J. Phys. Chem. 1992,96,9061-9068 (8) Pereira, R. R.; Zanette, D.; Nome, F. J . Phys. Chem. 1990, 94, 356. (9) Soldi, V.; Erismann, N.; Quina, F. H. J . Am. Chem. Soc. 1988, 110, 5137. (IO) Bunton, C. A.; Romsted, L. S.;Savelli, G. J . Am. Chem. Soc. 1979, 101, 1253. ( 1 1 ) Nome, F.; Rubira, A. F.; Ionescu, L. G. J. Phys. Chem. 1982, 86, 1881. (12) Stadler, E.; Zanette, D.; Rezende, M. C.; Nome, F. J . Phys. Chem. 1984,88, 1892. (13) Bunton, C. A.; Romsted, L. S.In Sofurion Behaoior of Surfacranrs: Theoretical and Applied Aspecrs; Mittal, K. L., Fendler, E. J., Eds.;Plenum: New York, 1982; p 975. (14) Broxton, T. J.; Sango, D. B. Aust. J . Chem. 1983, 36, 711. (15) Rodenas, E.; Vera, S.J. Phys. Chem. 1985, 89, 513. (16) Vera, S.;Rodenas, E. J . Phys. Chem. 1986, 90,3414. (17) Franm, J.; Dielunann, S.In Surfactants in Solution; Mittal, K . L., Lindman, B., Eds.; Plenum: New York, 1984.
9061
(18) Bunton, C. A.; Moffatt, J. R. J . Phys. Chem. 1986, 90,538. (19) Ortega, F.; Rodenas, E. J. Phys. Chem. 1987, 91, 837. (20) Neves, M. F. S.; Zanette, D.; Quina, F.; Moretti, M. T.; Nome, F. J . Phys. Chem. 1989,93, 1502. (21) Gonsalves, M.; Probt, S.; Rezende, M. C.; Nome, F.; Zucco, C.; Zanette, D. J . Phys. Chem. 1985, 89, 1127. (22) He,Z. M.; Loughlin, J. A.; Romsted, L. S.Bol. Soc. Chil. Quim. 1990,35,43. (23) Bunton, C. A.; Gan, L. H.; Moffatt, J. R.; Romsted, L. S.; Savelli, G. J. Phys. Chem. 1981,85, 1148. (24) Forrest, J.; Stephenson, 0.;Waters, W. A. J. Chem. Soc. 1946,333. (25) Nascimento, M. G.; Miranda, S. F.; Nome, F. J . Phys. Chem. 1986, 90, 3366. (26) Rezende, M. C.; Rubira, A. F.; Franco, C.; Nome, F. J. Chem. Soc., Perkin Trons. 2 1983, 1075. (27) Bowman, M. C.; Acree, F.; Cobert, M. K. J. Agric. Food Chem. 1960, 8, 406.
Microcaiorlmetric Investigation of the Solubilization of Water in Reversed Micelles and Water-in-Oil Microemulsions Gert Haandrikman, C.Janny R. Daane, Fred J. M. Kerkhof, Nico M. van Os, and Leo A. M. Rupert*i+ Koninklijke/Shell- Laboratorium, Amsterdam (Shell Research B. V.), P.O. Box 3003, 1003 A A Amsterdam, The Netherlands (Received: April 7, 1992; In Final Form: July 16, 1992)
Microcalorimetry has been used to study the solubilization of water in reversed micelles and water-in-oil (w/o) microemulsions. The systems were based on Aerosol OT (AOT) and two isomerically pure sodium alkylarenesulfonates as surfactants. The solubilization of water can be described in terms of hydration of the head group (ca. three H20molecules per AOT), swelling of the water droplet, and phase separation. A11 steps are endothermic, indicating that a gain in entropy is the driving force for solubilization. Above 60 ' C the hydration energy for AOT is strongly affected by temperature. The swelling of alkylarenesulfonatew/o microemulsions droplets in n-heptane is more endothermic than that of AOT w/o microemulsion droplets in n-heptane, indicating a significant influence of surfactant structure.
Introduction
In apolar solvents surfactants such as sodium bis(2-ethylhexy1)sulfosuccinate (Aerosol OT, AOT) form aggregates, which are able to solubilize large quantities of water in ~ i l . I -These ~ systems are often referred to as reversed micelles if the number of water molecules per surfactant molecule ( W) is smaller than 12 and as water-in-oil micro emulsion^^ if W is larger than 12. W/o microemulsions have a variety of possible and actual applications in fields such as biotechnology, separation processes, and preparation of m i c r o p a r t i ~ l e s Physicochemical .~~~~ properties of reversed micelles and w/o microemulsions have been investigated using several physical techniques. Emphasis has been placed, in particular, on the role of water, and most of the results have been interpreted in terms of the model of Zinsli," in which it is assumed that in a w/o microemulsion droplet two types of water exist: (i) water molecules bound to the head groups in the oilwater interface and (ii) free water, which behaves as bulk water. Fluores~ence,~~-~~ IR and R a m a ~ ~ ,NMR,17-22 ' ~ * ' ~ ESR,22and DSC measurements22*23 indicate that the physicochemical properties of AOT reversed micelles change strongly with water content if W is smaller than 6-10, For larger values of W,the behavior of the water is similar to that of bulk water. Light scattering and neutron scattering show that an increase in aggregate growth rate is observed at W > 12, which is indicative of the transition of reversed micelles to w/o m i c r o e m u l s i ~ n s . ~It~appears * ~ ~ that in this region the micellar size depends linearly on W.3*"26 Recently, it has been shown by Maitra and co-workers that, on the basis 'Present address: Shell Research Ltd., Thornton Research Centre, P.O.
Box 1, Chester CH1 3SH, England.
of IR measurements, it is better to distinguish three types of water molecules. Besides the bound water molecules and the free water molecules, 1-2 water molecules can be trapped between the head groups, having no hydrogen bond interaction with their surr o u n d i n g ~ . ~If~the ~ ~ water ' content of the system is increased still further, a phase boundary is p a s ~ e d . ~ *The . ~ ~exact location of the boundary and the phase types formed, Le., a w/o microemulsion with water or two microemulsions, depends on the oil and the t e m p e r a t ~ r e . ~ ~ , ~ ~ Although water-indud alterations in the structure of revenad micelles and w/o microemulsions have been studied in detail,'4J'-26 not much is known about the heat effects associated with these alterations. Thus, insight into the strength of various interactions in these systems is almost lacking. As far as the authors know, there have been only three invtstigations (all based on AOT) in which the heat effects associated with the solubilization of water in these systems have been m e a s ~ r e d . ~ Ref*~~ erences 30 and 3 1 show that the solubilization of water is endothermic, so driven by a gain in entropy. By applying a model to interpret the calorimetric data, D'Aprano et al. calculated that the solubilization of the first water molecules is associated with an endothermic heat effect of ca. 750 J/mol H 2 0 and that approximately six water molecules per surfactant molecule are bound to the interface." Here we report our microcalorimetric measurements on AOT-based reversed micelles and w/o microemulsions from 25 to 80 OC. The measurements were performed by titrating small quantities of water or w/o microemulsions into a solution of reversed micelles or w/o microemulsions. As a result, the heat effects associated with head group hydration and alterations in
0022-3654/92/2096-9061%03.00/0Q 1992 American Chemical Society
9062 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
the structure of the aggregate could be monitored directly. In an attempt to obtain more insight into the effect of surfactant structure, we conducted similar measurements on systems based on alkylarenesulfonates. In fact, surprisingly little has so far been done on systems other than AOT. The present microcalorimetric measurements show that significant differences exist between the two systems.
Experimental Section Materials. Sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT, AOT) was purchased from Fluka and purified according to standard procedure^.^^ After purification the AOT was dried in vacuo at 50 OC. Sodium 4-(7-tetradecyl)phenylsulfonate (7-4C14) and sodium 4,5-dimethyl-2-(3-dodecyl)benzenesulfonate (c-AXS) were synthesized as reported before.34 Toluene, xylene, n-heptane, and isoctane were obtained from Aldrich and stored on molecular sieves. The water content of the reversed micellar solutions was measured by using a Mitsibushi Moisturemeter CA.02. Microcalorimetric Measurements. The measurements were performed at 25,35,45,65,and 80 ‘C in a stirred LKB perfusion In a typical cell35installed in an LKB 2277 mi~rocalorimeter.~~ experiment 2 pL of water or w/o microemulsion was injected using a Hamilton Microlab M syringe into the perfusion cell, which initially contained 2 mL of a 1%w reversed micellar solution of the appropriate surfactant. After thermal equilibrium had been attained, the process was automatically repeated by injecting another 2 pL. Results and Discussion Solubhtion of Water in Reversed Micelles and w/o Microemulsiom. By titrating small quantities of water into a reversed micellar solution of AOT in isooctane ( W,nitisl= 0.26) we were able to observe the heat effects associated with the stepwise increase of the water content of the system (AH,).From Figure l a it can be seen that the first injection (Wfrom 0.26 to 6.9) induces a large heat effect. The heat effect associated with the second injection (Wfrom 6.9 to 13.5) is much smaller. With further injections AH,increases again and reaches a maximum at W = ca. 30. After the 10th injection (W= 66.2) an additional endothermic heat effect is observed (not shown). Since the system phase separates into two phases at W = 65,it is likely that this additional endothermic heat effect can be ascribed to the transport of water, oil, and surfactant and changes in the aggregate structure Amrding to Hou and Shah% during the phase separation p”. two microemulsions are formed in the phase separation process. In light of the above discussion it is reasonable to assume that the heat effect after the first injection is related to head group hydration, whereas the heat effect observed around W = 30 is related to the swelling of the microemulsion droplet. All heat effects are endothermic, which means that the solubilization process is driven by entropy. So contributions from the entropy of mixing, the gain in entropy due to fluctuations in the shape of the microemulsion and the gain in mobility of the surfactant molecule, including the counterion, are responsible for and water solubilization. Using (i) the approach of O~erbeek’~ Shah,28which in turn is based on the approach of Percus-YevicP and Carnahan-Starling,4I to calculate the entropy of mixing of hard spheres and (ii) the experimental data of Lang et a1.26at 25 “C, it can be concluded that the entropy of mixing (TASmIx) at W = 26 is in the order of 2.5 J/mol of H20. This is small as compared to the total enthalpy of water solubilization at W = 26 (see below, cf. Figure 2a). Consequently, contributions from shape fluctuations and increased surfactant mobility dominate the entropy term. From Figures la-c it appears that a change in temperature from 25 to 45 OC has only minor effects on how AH, is affected by W. When, however, the temperature is raised to 60 or 80 OC, AH, vs Wdiffers significantly from those at low temperature (Figure 1, parts d and e). To be able to compare the plots at the various temperatures with each other and with the data reported by DAprano et al.” the heat effects observed for each injection have
Haandrikman et al. TABLE I: Enthalpy of Hydration of AOT in Isooctane
AH (kJ/mol H,O) water molecule 1 2 3
4
25 O C 2.3 0.71 0.16 -0.03
65 O C 12.7 8.3 -0.75
been converted into the total enthalpy of solubilization (AiY), i.e., the enthalpy observed per mol water if an amount of water equal to W would have been added in a single injection, according to
where AH, is the total enthalpy of solubilization after n injections, Q,, is the Peat measured after the nth injection, Wi is the initial water content in mol, I, is equal to the mol of water added in each injection, and AHpl is the total enthalpy of solubilization for the system after the previous injection. By putting AHo = 0 all AH values are related to the initial state of the reversed micelles with W = 0.26. The results of these calculations are shown in Figure 2. In addition, Figure 2a shows the total enthalpy of solubilization reported by D’Aprano et al.31 It can be seen that our results agree qualitatively with those of DAprano et al., who added the water all at once in a flow experiment.” It can also be seen that a change in solvent from n-heptane to isoctane has only minor effects. The data collected in Figure 2 can be used to determine the effect of the temperature. To this end AH values obtained from Figure 2 at W = 10 and at the various temperatures are presented in Figure 3 as a function of temperature. This figure shows an abrupt change in AH vs T around 60 “C. Such an abrupt change in a property of reversed AOT micelles as a function of temperature has not previously been reported. Indeed the maximum temperature at which AOT-based systems have been investigated is 55 0C.1,4J3,17,26,42 Since the largest contribution to AH for W = 10 stems from the hydration of the head groups (see above), it is interesting to study this hydration process in more detail. Hydration of the AOT Head Group. To study the hydration process it is necessary to add water in such quantities that W increases less than one water molecule per surfactant per addition. In our experimental setup this can be achieved only by injecting a w/o microemulsion instead of pure water. However, in that case the measured heat effect has to be corrected for the heat effect associated with the reduction of the water content of the water droplet. It can be shown that the total enthalpy of solubilization can be calculated with a modified version of eq 1
where Siis the initial number of mol of surfactant in the perfusion cell, I, the amount of surfactant (in mols) added during each injection and AHj the total enthalpy of solution for the w/o microemulsion injected containing j water molecules per surfactant. Hence, the term l/(Si+ nl,) corrects for the change in surfactant concentration upon injecting the microemulsion, while the second and third terms of eq 2 correct for the heat effect of desolubilization of the microemulsion droplet. Since AHo = 0 and AHj can be obtained from Figure 2, AH,, can be calculated for each addition. Figure 4 shows the results of the experiments in which a microemulsion was injected into the reversed micellar solution at 25 and 65 OC in terms of AH,which equals S,AH,. It can be seen that the outcome of these titration experiments agrees well with the values for AH when the water is added in one single injection. It can be seen, too, that in both cases a maximum in AH is obtained at W = 1.0. Thus solubilization of the first water molecule leads to the highest loss in enthalpy. The enthalpy of solubilization, which is in this case identical to the enthalpy of
H20Solubilization Investigation a
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 9063
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Haandrikman et al.
9064 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 AH
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w Figure 2. Total enthalpy of water solubilization (AH) as a function of the number of water molecules per surfactant (W). System: AOT (1 % w) in isooctane at 25 (a), 35 (b), 45 (e), 65 (d), and EO OC (e). Also indicated in (a) are the data points obtained for AOT in n-heptane (A) and those
reported in ref 29 on the same system (0).
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 9065
H 2 0 Solubilization Investigation A H (kJ/mol H,O)
25
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w
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hydration, can be calculated for the first three to four water molecules from these curves (Table I). It appears that the hydration energy for each water molecule decreases stepwise. A similar observation has previously been made for ions in the gas phase43or for ion exchange resin^.^.^^ In fact, the main contributions to AH comes from the first three water molecules. This is in line with results obtained by fluorescence and NMR meawhich show that the reversed micelle has a rigid structure below W = ca. 3 and that other properties, such as the effective dielectric constant, change strongly around the same W v a l u ~ . l lThis ~ ~does ~ ~ not ~ mean that the interaction of additional I ; water molecules is completely negligible. Using DSC it has been ~ ~ .that ~~ shown that ca. six water molecules are ~ n f r e e z a b l e and 5.00 - I I two of them interact more strongly with the head group than the I remaining four water molecules.22 I I The hydration energy of the first water molecule (2.3 kJ/mol 2.50 of H20, Table I) is higher than that obtained by D'Aprano et al. I (750 J/mol of H20).31 However, in their model all bound water , molecules (ca. six) are treated equally, resulting in only a mean hydration energy. The value of 2.3 kJ/mol of H20hydration 0.00 : ' ' ' " energy is small and has the wrong sign with respect to the hy0 1 2 3 4 dration energy of Na+ in the gas phase (-100 kJ/mol of H20)43 w and is also small with respect to the strength of a hydrogen bridge F m e 4. Total enthalpy of hydration (AH) as a function of the number in bulk water (ca.17 kJ/mo14). This can be understood in terms of water molecules present (w)obtained by injection a w/o microof the structural model proposed by Z ~ n d e for l ~ hydrated ~ sodium emulsion at 25 and 65 O C . The initial water content W,is 0.26. The polystyrenesulfonate resins and later applied by Eicke to reversed 56.5 and amount of water present in the injected microemulsion (w) AOT micelles.48 On the basis of IR measurements Z ~ n d e l ~ ~ 23.2, respectively. Also shown AH obtained from a singlewas injection of suggested that the sodium ion forms a contact ion pair with the water (m, from Figure 2). sulfonate ion, Le., no solvent penetration between the two ions. The water molecules coordinate with their oxygen atom to the to 55 "C leads to a change in the head group conformation such cation, whereas both hydrogens are involved in two hydrogen that the trans conformation becomes more favorable than the bridges with adjacent sulfonate head groups. It is interesting to This in turn reduces the capacity of AOT to accept gauche note that a similar structure was obtained from X-ray diffraction hydrogen bonds.42 Apparently, this change in head group conon sodium diheptylsulfosuccinate dihydrate, a homologue of formation above 60 OC leads (see Figure 4) to an aggregate structure, in which indeed the strength or number of hydrogen AOT.9' The fact that the strength of a hydrogen bond between bonds is significantly different. water and the sulfonate group is lower than that of the bond Swelling of w/o microemulsions. From Figure 1 it can be seen between two water could account for the value of AH. Increasing the temperature from 25 to 65 OC results in an that at W > 12 AHs increases again. It appears that this increase increase in AH from 2.3 to 12.7 kJ/mol of H 2 0for the first water in AHs is accompanied by a second heat effect in the enthalpogram molecule (Table I). In addition, it seems that the decrease in AH (Figure 5 ) . Battistel and Luisi explained their kinetic measurebetween the first and second water molecules is less at 65 OC. The ments on the solubilization of water in terms of two steps: (i) fact that the hydration of the head group becomes more endoa rapid formation of macroemulsion droplets followed by (ii) a thermic suggests that the hydrogen bond interaction between the slow solubilization of the water into reversed micelles or w/o water and sulfonate head group becomes weaker. Using NMR microemulsion droplet^.^ In terms of their model the first peak measurements it has been shown that increasing the temperature in the enthalpogram (Figure 5 ) after the injection of water should
t P I
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9066 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
Haandrikman et al.
2.0 hr
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Figure 6. Total heat effect surfactant molecule (u).
8.0 hr
c-c
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(A2)
(Q)as a function of the surface area per
heat effects (QtOl) are plotted against the effective surface area. Indeed, the linear relation between Qla and u supports the idea that this second heat effect is due to a less effective screening of water-hydrocarbon interactions. The difference between the effective surface area (a) and the minimum area occupied by an AOT molecule (a&does not only increase because the total surface area increases faster with W than N does but also inmasea because water induces a change in head group conformation from gauche to t r a n ~ ~ ~ Jin*the J~*~ same way as an increase in temperature does (see above). a, is smaller for the trans conformation than for the gauche 0ne.19*50 The shift in the location of the maximum in AHg from W = 28 at 25 OC to W = 18 at 40 OC is probably the result of these superimposed effects of water content and temperature (Figure
Irtt
1).
W: 6.0 9.0 12.0 15.8 Figure 5. Part of the enthalpograms for AOT in isooctane at 25 "C(a, top) and for 0-AXS in n-heptane a t 80 "C (b, bottom). Shown are the second to the fourth injection. The arrows give a reference frame for the heat consumed in microwatt (pW) and the time in hours (h).
be due to the formation of a macroemulsion and the second peak should be assigned to swelling of the microemulsion droplet. Such an explanation agrees with the ob~ervation,'*~~*~~ based on light scattering measurements, that w/o droplets start to grow above W = ca. 12. Hence, this second heat effect is probably related to an increase in the effective head group area, resulting in a less effective Screening of unfavorable water hydrocarbon interactions. By taking togctk the equations that describe the relation between the surface and the volume of a sphere and the radius of that sphere, the effective surface area per AOT molecule (a) can be obtained as follows:28 (3)
with N being the number of AOT molecules per w/o microemulsion droplet and V, the volume of a water molecule (0.0299 nm9. Since it is known from fluorescence quenching measurements26that N depends in isooctane in a good approximation linearly on W N = 23(W- 10) + 140 (4) u can be calculated as a function of
45
33.4
W.In Figure 6 the measured
In summary, it appears that the heat effects for the solubilization in AOT-based systems can be understood in terms of two contributions: (i) hydration of the head group and (ii) swelling of the microemuLpion droplets. A further heat effect was observed when the system passes the boundary giving two micrmmulsion phases. To obtain insight into the question how far the above results are specific for AOT we performed some preliminary experiments on systems based on isomerically pure sodium alkylarenesulfonates. System Based 011 Sodipnr Alkylueaedfmtea. Reasonable amounts of water can be solubilized in o-Axs-bascd systems only above m.65 OC?l so that a c o m m with AOT-based systems is only possible in a limited range of temperatures. ' ' F 7 shows the heat effect observed for each water addition (AHg)and the total heat effect (AH) as a function of the amount of water in systems based on 0-AXS and 7d-CI4. In contrast to the AOT-basedsptcms, the +AXs based systems show a pronounced solvent effect. Whereas a change from isooctane to n-heptane AH,decreases has only minor effects with AOT (Figure a), strongly if n-heptane is replaced with an aromatic solvent (Figure 7a). In line with the above d i m i o n s on AOT it is likely that the increase in AHs above W re: 6 has to be assigned to swelling of the r c v d micelle/"cmulsion droplet. Although no clear sccond signal due to the swelling process can be dimmed in the enthalpogram (Figure Sb), the strong increase in the width of the signal indicatesthat a d heat effectispreaent. Themagnitude of tbe totalheat dfca sugegui that this second signal is m@ble for the increase in the total heat effect. Apparently, it is highly unfavorable to swell the w/o microemulsion droplet such that the effective distance between the xylenesulfonate head groups increases. Probably, this is due to favorable stacking interactions
The Journal of Physical Chemistry, Vol. 96, No. 22, I992 9067
H 2 0 Solubilization Investigation
a 50
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10
15
20
25
30
20
25
30
/--O--O
./O
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10
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Figure 7. Enthalpy of water solubilization per injection (AH,, a) and total enthalpy of water solubilization (AH,b) as a function of the number of water molecules per surfactant: ( 0 ) o-AXS in n-heptane, 80 O C , W,= 0; (X) o-AXS in p-xylene, 80 O C , W,= 3.50; (0)o-AXS in pxylene, 65 O C , W, = 3.48; (W) o-AXS in toluene, 80 O C , W,= 3.05; (A) 7-4-C,,, in xylene, 80 O C , W,= 1.52. Also indicated is AH vs W for AOT in isooctane at 80 O C (V).
between the aromatic moieties. Changing the solvent from nheptane to an aromatic one increases the amount of oil penetration between the head g r o ~ p . ~ *As- a~ consequence, ~ the intermolecular head group interactions decrease and a reduction in AHH, is observed. It is interesting to note that AHs is higher for toluene than for pxylene. Apparently, xylene reduces the intermolecular interaction more effectively than toluene. This could be due either to preferential solvation of the head group by xylene or to a disturbance of the packing in the interface by the additional methyl group. From Figure 7a it follows that the variation in AHs with Wis in xylene. It appears that a change similar for eAXS and 7-41-C~~ in temperature from 80 to 65 OC has a more pronounced effect than the change in head group. It therefore seems that a change in head group structure has only a minor influence on the swelling
of the microemulsion droplet. A comparison with AOT can be made via the total enthalpy of solubilization. From Figure 7b it can be seen that the heat of hydration is identical for o-AXS in n-heptane and AOT in isooctane, indicating a similar structure of the hydration shell. However, the systems differ in the swelling region. It is much more unfavorable to swell 0-AXS microemulsion droplets than AOT microemulsion droplets. This indicates once more that additional intermolecular head group interactions are present for o-AXS in n-heptane. Microcalorimetricmeasurements can be used also for gaining insight into the kinetics of water solubilization. The turbidity measurements conducted by Batistel and Luisi' revealed that the rate of water solubilization decreases with an increase in the amount of water present in the system. Batistel and Luisi related this to a decrease in the number of micelles. As already mentioned above, we observe two step for AOT. Most probably, fmt a rapid formation of a macroemulsion occurs (the first signal after injection, see Figure Sa) followed by a slower solubilization into w/o microemulsion droplets. From the enthalpogram it follows that it is the rate of this water solubilization into the droplets, i.e., the swelling of the droplet, in particular, that is reduced with water content. In the case of o-AXS, water solubilization takes about 4 times as long as with AOT. In addition, it is not possible to discriminate between the two processes. Hence, not only it is unfavorable to solubilize water into 0-AXS micelles from an enthalpic point of view but also the activation energy is higher than that for AOT-based systems. It is interesting to note that fourier-transform pulsed-field gradient NMR (FTPFG NMR) measurements show that w/o microemulsion droplets based on alkylarenesulfonates exchange rapidly water and surfactant^.^^ Hence, although intermolecular interactions between the arenesulfonate head groups in the water-oil interface reduce the rate of water solubilization, they still allow a rapid exchange of material. Therefore, it seems indeed that the actual swelling process is the unfavorable step. In conclusion, the solubilization of water in surfactant aggregates can be described into three steps: (i) hydration of the head group, (ii) swelling of the microemulsion droplet, and (iii) phase separation. In the case of AOT the strength of the interaction between the water molecules and the head group is strongly affected by temperature above 60 O C . This can be ascribed to a change in head group conformation. The results presented here stress the importance of varying the structure of the surfactant in order to obtain a better understanding of the formation of w/o microemulsions. Although for AOT the strength of the hydration interaction is at 80 OC similar to that obtained for o-AXS in n-heptane, a pronounced effect of the head group structure is observed for the swelling process. The swelling of o-AXS droplets in n-heptane is much more endothermic than in aromatic solvents or the swelling process of AOT in isooctane. A similar head group effect is observed for the rate of water solubilization. Water solubilization in o-AXS-based system is four times slower than in AOT-based systems. Clearly, variations in surfactant structure, such as introducing a benzene moiety, affect significantly the properties of w/o microemulsions. Further systematicvariations in the structure of the sodium alkylarenesulfonates should clarify how the surfactant structure affects the properties of w/o microemulsions.
References and Notes (1) Eicke, H.-F. In Inter/acial Phenomena in Apolar Media; Eicke, H.-F., Parfitt, G.D., Eds.;Surfactant Science Series, Vol. 21, Marcel Dekker: New York, 1987. (2) Luisi, P. L.; Straub, B. E. Reversed Micelles; Plenum: New York, 1984. (3) Huang, J. S.J . Surf. Sci. Technol. 1989, 5, 83. (4) Zulauf, M.; Eicke, H.-F. J . Phys. Chem. 1979, 83, 480. (5) Singh, A. K.; Sandorfy, C.; Fendler, J. H. J. Chem. Soc., Chem. Commun. 1990, 233. (6) Luisi, P. L.; Laane, C. Trends Biotechnol. 1986, 4, 153. (7) Battistel, E.; Luisi, P. L. J . Colloid Interface Sci. 1989, 128, 7 . (8) Robinson, B. H. Chem. Brit. 1990, 342. (9) Ossw-Asare, K.; Chaiko, D. J. J . Membr. Sci. 1989, 42, 215. (IO) Petit, C.; Lixon, P.; Pileni, M. P. J . Phys. Chem. 1990, 94, 1598. (11) Zinsli, P. E. J . Phys. Chem. 1979, 83, 3223.
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(12) Valeur, B.; Keh. E. J . Phys. Chem. 1979,83, 3306. (13) Wong, M.; Thomas, J. K.; Gritzel, M. J . Am. Chem. SOC.1976, 98, 2391. (14) Belletete, M.; Lachapelle, M.;Durocher, G. J . Phys. Chem. 1990, 94, 5337. (15) DAprano, A.; Lizzio, A.; Turco Liveri, V.; Aliotta, F.; Vasi, C.; Migliardo, P. J . Phys. Chem. 1988, 92, 4436. (16) Jain, T. K.; Varshney, M.; Maitra, A. J . Phys. Chem. 1989,93,7409. (17) Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. Soc. 1977.99, 4730. (18) Martin, C. A.; Magid, L. J. J . Phys. Chem. 1981, 85, 3938. (19) Maitra, A. J . Phys. Chem. 1984,88, 5122. (20) Kim, V.; Frolov, Yu. G.; Ermakov, V. 1.; Pak, S. E. Kolloidn. Zh. 1987, 49, 1067. (21) Heatlev. F. J . Chem. Soc.. Faraday Trans. I 1989.85.917. (22) H a w c H . ; Haering, G.; Pande, A.;-Luisi, P. L. J . Pkys. Chem. 1989, 93,1869. (23) Boned, C.; Peyrelassc, J.; Moha-Ouchanne, M. J . Phys. Chem. 1986, 90,634. (24) Eicke, H.-F.; Rehak, J. Helu. Chim. Acra 1976, 59, 2883. (25) C a b , C.; Delord. P. J. Appl. Crysf. 1979, 12, 502. (26) Lang, J.; Jada, A.; Malliaris, A. J . Phys. Chem. 1988, 92, 1946. (27) Eicke, H.-F. In Microemulsions; Robb, I. D., Ed.; Plenum: New York, 1982. (28) Hou, M.-J.; Shah, D. 0. Longmuir 1987, 3, 1086. (29) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J . Chem. Soc., Faraday Trans. I 1987,83,985. (30) Eicke, H.-F.; Kubik, R. Faraday Discuss. Chem. Soc. 1983, 76, 305. (31) DAprano, A.; Liuio, A.; Turco Liveri, V. J. Phys. Chem. 1987, 91, 4149. (32) Goto, A.; Yoshioka, H.; Kishimoto, H.; Fujita, T. Thermochim. Acta 1990. 163. 139. (33) B6rkovec. M.; Eicke, F.-H.; Hammerich, H.; Das Gupta, B.J . Phys. Chem. 1988, 92, 206.
(34) van Os, N. M.; Kerkhof, F. J. M.; van Ginkel, R.; Daane, G. J. R.; Haandrikman, G. J . Colloid Interface Sci. Submitted for publication. (35) Nordmark, M. G.; Laynez, J.; Schoen, A.; Suurkuusk, J.; Wadsoe, I. Biochem. Biophys. Merh. 1984, 10, 187. (36) Wadsoe, I. Thermochim. Acra 1985,85, 245. (37) Milner, S.T.; Safran, S.A. Phys. Rev. A 1987, 36, 4371. (38) Huang, J. S.;Milner, S.T.; Farago, B.; Richter, D. Phys. Rev. Leu. 1987, 59, 2600. (39) Overbeek, J. Th. Faraday Discuss. Chem. Soc. 1987, 65, 7. (40) Percus, J. K.; Yevick, G. J. Phys. Rev. 1958, 110, 1. (41) Carnahan, N. F.; Starling, K. E. J . Chem. Phys. 1969, 51, 635. (42) Maitra, A. N.; Eicke, H.-F. J. Phys. Chem. 1981, 85, 2687. (43) Dzidic, I.; Kebarle, P. J . Phys. Chem. 1970, 74, 1466. (44) Glueckauf, E.; Kitt, G. P. Proc. R . Soc. London 1955, A228, 322. (45) Dickel, G.; Degenhart, H.; Haas, K.; Hartmann, J. W. Z . Phys. Chem. Neue Folge 1959, 20, 121. (46) Luck, W. A. P. In The Hydrogen Bond, Schuster, P., Zundel, G., Sandorfy, C., Eds.; North Holland: Amsterdam, 1976; Vol. 111. (47) Zundel, G. Hydration and Intermolecular Interaction; Academic Press: New York, 1969. (48) Eicke, H.-F.; Christen, H. Helu. Chim. Acra 1978, 61, 2258. (49) Lucassen, J.; Drew, M. G. B. J. Chem. Soc., Faraday Trans. I 1987, 83, 3093. (50) Maitra, A.; Dinesh, Varshney, M. Colloids Surf. 1987, 24, 119. (51) Daane, G. J. R.; Rupert, L. A. M. Unpublished data. (52) Bratt, P. J.; Choudhury, H.; Chowhurry, P. B.; Gillies, D. G.; Krebber, A. M. L.; Sutcliffe, L. H. J . Chem. Soc.,Faraday Trans. 1990,86,3313. (53) Atik, S. S.;Thomas, J. K. J . Am. Chem. Soc. 1981, 103, 7403. (54) Keh, E.; Valeur, B. J. Colloid Inferface Sci. 1981, 79, 465. (55) Atik, S.S.;Thomas, J. K. Chem. Phys. Lett. 1981, 79, 351. (56) Binana-Limbele, W.; van Os,N. M.; Rupert, L. A. M.; Zana, R. J . Colloid Interface Sei., 1991, 144, 458. (57) Datema, K. P.; Bolt-Westerhof, J. A.; Jaspers, A.; Daane, J. G. R.; Rupert, L. A. M. Magn. Reson. Chem. 1992, 30,760.
Probing the Internal Dynamics of Reverse Micelles Formed in Highly Compressible Solvents: AerosoCOT in Near-Critical Propane Jing Zbmg and Frank V. Bright* Department of Chemistry, Acheson Hall, State University of New York at Buffalo, Buffalo, New York 14214 (Received: May 8. 1992; In Final Form: July 17, 1992)
In normal liquids, the rate of water reorganization within the interior of AOT (sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT) reverse micelles in a strong function of water loading and temperature. In this report, we discuss our most recent efforts to understand the internal dynamics of reverse micelles maintained in highly compressible solvents (nearcritical propane). By using steady-state and time-resolved fluorescence spectroscopy, we report the first evidence for pressureassisted control of the rate of solvent relaxation within the core region of a reverse micelle maintained in a highly compressible fluid. Three factors are taken into account in this study: amount of water loading, temperature, and bulk fluid density. The key conclusion is that the continuous phase density can influence the rate of solvent reorientation within the interior of AOT micelles formed in near-critical propane.
Introduction The fact that AOT reverse micelles form in near-critical and supercritical fluids has sparked significant interest.’-22 Following the initial report,’ many research groups have focused on understanding the phase behavior and structure of reverse micelles maintained in highly compressible solvents.2-22Over the past 5 years, many spectroscopictechniques (e.g., IR, UV-vis, dynamic light scattering, and fluorescence) have been used to probe the AOT system.’-22 To date, however, there have been only a handful of studies aimed at elucidating the dynamics within these microheterogeneous ~ y s t e m s . ’ ~ J ~ Time-resolved fluorescence techniques offer extremely high sensitivity and simultaneously provide a means to access kinetic i n f o r m a t i ~ n . ~Thus, ~ - ~ ~they are used to study extremely fast processes (e.g., solvation).25.26For example, we recently showed that time-resolved fluorescence can be used to probe the nanosecond reorganization of water within AOT reverse micelles Author to whom all correspondence should be addressed.
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formed in liquid n-he~tane.~’We reported on the rate that water within the core region of the reverse micelle could reorganizt about a solute. In these studies, a well-characterized28-46fluorescent solute, 1-anilino-8-naphthalenesulfonicacid (1,8-ANS), was used to determine how the local environment within the reverse micelle was affected by temperature and water loading. From this work we arrived at several interesting conclusions. First, there were two distinct solvent relaxation rates. second,the relaxation rates increased as we increased water loading. Third, temperature has a significant affect on the reorganization process. Fourth, temperature-depcndcnt studies provided a convenient means to study the effects of water loading on the energetics of the solvation process. However, because liquids are so incompressible, it was problematic to study the effect of continuous phase density on the internal micelle dynamics. Yazdi et al.” used 1,8-ANS to probe the microstructureof AOT reverse micelles formed in near-critical and supercritical alkanes. They showed that the fluorescence spectra of 1,8-ANS in AOT/ethane do not shift with fluid density in the absence of water. 0 1992 American Chemical Society
