takes place in C12E7/bmimBF4mixture as a result of the reduced
solvophilicity Fig.2.Plot of H against the weight fraction of C12E7in C12E7/bmimBF4solution,x.
H is expressed in J per gram of mixture.Solid curve represents the best-fit curve of quadratic function to data
points.
Fig.3.Plot of T c as a function of the surfactant concentration.Surfactants are C12E5 (squares),C12E6(circles),and C12E7(triangles).Gray curves represent the cloud points in aqueous solution for C12E5(solid line),C12E6(dashed line),and C12E7 (dotted line).
of the surfactant due to the temperature increase,and the tem-perature at which droplets start to form corresponds to the cloud point of the surfactant solution in bmimBF4.Cloud points,T c,of C12E7in bmimBF4were determined as a function of composition of the mixture by optical microscopic observation.For homologous surfactants with shorter POE chain or longer hydrocarbon chain, it was found that the clouding temperature is lowered.In these cases,cloud points were determined by visual observation as men-tioned in Section2.
The plots of T c as a function of the surfactant concentration are shown in Fig.3for C12E5,C12E6,and C12E7in bmimBF4.The cloud point of35%C12E6solution in bmimBF4has been reported to be88◦C(supporting information in Ref.[9]).Our result ob-tained for the same system at the corresponding concentration
486T.Inoue,T.Misono/Journal of Colloid and Interface Science326(2008)
483–489
Fig.4.Plot of T c as a function of the surfactant concentration.Surfactants are C14E6 (squares),C12E6(circles),and C10E6(triangles).Gray curves represent the cloud points in aqueous solution for C14E6(solid line),C12E6(dashed line),and C10E6 (dotted line).
is57◦C,which is considerably lower than the reported value in Ref.[9];the reason of this discrepancy is not clear.It can be seen in Fig.3that T c increases with the increase in POE chain length. The stronger the solvophilicity is,the higher the cloud point tem-perature becomes.Thus,the results of Fig.3clearly demonstrate that POE chain is responsible for solvophilicity of the surfactants in the ionic liquid,just like for hydrophilicity in aqueous system. On the other hand,as can be seen in Fig.4,T c decreases with the increase in hydrocarbon chain length of the surfactants.This indi-cates that hydrocarbon chain is responsible for the solvophobicity of the surfactants in the ionic liquid,just like for hydrophobicity in aqueous system.
Imidazolium based RTILs such as bmimBF4exhibit a multiple hydrogen-bonding interaction with solute molecules;i.e.,bmim cation acts as a hydrogen-bond donor through H atoms attached to imidazolium ring,while anion species like BF−
4
acts as a hydrogen-bond accepter[14,15].It has been frequently emphasized that the property as a hydrogen-bond accepter of the counter anion is more important in the solute–solvent interaction in RTIL systems[17,18]. However,in the present case,the oxygen atoms in POE chain of the surfactants must play a role as a hydrogen-bond accepter,and hence,a hydrogen-bond may be formed between imidazolium H atoms in bmim cation and O atoms in POE chain;the hydrogen-bond formation between O atom in methanol and imidazolium H atoms has been suggested by MD simulation[16].Apart from the POE oxygen,the terminal OH group in POE chain is likely to form
hydrogen-bond with BF−
4
ion.Due to these hydrogen-bond interac-tions,the solvation around POE chain of the surfactant would take place,which must be the origin of the solvophilicity of the POE chain in bmimBF4solution.In general,the strength of a hydrogen-bond interaction reduces with the increase in temperature.Thus, desolvation of the POE chain would occur at elevated tempera-ture,which brings about the reduced solubility of the surfactant molecules,and hence,induces a phase separation;this is just a similar scenario to the case of aqueous solution of POE-type non-ionic surfactants.This interpretation coincides with the POE chain length dependence of the cloud point temperature as the case of aqueous system
does.Fig.5.Turbidity change with temperature increase obtained for10and20%C12E6 in bmimBF4.
parison of aqueous and bmimBF4systems
It is of interest to compare the cloud points obtained for the bmimBF4solution with those in aqueous system.The cloud point curves obtained for the same surfactants in aqueous solutions[19] are drawn by gray lines in Figs.3and4.It is common to the two systems that with the increase in the surfactant concentra-tion,T c once decreases and then increases through a minimum temperature.However,the minimum T c appears at much lower concentration for aqueous system than for bmimBF4system,and the increase in T c after the minimum is much steeper for aque-ous system compared with bmimBF4system.The difference in the composition of minimum T c may be attributed to the difference in a molecular volume between the two solvents;water molecule is much smaller than bmimBF4ion pair.When we take into account the analogy between cloud point phenomenon and a liquid–liquid phase separation occurring in polymer solution(see Section3.5), the concentration providing minimum T c is expected to become lower for the solvent with smaller molecular volume[20].In ad-dition to the difference in the concentration of minimum T c,it is noticeable that the chain length dependence of T c is stronger in bmimBF4system compared with aqueous system.In particular,the difference in the effect of hydrocarbon chain length on T c between the two systems is quite remarkable(see Fig.4).We can compare in Fig.4the cloud point behavior in bmimBF4and that in aqueous solutions for three surfactant species with the same POE chain but different hydrocarbon chains.The three surfactants must have the same solvophilicity for bmimBF4and the same solvophilicity for water(hydrophilicity),since their POE chain lengths are the same. Then,the difference in the cloud points in each solvent would come from the difference in the solvophobicity of their hydrocar-bon chains.A much larger chain length dependence of T c obtained for bmimBF4system means that the solvophobicity of hydrocarbon chain against bmimBF4is much more sensitive to its length than that against water.This implies that the origin of the solvopho-bicity of hydrocarbon chain might be somewhat different between the two solvents,although the detail is not clear at the present time.Otherwise,a parallel trend would be expected for the cloud point behavior of the surfactants with different chain lengths in both solvents,bmimBF4and water.
3.4.Micellar growth associated with the temperature rise
It is known that the micelles of POE-type nonionic surfactants in aqueous solution grow in size with the increase in temperature until the cloud point is reached.We examined whether a similar event occurs in RTIL solution.As is shown in Fig.5,the scattered-light intensity from C12E6solution in bmimBF4increases gradually with the increase in temperature,and it increases steeply when
T.Inoue,T.Misono /Journal of Colloid and Interface Science 326(2008)483–489
487
(a)
(b)
Fig.6.Plots of viscosity for pure bmimBF 4(squares)and 20%C 12E 6solution in bmimBF 4(circles)against temperature (a)and specific viscosity of the solution as a function of temperature (b).
temperature approaches the cloud point temperature.The critical micelle concentration of C 12E 6in bmimBF 4is reported [9]to be 57mmol /dm 3which corresponds to 2.2wt%(density of bmimBF 4is 1.15g /cm 3[21]).Thus,the micelles are present in the solu-tions subjected to the turbidity experiments.The turbidity change shown in Fig.5suggests the steep increase of the micellar size near T c .In parallel with the turbidity behavior,the specific viscos-ity of C 12E 6solution in bmimBF 4also increases rapidly with the approach of temperature to T c ,as is shown in Fig.6.This supports the micellar growth induced by temperature rise.More definite evidence for the micellar growth is given by a hydrodynamic diam-eter derived from DLS experiments.Fig.7illustrates the variation of the hydrodynamic diameter with temperature rise obtained for 20wt%C 12E 6solution in bmimBF 4.The diameter obtained at 25◦C (4.6nm)is the same as the value reported in literature [9]for 9–18%C 12E 6in bmimBF 4at 25◦C.Fig.7shows that the micelles of approximately 4nm in diameter at 20◦C become much larger micelles with the diameter of about 40nm just below T c .In con-clusion,similarly to the case of aqueous system,a steep growth of micelles takes place for POE-type nonionic surfactants in bmimBF 4when temperature approaches the cloud point temperature.This micellar growth may be attributed to the reduced solvophilicity
of
Fig.7.Plot of hydrodynamic diameter of the particles in 20%C 12E 6solution in bmimBF 4as a function of temperature.
POE chain at elevated temperature.The reduced solvophilicity of POE chain would result in enhanced attractive inter-micellar inter-action,which leads to a fusion of small micelles to form bigger ones.
3.5.Analysis of cloud point curve on the basis of the Flory–Huggins model for phase separation of polymer solution
Cloud point curve is regarded as a mutual solubility curve with lower critical solution temperature (LCST).At the tempera-ture above the cloud point curve,two liquid phases,one is a dilute solution and the other is concentrated one,coexist in equilibrium,and the concentrations of each phase is given by a tie line.A simi-lar situation is seen in polymer solution when the solvent changes from a good solvent to a poor solvent depending on temperature;usually,the mutual miscibility curve in polymer solution exhibits upper critical solution temperature (UCST).Thus,the cloud point curve of nonionic surfactants has been frequently analyzed by ap-plying the Flory–Huggins model developed for the phase separa-tion of polymer solution [19,22–25].This treatment is based on the analogy between surfactant micelle and polymer.
When the two solution phases with different polymer (surfac-tant)concentration coexist in equilibrium,the chemical potential of the component in the two phases should be equal to each other according to the condition of phase equilibrium.That is,
μi =μ i ,
(4)
where i stands for solvent (1)or solute (2),and prime is used to represent the concentrated phase.According to the Flory–Huggins theory,the chemical potential of solvent in a polymer solution is expressed by Eq.(5)being derived based on the lattice model of solution [20,26].
μ1=μ01
+RT
ln (1−φ2)+
1−
1N
φ2+
ω12
RT
φ22
,
(5)
where μ01is the standard chemical potential of the solvent,φ2represents the volume fraction of polymer (or micelle)in solution,N the number of segments (or micellar aggregation number),R the gas constant,T the absolute temperature,and ω12the interaction energy parameter which corresponds to the energy difference be-tween the segment (or surfactant)–solvent pair and the sum of the segment–segment (or surfactant–surfactant)pair and solvent–solvent pair.In ω12,the coordination number,i.e.,the number of
488T.Inoue,T.Misono/Journal of Colloid and Interface Science326(2008)483–489
nearest neighbor sites of the segment(or surfactant),is also in-
cluded.
The interaction parameterω12should be regarded as free en-
ergy,taking into account the possible entropy effect associated
with the pair interaction[20,26].The free energy of interaction is
composed of an enthalpic contribution(H12)and an entropic con-
tribution(S12)and is expressed by
ω12=H12−T S12.(6)
Substituting Eq.(6)into Eq.(5),and equating the two expressions
of the chemical potential for dilute and concentrated solutions ac-
cording to Eq.(4)followed by appropriate rearrangement,one can
obtain the following expression for T c:
T c=
−H12(φ 2
2
−φ2
2
)
R[ln1−φ
2
1−φ2
+(1−1
N
)(φ
2
−φ2)]−S12(φ 2
2
−φ2
2
)
.(7)
Equation(7)demonstrates that the temperature T c at which the two solution phase with the concentrationsφ2andφ 2coexist is determined by the three parameters,N,H12,and S12.We analyzed the experimental cloud point curves in terms of Eq.(7)as men-tioned bellow.
For micellar system,N is approximately regarded as the micel-lar aggregation umber at the temperature close to T c.We roughly estimated the N value for C12E6micelles near T c using the molec-ular volume of the surfactant and the hydrodynamic diameter determined from DLS experiments under the assumption of spher-ical micelles.The molecular volume is estimated to be0.82nm3 according to the procedure described in literature[27],and the micellar volume estimated from D ≈40nm is3.3×104nm3, which results in N≈40000.This value must be overestimated, since the hydrodynamic diameter reflects the volume of particles including solvating molecules.We adopted N=10000for all sur-factant species;the value of T c calculated from Eq.(7)is practically independent of the N value for the case of such large values as104. The volume fraction,φ2andφ 2,of the surfactant in two coexist-ing phases was approximated by weight fraction,and they were read off from the curve drawn smoothly through experimental data points.The values of two adjustable parameters,H12and S12,were determined so as that the values of T c calculated from Eq.(7)fit to the experimental cloud point curve;thefitting was checked by vi-sual inspection.The cloud point curves calculated from Eq.(7)are compared with experimental ones for C12E m in Fig.8.The values of H12(in kJ/mol)and S12(in J/(K mol))used for the calculation are−11.1and−44(C12E5),−20.1and−67(C12E6),and−28.7 and−88(C12E7),respectively.The agreement of the calculated T c with experimental results is satisfactory.
H12and S12are defined as the enthalpy and entropy changes associated with the process that a surfactant micellar solution is formed from the pure surfactant micelles and the solvent;“pure surfactant micelle”is hypothetical state,but may be approximated by pure surfactant liquid.Negative sign of H12and S12means that the contact of the surfactant with solvent,bmimBF4,is en-thalpically favorable but entropically unfavorable compared with the surfactant–surfactant contact and solvent–solvent contact.This corresponds to a picture that the surfactant POE chain is solvated when the surfactant is mixed with solvent to form micellar solu-tion;the solvation results in the release of energy,i.e.,enthalpy gain,and ordered structure of solvating molecules,i.e.,entropy loss.According to this treatment,the cloud point phenomenon is interpreted as follows.Chemical potential of solvent,and also of solute,increases with the increase in temperature due to nega-tive S12term,and the increasing rate depends on the composition. Then,when a solution with a certain concentration,say,dilute so-lution,is heated,the chemical potential of the two components becomes equal to those in a concentrated solution at a
certain parison of the experimental cloud points(circles)for C12E m in bmimBF4 and those calculated according to Eq.(7)(cross symbols).The calculation was made using the following parameter values;N=10000,H12(kJ/mol)and S12(J/(K mol)) are−11.1and−44(C12E5),−20.1and−67(C12E6),and−28.7and−88(C12E7).
temperature.In other words,Eq.(4),the condition of phase equi-librium,is fulfilled at this temperature,and hence,the concen-trated solution phase starts to separate from the dilute solution phase at this temperature.Thus,the origin of the cloud point phe-nomenon is unfavorable entropic(negative S12)contribution to the surfactant–solvent interaction.The unfavorable contribution from entropy term increases more and more with the increase in tem-perature.This may correspond to the weakening of the hydrogen-bond interaction between POE chain of the surfactant and solvent associated with the temperature increase.
4.Conclusion
Present work has revealed the existence of a cloud point phe-nomenon in the solution of POE-type nonionic surfactants in bmimBF4,a typical RTIL,as well as in aqueous solution.The depen-dence of T c on the POE chain length and on the hydrocarbon chain length has proved that the POE chain/hydrocarbon chain brings about the solvophilicity/solvophobicity to the surfactants in RTIL. The appearance of a minimum in T c vs concentration plot is com-mon to both RTIL and aqueous systems.Micellar growth occurring with temperature rise up to T c is also common.A remarkable dif-ference between the two systems is the dependence of T c on the lengths of POE chain and hydrocarbon chain of the surfactants;in particular,the effect of hydrocarbon chain length on T c is much stronger for RTIL system than for aqueous system.Another notice-able difference is that no mesophase is formed in the RTIL system; it is usual for the case of aqueous system that lyotropic liquid–crystalline phases such as H1(hexagonal),V1(bicontinuous cubic), and Lα(lamellar)phases appear in high concentration region of the surfactants[28–30].Such differences in the behavior of self-aggregation of surfactants between in RTIL solution and in aqueous solution must be attributed to the difference in the interaction mode of the solvent with hydrocarbon chain and/or POE chain of the surfactants between the two systems.The present work would。