J. Phys. Chem. B 1999, 103, 5977-5983
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Microcalorimetric Investigation of AOT Self-Association in Oil and the State of Pool Water in Water/Oil Microemulsions Pinaki R. Majhi and Satya P. Moulik* Department of Chemistry, Centre for Surface Science, JadaVpur UniVersity, Calcutta 700 032, India ReceiVed: September 24, 1998; In Final Form: April 25, 1999
The reverse micelle formation of AOT in n-heptane, n-octane, isooctane, n-decane, and eucalyptol has been microcalorimetrically studied. From the thermograms, the critical micellar concentration (cmc) and the energetic parameters of standard enthalpy and free energy and entropy of micellization (∆Hm, ∆Gm, and ∆Sm respectively) have been estimated. From the microcalorimetric titration of AOT/oil mixtures with water up to the phase separation point, the enthalpy of water dispersion (∆Hd) in the amphiphile/oil medium forming a microemulsion has been also determined. The water dissolution process has shown endothermic enthalpy changes in three distinct stages with an initial enthalpy maximum (except for the AOT/eucalyptol system). ∆Hd has been estimated from the solubility limit, and the entropy of dispersion (∆Sd) has been consequently obtained from the Gibbs equation. The concentration of the dispersed microdroplets required in the above calculations has been estimated by measuring the droplet size by the DLS method and the known composition of the solution.
Introduction Compared to normal micelles, thermodynamic studies on reverse micelles are limited. For a comprehensive understanding of the behavior of micelle formation, such studies are essential. Aerosol-OT (sodium bis(2-ethylhexyl)sulfosuccinate, AOT) is a versatile anionic surfactant very frequently used in the formation of reverse micelles and microemulsions.1 It hardly needs a cosurfactant for the formation of the later. We have liberally used AOT for physicochemical investigations of micelles and microemulsions.2-13 The dissolution of water in surfactant/oil medium leading to microemulsion formation is an important aspect of the study of microheterogeneous systems. The dissolved water yielding nanodispersion of water droplets in an oil medium may undergo changes in its physical state.14-18 Association of the dispersed droplets in the oil medium may also occur.19-22 There are reports on the thermodynamics of formation of reverse micelles and water dissolution in nonpolar media (oil), leading to microemulsion formation.7,9,13,23-28 The microcalorimetric measurements in this respect are scanty. Very recently, the thermodynamics of normal micelle formation has been accurately assessed by using the OMEGA ITC titration microcalorimeter;29-32 its use in the formation of reverse micelles and microemulsions remains (to our knowledge) unattended. The present investigation aims to explore this area using AOT and several hydrocarbons (n-heptane, n-octane, n-decane, isooctane) and a water-immiscible compound eucalyptol, which is the major component of eucalyptus oil. We have very recently reported the micellization behavior of AOT in whole eucalyptus oil.12 The dispersed microwater pools in microemulsions and the physical state of water therein are considered comparable with the state of water present in the pockets of cell membranes.33 It is also important from the standpoint of catalytic activities and reactivities of enzymes in nonaqueous solution environments.34,35 The thermodynamic * Fax: 91-33-473-4266. Phone: 91-33-483-8411. E-Mail: dcsoesju@ giascl01.vsnl.net.in.
study of water dissolution in the AOT/oil mixture herein undertaken is expected to offer information in this respect. Experimental Section Materials. The surfactant AOT was a 99% pure Sigma product. By the Karl-Fisher method, the water content of the material was found to be 3% w/w. The hydrocarbons (n-heptane, n-octane, isooctane, and n-decane) were A. R. grade products of S.D. Fine Chemicals. Their purity was checked by measuring the boiling points and the refractive indices. Further purification was not necessary. The compound eucalyptol (or cineol) of chemical name 1,3,3-trimethyl-2-oxabicyclo[2.2.2]octane was a product of 99% purity obtained from Fluka. It was also used as received. The water used for microemulsion formation in the dissolution measurements was doubly distilled. All measurements were taken at 30 ( 0.001 °C. Calorimetric Measurements. An OMEGA ITC microcalorimeter from Microcal, Northampton, MA, was used for the thermometric measurements. It can record heat flow as low as 10 µcal/s. The minimum heat flow recorded in a single injection was 50 µcal/s, which was well above the sensitivity limit of the instrument. For reverse micelle formation, 1.325 mL of oil was taken in the calorimeter cell into which temperatureequilibrated AOT solution in oil was injected successively for a predetermined number of installments programmed in the instrument as reported in the recent literature.29-32 The heat flow per injection and the corresponding enthalpy change per mole of injectant were obtained using the ITC software. For the water dissolution process, 1.325 mL of oil with a fixed concentration of AOT (ca. 0.20 mol dm-3) was taken in the cell, and water was added successively in aliquots as mentioned above. The heat flow per injection and the enthalpy change per mole of injectant were obtained as stated above. The enthalpy change per mole of water addition in AOT/oil medium just up to the predetermined point of phase separation was also obtained from a single-step operation procedure. DLS Measurements. The particle size of the microwater droplets of a microemulsion was determined in a dynamic light
10.1021/jp9838590 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999
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Figure 1. Microcalorimetric mesurements on AOT self-association in octane at 303 K. Top: Heat flow with time-injecting aliquots of 29.894 mM AOT in 1.325 mL of octane. Middle: Enthalpy change per mole of injectant vs [AOT] corresponding to the aliquots represented above. Bottom: Differential plot of enthalpy vs [AOT]. The cmc and ∆Hm values are indicated in the diagrams.
scattering instrument from Otsuka Electronics. The light source used was a 632-nm neon laser, which acted on microemulsion samples (filtered a couple of times using a 0.45-µm Millipore microfilter and taken in a cylindrical cell). Measurements were taken at 90°, and the intensity data were processed in a computer. Essentially, the instrument measured the diffusion coefficient (D) of the dispersed droplets and evaluated the hydrodynamic diameter (dh) in terms of the Stokes-Einstein equation,
dh )
6πηkT D
(1)
where η, k, and T are the viscosity coefficient of the medium, the Boltzman constant, and the absolute temperature, respectively. Results and Discussion Formation of Reverse Micelles. In Figures 1-3, the heat flow, the enthalpy change per mole of injectant, and the differential plot of the enthalpy change are depicted. The locations of the critical micellar concentration (cmc) are
Figure 2. Microcalorimetric mesurements on AOT self-association in isooctane at 303 K ([AOT] ) 23.76 mM). The explanation for the three sections is as in Figure 1.
indicated on the enthalpy and differential plots following the recently reported microcalorimetric procedure adopted on normal micelle formation.31,32 Since the injectant concentration is greater than the cmc, the initial heat flow is due to micellar dilution followed by demicellization in water present in the cell. When the concentration of AOT in the medium reaches the cmc, the demicellization process is absent and the heat change corresponds to the micellar dilution. The course of the enthalpy change takes a sharp turn and practically comes to a halt (mild variations are found because of the decreasing dilution of the micellar solution (injectant) transferred to the receiving solution in the cell). The difference between the final and initial turns (represented by the vertical arrow in the middle section in each figure) is then the enthalpy of micellization (∆Hm). The evaluation is sharp and accurate. Both the cmc and ∆Hm are obtained from the same run, which makes the method convenient. The cmc and ∆Hm values are found to decrease with increasing carbon number in the hydrocarbon chain; isooctane is out of this trend. Similar was the macrocalorimetric observation reported earlier.3 But the present values are smaller than the earlier report, except for isooctane, where the two findings have good agreement. We consider the present results to be more accurate because of the use of our sophisticated technique. Taking the counterions (Na+ ions) to be all bound to the AOT anions in the tiny water compartment of the reverse micelles (this is acceptable since 90% counterion binding has been reported for AOT normal micelles),3 the free energy of
Microcalorimetry of AOT Self-Association
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TABLE 1: CMC and Energetic Parameters for the Reverse Micelle Formation of AOT in Different Oils at 303 K solvent
106cmc/mol fract
∆Hm/kJ mol-1
-∆Gm/kJ mol-1
∆S/J mol-1 K-1
heptane octane isooctane decane eucalyptol
9.72 5.58 15.30 4.01 10.08
-8.60 15.44 10.32 18.35 -0.638
58.2 59.67 55.9 62.6 57.97
164 248 218 220 189
Figure 3. Microcalorimetric mesurements on AOT self-association in eucalyptol at 303 K ([AOT] ) 33.09 mM). The explanation for the three sections is as in Figure 1.
micellization has been calculated by
∆Gm ) 2RT ln cmc
(2)
expressing the cmc in mole fractional units and ∆Sm then follows from the Gibbs equation,
∆Sm ) (∆Hm - ∆Gm)/T
(3)
The calculated ∆Gm and ∆Sm values are also given in Table 1. The signs of ∆Gm and ∆Sm are negative and positive, respectively. The positive ∆Sm accounts for the ultimate disorder during the self-assembling process of AOT molecules in the oil medium. The removal of oil molecules to create room for the residence of the polar surfactant headgroups in the form of micelles is considered to be associated with a positive entropy change. The observation of T∆S > ∆Hm suggests that the reverse micelle formation of AOT is an entropy-directed process. In terms of ∆Sm, the normal and reverse micellization processes behave in an identical manner. The entropy-directed micellization process is, therefore, universal.
Figure 4. Enthalpy change by injecting aliquots of water in 1.325 mL of 205.495 mM AOT solution in heptane at 303 K. A: Bottom: Enthalpy change per mole of water per addition (∆Hp) vs ω. The broken vertical line indicates the limit of water solubility. The changes in the organization of water in the micropool are indicated on the figure by arrowheads. Top: Enthalpy change per mole of water on a cumulative basis (∆Hc) vs ω.
Enthalpy of Water Dispersion in AOT/Oil Medium. There are only limited thermodynamic studies of water dispersion in amphiphile/oil media yielding microemulsions.27,28 The microcalorimetry with stepwise addition of water as has been done in this study is rare. The enthalpy changes per mole of water (injectant) addition in the multiple steps in the binary mixtures of AOT with n-heptane, n-octane, isooctane, n-decane, and eucalyptol are presented in Figures 4-8. In general, an initial rise in ∆Hm has been observed for all the oils except for eucalyptol. The maximum rise corresponds on average to [water]/[AOT] (ω) ) 1.0; individually, the ω values are 0.61, 1.2, 0.81, and 0.6 for n-heptane, n-octane, isooctane, and n-decane, respectively. The water dissolution process for all the cases is endothermic. In Figures 4-8, the bottom section constitutes the enthalpy change per mole of injectant (water) per addition (∆Hp) plotted against the [water]/[AOT] mole ratio (ω) that helps to reveal the intricacies of the water dispersion process. In the top sections of the figures, the enthalpy change per mole of injectant (water) calculated on the cumulative basis (∆Hc) is plotted against ω, which is equivalent to reports on
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Figure 5. Enthalpy change by injecting aliquots of water in 1.325 mL of 212.223 mM AOT solution in octane at 303 K. The explanations for bottom and top are the same as in Figure 4.
Figure 6. Enthalpy change by injecting aliquots of water in 1.325 mL of 208.475 mM AOT solution in isooctane at 303 K. The explanations for bottom and top are the same as in Figure 4.
AOT/isooctane and a few other systems by batch microcalorimetric measurements.16-18 It is less revealing than the plots presented in the bottom section, where the maximum shifts toward higher ω. The reported maximum at ω ) 2 for the AOT/ isooctane system by Goto et al.16-18 is in fair agreement with our result of 1.82. The results in the bottom section reveal that on the whole, the enthalpy variations occur in stages. The first stage constitutes a sharp increase in ∆Hd within a low value of ω, the second stage shows a sharp decline in ∆Hd up to ω ) 3-5, and there is a third stage of shallow decline up to ω ) 35-43. Thereafter, the enthalpy change is virtually zero. It is thus imperative to conclude a possible existence of more than one state of water in the micropool depending on ω. The enthalpy changes for all three stages are shown on the graphs and are presented in Table 2. In this context, the AOT/eucalyptol system is different (Figure 8). A maximum in ∆Hd is not observed. But there are also three distinct stages in the ∆Hd of water dissolution; they correspond to ω values of 3.17, 9.5, and 18, respectively. The maximum water intake forming a singlephase solution is at ω ) 18, indicated by the broken vertical line in the graph. Similar markings are also shown in other graphs of the bottom section a in Figures 4-7. Different states of water have been reported in AOT containing water/oil (w/o) microemulsions.16,36,37 Six water molecules per SO3- group of AOT (up to ω ) 6) are considered to be rigidly held; thereafter, the rigidity declines up to ω ) 15, and afterward bulk solvent property prevails. Goto et al.16,17 have supported the presence of three states of water in the micropool. Aliotta et al.38 and Correa et al.39 are in favor of two states. This has been criticized by Gu et al.26 Distinct evidence in
support of more then one aggregation state of water was not obtained from the macrocalorimetric measurements by Ray et al.7 The initial rapid change in the endothermic ∆Hd observed in Figures 4-7 has suggested a continuously changing interaction of water with the ionic headgroup of AOT and Na+ ions. This also includes the change in the aggregation state of the AOT and the associated water molecules. The penetration of water molecules crossing the interfacial water/oil barrier has contributed endothermicity to the process by counterbalancing the exothermicity coming from ionic interaction and amphiphile organization. The onward water solubilization has produced solvated water of decreasing rigidity with increasing ω. The associated enthalpy change is also in decreasing magnitude; the interfacial barrier has been lowered by AOT adsorption. The overall dissolution process on the whole remains endothermic, and the enthalpy difference in going from the beginning to the end of a stage (final heat - initial heat) is positive for the first stage and negative for the two successive stages (indicated on the graphs and given in Table 2 as well). Prior to acquiring bulk property, more than one state of water in the water/AOT/ n-heptane microemulsion has been recently reported from solvation dynamic studies.40,41 In the process, energetics, droplet clustering, interfacial amphiphile rearrangement, etc., also contribute their share, particularly at ω > 6. From batch microcalorimetry, three states of water have been also reported for AOT-formulated w/o microemulsions (reverse micelles) using oils, viz., isooctane, chloroform, benzene, and cyclohexane.16-18 The occurrence of bulk water has been reported therein to be at ω > 15, which we have found to extend much beyond, i.e., up to ω = 43. If we compare the results shown in
Microcalorimetry of AOT Self-Association
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Figure 7. Enthalpy change by injecting aliquots of water in 1.325 mL of 209.353 mM AOT solution in decane at 303 K. The explanations for bottom and top are the same as in Figure 4.
Figure 8. Enthalpy change by injecting aliquots of water in 1.325 mL of 213.798 mM AOT solution in eucalyptol at 303 K. The explanations for bottom and top are the same as in Figure 4.
the top section of Figures 4-8 with that of the bottom section, the sensitivity of the enthalpy variation is found to be much reduced in the former. This is the reason for the difference between the observations of Goto et al.16-18 and ours, particularly for ω > 6. For a better revealing of the states of the formed systems, their compositions in moles are presented in Table 3. The extent of bound and free water has been calculated taking the hydration number of AOT (i.e., per SO3- ion) as six. It is seen from the table that except for the system water/AOT/eucalyptol, all the water in the reverse micellar state remains bound to the AOT up to the second stage of the addition process (i.e., at ω2). At the third stage (ω3), a major part of the added water remains free. Among the different oils used, the extent of free water in the pool follows the trend n-heptane > isooctane > n-decane > n-octane > eucalyptol. This is also the trend of maximum solubility of water in the mixed three-component systems (shown subsequently in Table 3). For eucalyptol and n-decane, the third stage of the enthalpy of water dispersion and the maximum solubility are almost equal; this is reasonably different for n-heptane, isooctane, and n-octane. The enthalpies of water dissolution in AOT/n-heptane,7 AOT/ cinnamic alcohol,13 and AOT/CHCl342 have been reported to be endothermic. A detailed study on the energetics of water dissolution in AOT/alkanol media (also endothermic) has been made by Ray and Moulik.9 The experiments were performed in a TRONAC 458 Isoperibol macrotitration calorimeter in a single-step addition; multiple-step additions as done in this study were not possible there. The endothermicity of the present results is in line with the above-mentioned findings.
TABLE 2: ω-Dependent Structural Transitions and Enthalpy Changes for the Corresponding States Formed During Water Dispersion in AOT/Oil Systems Forming Reverse Micelles/Microemulsions at 303 K [H2O]/[AOT] heptane octane isooctane decane eucalyptol
enthalpy/kJ mol-1
ω1
ω2
ω3
+(∆Hd)1
-(∆Hd)2
-(∆Hd)3
0.61 1.2 0.81 0.6 3.17
3.45 4.40 4.16 3.4 9.5
43 33.4 41.0 37.0 18.0
0.152 0.143 0.342 0.116 -0.849
0.191 0.243 0.246 0.360 0.148
0.030 0.027 0.033 0.055 0.096
One of the objectives of the present study is to understand the states of aggregation of water in the micropools formed. A detailed study using different AOT/oil compositions has not been done. To supplement the results cited above, single-stage addition of water leading to the final composition (a representative plot is shown in Figure 9) has been performed to evaluate the other thermodynamic parameters, i.e., the free energy of dispersion (∆Gd) and the entropy of dispersion (∆Sd), following the rationale adopted in earlier reports considering the maximum water solubilization (prior to phase separation) as the solubility limit of the dispersed water in the form of microdroplets:
∆Gd ) -RT ln Xd
(4)
∆Sd ) (∆Hd - ∆Gd)/T
(5)
In this calculation, the mole fraction of microdroplets (Xd) has been calculated from the solution composition and the droplet size determined by the DLS method. The results are presented
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TABLE 3: Compositions of the Mixed Systems in Molesaat Different Stages of Water Addition to Form Reverse Micelles/ Microemulsions at 303 K mol of bound water × 104
mol of components system
103 (oil)
104 (AOT)
103 (water)
ω1
ω2
water/AOT/heptane water/AOT/octane water/AOT/isooctane water/AOT/decane water/AOT/eucalyptol
9.04 8.15 8.02 6.8 7.94
2.72 2.81 2.76 2.77 2.83
10.9 8.78 10.53 9.61 5.17
1.67 3.33 2.22 1.67 8.89
9.44 12.2 11.38 9.44
a
ω3
mol of free water × 103 ω1
ω2
ω3
0.94
9.31 7.09 8.88 7.95 3.47
The bound and free water were calculated taking six as the hydration number of AOT (mole/mole); see text.
TABLE 4: Energetics of Water Dispersion in AOT/Oil Media, and the Diffusion Coefficient (D) and Polydispersity Index (PDI) of Dispersed Droplets at 303 K dh solvent
ωa
exptl
calcdb
∆Hd/J mol-1
∆Gd/kJ mol-1
-∆Sd/J K-1 mol-1
107D/cm2 s-1
PDI
heptane octane isooctane decane eucalyptol
73 59.5 65 44 25
21.8 20.0 29.4 20.5
21.3 17.8 19.3 13.8
2.72 24.90 18.12 7.20 260
32.7 33.8 36.4 33.6
108 111 120 111
5.29 3.09 2.89 2.35
0.458 0.338 0.418 0.392
a
Maximum state of water dispersion. b Calculated according to the relation43 dh ) 2(1.18 + 0.13ω).
Figure 9. Heat flow with time for single-step water dispersion in AOT/ oil medium at 303 K. Heptane: 53 µL of water was added in 1.325 mL of 31.06 mM AOT solution in the oil. Octane: 40 µL of water was added in 1.325 mL of 28.72 mM AOT solution in the oil.
in Table 4. The detailed DLS measurements have been taken at 90°. Measurements taken at angles, both lower and higher, other than 90° have yielded results within close agreement. The choice of 90° in the DLS measurements is, therefore, not arbitrary. The attractive interaction among the nanodroplets of the w/o type may lead to their aggregation and coalescence; the
information derived from DLS measurements may then be that of the transformed droplets. But the dimensions herein realized are reasonably small. A straightforward calculation from ω on the basis of the relation proposed by Ray et al.43 has revealed droplet dimensions comparable with the measurements, except for the systems water/AOT/isooctane and water/AOT/n-decane, where the results are lower than the experimental values. This may be due to droplet growth by attractive interaction. In the experimental systems, the nanodroplets have, therefore, retained their individual identities: they may have only loosely clustered but have not coalesced. The calculated dh values are given in column 3 of Table 4. The DLS-determined hydrodynamic diameters are thus useful to estimate the droplet mole fractions for use in the equation. ∆Gd and ∆Hd are very much off balance so that the solubilization process essentially becomes entropy controlled. The overall system is ordered. Association, organization, and orientation of AOT and water molecules as well as the oil contribute considerable ordering effects compared to the disordering effect of water dispersion, ultimately yielding sizable negative entropy and insignificant enthalpy. The above energetic assessment could not be performed for the system with eucalyptol due to the very feeble scattering of the dispersed water droplets subjected to DLS measurements. The droplet size appeared to be below 3 nm (the lower measurable limit of the instrument). Without the availability of dh values, a thermodynamic calculation has remained pending. The ∆Hd in the AOT/ eucalyptol system is far greater than those for the rest of the AOT/oil systems. In Table 4, along with the energetic parameters, the droplet diffusion coefficient (D) and polydispersity index (PDI) are also given. The PDI values are all greater than 0.1 (requirement for monodispersity) and fall in the range of 0.34-0.46; the droplets in the microemulsions are fairly polydisperse. In Figure 10, the distribution of the droplets determined by the DLS method assuming spherical shape is displayed. The polydispersity of the droplets is obvious from this figure. Conclusions (1) The microcalorimetric method is suitable for the determination of the cmc and energetics of reverse micelle formation. Like normal micelles, reverse micelle formation is also entropy directed.
Microcalorimetry of AOT Self-Association
Figure 10. Representation of percent particle size distribution (%PSD) as a function of particle diameter (dh) for different microemulsion systems. A: water/AOT/n-heptane. B: water/AOT/n-octane. C: water/ AOT/isooctane. D: water/AOT/decane.
(2) The enthalpy of water dissolution in AOT/oil forming microwater pools (or microemulsions) is ω dependent. The dissolved water may exist in three states; the first transition occurs virtually at ω ) 1.0 (3) The dispersion of a water-forming microemulsion is also an entropy-controlled process. Acknowledgment. The work has been performed in a research program supported by the DST, Government of India. References and Notes (1) Moulik, S. P.; Mukherjee, K. Proc. Ind. Natl. Sci. Acad, 1996, 62, 215. (2) Ray, S.; Paul, S.; Moulik, S. P. J. Colloid Interface Sci. 1996, 183, 6. (3) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. J. Phys. Chem. 1994, 98, 4713. (4) Mukherjee, K.; Moulik, S. P.; Mukherjee, D. C. Langmuir 1993, 9, 1727. (5) Moulik, S. P.; paul, B. K.; Mukherjee, D. C. J. Colloid Interface Sci. 1993, 161, 77. (6) Gupta, S.; Mukhopadhyay, L.; Moulik, S. P. Colloids Surf. B: Biointerfac. 1994, 3, 19. (7) Ray, S., Bisal, S. R.; Moulik, S. P. Langmuir 1994, 10, 2507. Ray, S.; Moulik, S. P. Langmuir 1994, 10, 2511. (8) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. Int. J. Chem. Kinet. 1994, 26, 1063. (9) Ray, S.; Moulik, S. P. J. Colloid Interface Sci. 1995, 173, 28. (10) Mukherjee, L.; Mitra, N.; Bhattacharya, P. K.; Moulik, S. P. Langmuir 1995, 11, 2866. (11) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. Bull. Chem. Soc. Jpn. 1997, 70. (12) Majhi, P. R.; Mukherjee, K.; Moulik, S. P. Langmuir 1997, 13, 3284. (13) Mukhopadhyay, L.; Mitra, N.; Bhattacharya, P. K.; Moulik, S. P. J. Colloid Interface Sci. 1997, 186, 1.
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