Langmuir 1993,9, 1727-1730
1727
Thermodynamics of Micellization of Aerosol OT in Polar and Nonpolar Solvents. A Calorimetric Study Kallol Mukherjee and S. P. Moulik* Department of Chemistry, Jadavpur University, Calcutta 700032, India
D. C. Mukherjee Department of Pure Chemistry, Calcutta University, Calcutta 7oooO9, India Received January 14,1993 The critical micelle concentration (cmc) and the thermodynamics of micellization of sodium bis(2ethylhexyl) sulfosuccinate (AOT) in four polar solventa (water, ethylene glycol, propylene glycol, and formamide)and seven nonpolar solventa (hexane,heptane, octane,isooctane,decane,dioxane,andbenzene) have been determined by spectrophotometry and calorimetry. Normal and reverse micelle formation processes have taken place in the two categories of solventa. The cmc values in all the solventahave been found to be on the order of m mol dm4 with a systematic trend in alkanes. The polar solvent media aa well as dioxane and benzene show positive enthalpies of micellization. The alkanes show a negative enthalpy of micellization,decane being an exception to this. The entropies of micellization of these two categories of solventa are also of opposite signs. A nice correlation between the enthalpy and entropy of micellization encompassing all the solvents has been observed. Introduction In polar solventmedia, surfactants form normal micelles with their nonpolar tails inside the core and the polar (or ionic) head groups exposed out toward the solvent. In nonpolar solvents, even with a trace of water, a reverse orientation of the head and tail parts takes place with the formation of reverse micelles which may grow in size by absorbing water. Nonaqueous media can be both polar and nonpolar to yield both normal and reverse micelles in them.14 Studies on the reverse micelles of surfactants in nonaqueousmedia are nonetheless important for they can influence redox processes,photoinduced electron transfer processes, enzyme activities, etc. Aerosol OT (sodium bia(2-ethylhexyl) sulfosuccinate, AOT) is a versatile ionic surfactant which is widely used in the chemical and biophysical works.w It is soluble in polar and nonpolar solventa. It is a well-studiedsurfactant and can form microemulsions with various kinds of oil alone and in conjugationwith acosurfactant.&16Although substantial reporta exist on the formation of normal and (1) Ruckenatein, E.; Nagarajan, R. J. Phys. Chem. 1975, 79, 26222626. (2) Micellization, Solubilization and Microemulsion; Mittal, Ed.; Plenum: New York, 1977; p 133. (3) Wong, M.; Tho-, J. K.; Nowak, T.J. Am. Chem. SOC.1976,98, 2391-2397. (4) Eicke, H. F. Top. Curr. Chem. 1980,87,86-145. (5) Eastoe, J.; Robineon, B. H.; Vieeer, A. J. W. G.;Steytler,D. C. J. Chem. SOC.,Faraday. Trona. 1991,87,1899-1903. (6) Candu, F.; Leong, Y. S.;Pouyet, G.;Candu, S.J. Colloid Interface Sci. 1984,101,167-183. (7) Surfactants by Cyanamid; American Cyanamid Co.: Wayne, NJ, Jan 1983. (8) Tamamushi, B.; Watanabe, N. Colloid Polym. Sci. 1980,258,174178. (9) Huang, J. S.;Kim, M. W. SOC.Pet. Eng. J. 1984,24, 197. (10) Kotlarchyk, M.; Chem, 5.H.; Huang, J. 5.Phys. Rev. A 1983,28, 508-511; 1984,29,2054-2069. (11) Huang, J. S.;Safran, S. A.; Kim, M. W.; Greet, G.S.;Kotlarchyk, M.; Quink,N.Phys. Rev. Lett. 1984,53,592-695.
(12)Borkovec, M.; Eicke, H. F.; Hammerich, H.; Das Gupta, B. J. Phys. Chem. 1988,92,206-211. (13) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989,93, 10-12. (14) Bisal, S. R.; Bhattacharya, P. K.; Moulik, S. P. J. Phys. Chem. 1 o. 350-355. - -9-9 -, .w . ,. -- - -- -. (15) Mukhopadhyay, L.;Bhattacharya, P. K.; Moulik, S.P. Colloids Surf. 1990,50, 295-308.
reverse micellesof AOT in polar and nonpolar so1vents,17-26 references on the thermodynamics of micelle formation (particularlyreverse micelle formation)of AOT in different solvents are rare. In general reporta on the energetics of reverse micelle formation of surfactants are indeed limited. Exploration of this area is, therefore, of significant importance. In relation to our work on microemulsions wing AOT,lC16we have been interested in the thermodynamics of micelle formation of AOT in both polar and nonpolar media. In thispresentation, determinations of the critical micelle concentration(cmc)of AOT by spectrophotometry and the cmc and thermodynamics of micelle formation of AOT by calorimetry in 10 solvent media (water (Wa), formamide (Fa), ethylene glycol (Eg) , propylene glycol (Pg), hexane (Hx), heptane (Hp), octane (Oc), isooctane (i-Oc), benzene (Bz),and dioxane (Dx))have been documented. Calorimetricinvestigations on micellar syeteme are only limited. A detailedthermodynamicstudy on AOT in a number of polar and nonpolar solvents adopting the calorimetric method has not been done in the past. Experimental Section Material13and Methods. Aerosol OT (AOT)of 99%.purity wae obtained from Sigma. Ita purity wae checked followmg the procedure recommended by Magid et dBThe water content of the sample as determined by the Karl Fiecher titration method (16) Pal, B. K.; Moulik, S.P.; Mukherjee, D.C. Indian J. Chem. 1991, 28,174-183. (17) Williame,E.F.;Woodbury,N. Y.;Dixon, J. K.J.Colloid Interface Sci. 1967, 12, 452-459. (18) Kitahara, A.; Kobayaehi, T.; Tachibana, T.J. Phys. Chem. 1962, 66,363-365. (19) FontelI, K. J. Colloid Interface Sci. 1973,44, 318-329. (20) Mukherjee, P.; Mysele, K. Critical Micelle Concentrationu of Aqueous Surfactant System; NBS: Washington, DC 1971; NSRDSNBS 36. (21) Pal, M. K.; Pal, P. K. J. Phys. Chem. 1990,94,2667-2659. (22) Stilbs, P.; Lmdman, B. J. Colloid Interface Sci. 1984, 99, 2293. (23) Kotlarchyk, M.; H u m , J. S.; Chen, S. H. J. Phys. Chem.1986, 89,4382-4386. (24) Kim, V.; Pak, 9. E.; Kharlamove, 1. M.; Frolov, Y. G.Kolloidn. Zh. 1986,48,830-831.
(25)Vos, K.; Lanne, C.; Vieser, A. J. W. G.Photochem. Photobiol.
1987,45,863-878.
0743-7463/93/2409-1727$04.00/00 1993 American Chemical Society
Mukherjee et al.
1728 Langmuir, Vol. 9, No. 7, 1993 Table I. Critical Micelle Concentration of AOT in Different Solvent Media at 303 K and the Specific Heats (C# of the Media Cp/(Jmol-' deg-1) cmc x 109/(mol dm-9) solvent spectrophotometry calorimetry present study lit.f 2.23 77.1 75.16h Wan 107.94 107.6' 1.80 Fa 155.9 149.7* 1.50 Eg 0.90 172.8 176.4e 5.60 pl3 3.00 147 149.lj 1.80 Dx 1.60 129.4 135.6' 0.86 BZb 1969 206.2 1.09 HI 224.7k 229.3 1.10 1.00 HP 1.00 254' 263.3 oc 236.4' 234.4 0.84 i-Od 1.1 314.e 320.9 0.73 DCd cmc 6.0 X 10-9mol dm- ,conductance) and 2.2 X 1V mol dm(surfacetension), present study. * cmc = 0.9 X 109 mol dm-9 by dye absorption." e cmc = (0.649)X 109 mol dm4 by positron annihilation."dcmc = 0.73 X 10-9 mol dm4 by X-ray scattering.29 a Reference 30.f298 K. The standard deviation in Cp is *4.6%. &borne; Stimson; Ginniinge. E. S. J. Res. 1939,23,238. Skold, R.; Suurkwsk, J.; Wadso, I. J. Chem. Thermodyn. 1976,8,1076 1080.jBarta, L.;Kooner, Z.S.; Hepler, L. G.; Desgranges, G. R.; Grolier, J. P. F. Can. J. Chem. 1989,67(3), 12251229. Handbook of Chemistry and Physics, 55th ed.; CRC P r m , Inc.: Cleveland, OH, 1974-75; p D-141. ~
~~
-
f
A (nm)
Figure 1. UV spectra of AOT in different organic solvente at 298K: Eg, nos. I-VII correspond to 0.77,1.65,2.28,3.26,4.2,7.0, and10.0mmoldm-9;Hp,nos.I-Vcorrespondto0.5,1.0,2.0,6.0, and 10.0 m mol dm4; Dx, nos. I-VI1 correspond to 1.2,2.4,2.8, 3.0, 4.0,4.6,and 5.0 m mol dm4. 0.5
was 3% (g/g). The sample had a [waterJ/[AOTl mole ratio of 0.76. A ratio of 0.7 has been reported earlier." The nonaqueous solvents were either Analar BDH or GR E. Merck. Their purity was tested by the determination of the density, refractive index, boiling point, and specific heat. The results were in good agreement with the literature values. Care was taken for dioxane, ethylene glycol, and propylene glycol to avoid unnecessary exposure to air. Spectral measurements were taken in a Shimadzu UVdlOA double-beam spectrophotometer using 1-cm silica cuvettes with pure solvents as the controls at 303 f 0.1 K. A TRONAC 458 isoperibol calorimeter was used for the thermodynamic study. The procedure of measurements was the A concentrated solution of AOT same as described in a particular solvent was taken in the buret and was slowly added to the pure solvent in the titrating vessel, both immersed in aconstant-temperature water bath maintained at 303 f O.OOO1 K. The aseociated heat was recorded in terms of voltage. The data were processed in the prescribed w a P * to evaluate the cmc and the enthalpy. The heat capacities of the solvents were determinedn separately.
1
/
P P I
0.0
C x lO'//mol dw?
C x IO)/mol dm'
Figure 2. Optical density (OD) w concentration profies of AOT in different solvents at 230 nm and 298 K. The break points in the plots correspond to the cmc.
(Hxbeing an exception). The resulta, therefore, provide evidence in favor of good purity of the solvents. Critical Micelle Concentration of AOT. A convenient measured physical property of a surfactant in a medium can provide evidence in favor of the cmc. Both spectrophotometry and calorimetry provide evidence of micelle formation of AOT in various solvents. Figure 1 depicts representative W spectra of AOT in a couple of solvents. The absorptionmaxima intensifywith increased concentration of the solute with a gradual red shift. The absorption versus concentration profile (close to the absorption maximum in each case) is h e a r and demonstrates a break (Figure 2) that corresponds to the cmc of the surfactant. Spectral determination of the cmc of AOT in severalnonaqueous solventshas been made in the pa~t.8~ The earlier determinedm cmc's in Bz,Dc, and i-Oc agree (26)Luisi, P.L.;Magid, L.J. CRC Crit. Rev. Biochem. 1986,20(4), fairly with the present measurements. The determination 409.474. (27)Dae,M.L.;Bhattacharya,P.K.;Moulik,S.P.;Dae,A.R.Longmuir of the cmc of AOT in other solvent media (Fa, Eg, Pg, Dx, Hx, Hp, and Oc, employed in this study) are not found in 1992,8,2135-2139. (28)Jana, P.K.;Moulik, S . P.J. Phys. Chem. 1991,95,9626-9532. the literature. The reaults presented in Table I ale0include (29) Kreaheck, G. C.; Hargraves, W. A. J. Colloid Interface Sci. 1974, the cmc's estimated in an aqueousmedium by the methods 48,481-493. Results and Discussion Specific Heats of the Solvents. The measured molar specific heats at 303 K of the solvents are presented in column 4 of Table I. The literature values at 298 K are given in column 5. The differences may be attributed in part to the 5 K difference in the temperatures of the two sources of measurements. The C, value of Pg is rare in the literature. We found the C, valuem for propane-1,b diol to be 176.4 J mol-1 K-I (identified in column 5 of Table I). The measured value 172.8 J mol-' K-' for Pg closely agrees with ita isomer at 298 K. Apart from the low-temperature effect, the uncertainty in the measurementa (footnote g in Table I) may fairly account for the discrepancybetween the present and the literature reports
(30)Nichols, N.;Skold,R.; Spink,C.; Wadso, I. J. Chem. Thermodyn. 1976,8,993-999.
(31)Muto, 5.;Meguro, K . Bull. Chem. SOC.Jpn. 1973,46,1316-1320.
Micellization of Aerosol OT
Langmuir, Vol. 9, No. 7, 1998 1729 Table 11. Thermodynamic Parameters. for the Mioellization of AOT in Variour Solventr at 808 K solvent -AGmo/(kJ mol-') AHmol(kJ mol-') ASmo/(J mol-' K-1) Wa 15.6 2.97 61.0 16.1 0.18 53.6 El3 13.4 1.15 48.1 pl3 16.3 Fa 0.21 64.5 16.8 Dx 0.65 63.6 BZ 17.9 20.90 128.3 HI 17.3 -84.80 -222.0 17.5 -76.90 -194.8 HP 17.4 oc -28.70 -37.6 17.6 i-Oc -47.7 -99.1 Dc 18.3 9.7 92.4 a Standard deviation in AGmO, AHm", and ASmo are *2%, +5%, and *7%.
i-Oc
10
In the present context, both normal and inverted micelles form in the studied media. On the basis of the calorimetricallydetermined cmc values, the standard free energyof micellition (AGmO) is calculated by the relation AGm0 = RT In cmc
I
2
t
h T l m e6
I
8
,
iocm
Figure 3. Representative thermograms of AOT addition in different solvents at 303 K. S and E indicate the s t a r t and end of a run. Scale divisions are in centimeters: ordinate, 2.55 cm = 1 mV (for Oc and Dc, 25.6 cm = 1 mV);abscissa, 1 cm = 1 min.
of conductance and surface tension; the former is higher than the latter. Thermograms presented in Figure 3 also support association of the surfactant molecules forming micelles. The breaks in the thermal eventsare distinct. Calorimetric determination of the cmc has been infrequent.28*29If adopted, it can provide information about the cmc as well as the enthalpy of micelle formation from a single run.28@ Calorimetric determination of the cmc of AOT has not been made in the past. The cmc's by calorimetry agree with those determined by spectrophotometry. A systematic but mild variation (decrease) of the cmc of AOT with the carbon numbers of the alkanes has been observed. The noncompatibility of the polar head groups with the nonpolar medium responsible for the association of the surfactants increases with the chain length, resulting in easier micelle formation. The cmc's of AOT in the polar media for normal micelle formation have been of the same order of magnitude as those for the reverse micelles in nonpolar media. A systematic trend of the cmc with respect to the polarity of the medium has not been observed. Energetics of Micellization. The calorimetrically determined standard enthalpies of micellization, A?€mo, of AOT obtained in the studied solvent media are presented in Table 11. The alkanes (except Dc) yield negative enthalpies whereas the other media yield positive enthalpies. On the whole the amphiphilic molecules assemble with release of heat. In polar nonaqueous media (Fa, Eg, Pg), micelle formation requires weakening of the dipolar interactions and hydrogen bonding among the solvent molecules with absorption of heat followed by its release as a result of the association of the AOT molecules among themselves. The overall balance is endothermic. The endothermicity in Bz and Dx does not really fit into the above rationale. Molecules of these compounds can only undergo weak dispersion interactions among themselves. The endothermic heat of micellization of AOT in these solvents is, therefore, less understood.
(1) In nonpolar solvents ionic dissociation does not arise so that RT In cmc is the appropriate measure of AGm0. In the polar media (Wa, Fa, Eg, Pg), counterion dieeociation only to is possible and AGmO should not be RT In cmc. From the conductance study of AOT in an aqueous medium, the counterion dissociation of the AOT micelle has been estimatedBa2to be 4 0 % . Although Eg and Pg have lower polarities and Fa has a higher polarity than water, the same extent of counterion dissociation has been assumed in them. The 10% ion association increases= the AGmO by 10% which is considered insignificant. For a comparisonof the effectivityof thesolvents, therefore, the free energy of micellization has been estimated from RT In cmc. The use of AGmO and A H m o in the Gibbs-Helmholtz equation yields the entropy of micellization ASm". Normally, micellization in an aqueous medium is associated with a positive entropy change which is mainly due to the melting of the niceberg"or "flickering cluster" that arises out of the hydrophobic effect of the amphiphilic parta of the surfactant molecules.ga On the basis of the proposition that the free energy of micellization is equal to RT In cmc, the calorimetric method can thus provide a completeanalysisof the thermodynamicsof micellization. Except Bz and Dc, the positive entropy values in all the other media (Wa, Eg, Pg, Fa, and Dx) are more or less equal. The negative entropies in Hx, Hp, Oc, and i-Oc have a systematictrend with the hydrocarbonchain length. The manifestation of negative enthalpy with negative entropy and positive enthalpy with positive entropy ie a striking feature of the alkanes and other solvents. The polar solvents Wa, Et, Pg, and Fa all produce positive enthalpy and entropy of micellization which is excepted. The endothermic melting of the ordered polar solvent molecules around the nonpolar tail of AOT is greater than the subsequent exothermic association of the molecules to form micelles. The resulting disorderedstate is correctly reflected as the positive entropy. For the nonpolar media, we introduce a concept on the state of residence of AOT molecules in them. Analogous to the formation of an iceberg around the nonpolar tails of surfactant molecules in an aqueousmedium,an assemblyof nonpolar molecules in the vicinity of -SOa-Na+ head groups of AOT in an (32) Evans, H. C . J. Chem. SOC.1956,679686. (33) Tanford, C. The HydrophobicEffect. Formation Of Micelle8 and Biological Membranes; Wiley New York, 1980.
Mukherjee et al.
1730 Langmuir, Vol. 9, No. 7, 1993 alkane medium is envisaged. The moisture content of AOT imp& a mole per mole hydration of 0.7. The water molecules are, therefore, rigidly held with the AOT head group which can fixM six molecules of water by the 4 0 3 group and another five molecules by the Na+ ion. The water of hydration, therefore, forms the core of the reverse micelles. Its physicalstate is hardly affectedduringmicelle formation. The micellization in alkanes and other nonpolar solvents may occur through (1) endothermic deorganization of the associated solvent molecules in the vicinity of the polar head groups of the surfactant monomersand (2) exothermicassociation of themonomers, forming micelles. The second process is considered to have more or less the same thermodynamic magnitude in all the alkane environments while the first process contributes variable enthalpies depending on the alkane chain length. Higher alkanes conveniently assemble around the surfactant head groups (flickering cluster analogue of nonpolar solvent molecules) so that the AH,' increases with the chain length (due to greater absorption of heat for deaggregation). The AHm' values in Bz and Dx are positive. The clustering of these solvent molecules around the AOT head group appears to be more ordered and to contribute a higher endothermic heat for process 1, resulting in an overall positive AH,'. In terms of the two processes mentioned above, an analysis of the measured AHm' can be made in the following way. Let dH1' be the enthalpy per CH2 group of the alkanes for endothermic process 1 and A H 2 ' be the exothermic enthalpy for aggregation process 2. The AH,' values presented in Table II for the alkanes increasewith C,, and AH,' is positive for Dc. It is a linear function of C, and is zero at C, = 9.5 (plot not shown), which suggests that the enthalpic contribution of the f i t of the two processes is constant and the other is variable. Since AHm' heads toward positive values with C,, the exothermic heat of process 2 (AH2") is taken to be constant considering the same aggregation numbers for the micelles in the alkanes. The overall enthalpy of micellization AH,' of AOT can then be related as AHH,' = c, dHl'
+ AH2'
(2)
where C, is the number of CH2 groups in the alkane. As mentioned above,at C, = 9.5,C, d H l o exactlycompensates AI&'. For an increase in C, by 0.5 unit (as in Dc), the enthalpy increase is 9.7 kJ mol-l, i.e.,
-240
-180
-120
180
-60 -50 T,,p
.303
n
HI
Figure 4. Enthalpy and entropy compensation plot for micellization of AOT in different solvents at 303 K. Table 111. C. dHlo and AHzo An Correlated in Equation 2 for AOT Micellization in Alkanes at 303 K Cnd.H1O/ -AHaol Cnd.Hlo1 -AHa"I mlvent (kJ mol-') (kJ mol-') solvent (kJ mol-') (kJ mol-') Hx 116 201 i-Oc 156 202 HP 136 213 Dc 194 194 oc 155 184 av: 199
AH,' helps to evaluate AH2' for the alkanes from eq 2. The results are presented in Table 111. The average A H z 0 for all the alkanes is 199 kJ mol-I. The entropiesof micellization A S,' are positivefor polar solvents and negative for nonpolar solvents. Dc is an exception where the entropy change is positive similar to the polar solvents. Among the polar solvents both A?€mo and ' & A follow the order Eg < Fa < Pg < Wa. The alkanes show a systematic trend of increased AHm' and AS," with their chain length. The reverae trend of ' & A for the process of reverse micelle formation in the alkanes with C, < 10 is noteworthy. Dx, Bz, and i-Oc have individual characteristics in this respect. The processes of both normal and reverse micellization in the polar and nonpolar media have been found to be nicely controlled by the combined effects of enthalpy and entropy. Since AGm' values in all the solvent media fall in anarrow range (Table111,a compensation between AHm" and ASm' is expected. An enthalpy-entropy compensation plot is presented in Figure 4. The compensation temperature is 314.3 K compared to the experimental temperature 303K. Both the normal and reverae micellization processes are covered by the same scheme. An enthalpyentropy compensationeffect has also been found in normal micellar systems in the past.3s So far this has not been demonstrated on reverse miceller systems.
0.5 dHlo = 9.7 kJ mol-' This means that 19.4 kJ mol-' is the endothermic contribution of a single CH2 group in the seriesfor process 1. On the basis of dH1" = 19.4 kJ mol-', the measured
Acknowledgment. Financial support fromthe Council of Scientificand Industrial Research,Governmentof India, to K.M.in the form of a Junior Research Fellow to carry out the work is thankfully acknowledged.
(34)Wong, M.;Thomas,J. K.; Nowak, J. J. Am.Chem. SOC.1977,99, 4730-4736.
(36)Koichiro, A.;Takayuk, N.; Kunio, F.; Mamovu, M.; Koichi, H. Yukagaku 1984,33,20.
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