Langmuir 1994,10,2511-2515
2511
Dynamics and Thermodynamics of Aerosol OT-Aided Nonaqueous Microemulsions S. Ray and S.
P.Moulik*
Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta 700 032, India Received May 4, 1993. I n Final Form: March 10, 1994@ The phase behavior, conductance, and viscous behavior of nonaqueous microemulsions formed by the dimethylformamide combinationofthe solvents[formamide(FA),ethyleneglycol (EG),propyleneglycol(PG), (DMF),and dimethylacetamide (DMA)]and oils [heptane (Hp), octane (Oc),isooctane (i-Oc),xylene (Xy), and toluene (Tl)] in presence of aerosol OT (AOT, sodium 1,4-bis(2-ethylhexyl)sulfosuccinate) have been studied. The ternary phase diagrams of the nonaqueous solvent systems with i-Oc are more or less similar; the single phase areas have fairly large viscous zones toward the amphiphile end, that of FA/AOT/i-Oc being significant. Both viscosity and conductance have demonstrated percolation and internal structure formation. The intrinsic viscosity and Huggin's constant at constant solvent/AOT mole ratio ( w ) for a number of systems have supported spherical (or minorly ellipsoidal)nonsolvated dispersions. Except FA, all the other solvents obeyed the viscosity equations of Vand, Moulik, and Eiler. The thermodynamics of a solution of the nonaqueous solvents in AOT/i-Oc medium resulting Winsor IV microemulsification has been calorimetrically studied. The FA/AOT/i-Oc system showed exothermicity, whereas the systems of EG, PG, DMF, and DMA with i-Oc and AOT exhibited endothermicity. The enthalpies of solution are significantly low and the specific heats of the resultant mixtures are very close to one another.
Introduction The majority of the studies in microemulsion utilize water as the polar component. Its dispersion in oil or dispersion of oil in it in presence of suitable amphiphiles (surfactants and cosurfactants) are considered, and structure and dynamics of the systems are investigatedl-'j along with their prospects as media for the purposes of enhancing oil recovery and studying reaction kinetics and reaction equilibria.'-1° Several reports"-14 on the enthalpy of microemulsion formation are also available. In recent years, attempts have been made to prepare and study waterless micro emulsion^.^^-^^ In this effort, water has been replaced by polar nonaqueous solvents (viz., ethylene glycol,formamide, etc.) and the preparations are essentially oil continuous. It has been reported that such preparations can be a useful media for organic reactions such as Diels-Alder and ~ t h e r . ~ OAlthough -~~ nonaqueous (or waterless) microemulsions may have this and other prospects, the literature on different aspects of Abstract Dublished in Advance ACS Abstracts. June 15.1994. (1)Blum, F:D.; Pickup, S.; Ninham, B.; Chen, S. J; Evans, D. F. J. Phys. Chem. 1985,89,711. (2)Borkovec, M.; Eicke, H. F.; Hammerich, H.; Dasgupta, B.J. Phys. Chem. 1988.92. 206. (3)Jada, A.; Lang, J.;Zana, R.; Makhloufi, R.; Hirsch, E.; Candan, S. J. J. Phys. Chem. 1990,94,387. (4)Bisal, S. R.; Bhattacharya, P. IC;Moulik, S. P. J. Phys. Chem. 1990,94,350. (5)Kahlweit, M.; Strey, R.; Firman, P.; Hasse, D. Langmuir 1985, 1, 281. (6)Peyrelasse, J.; Moha-Ouchane, M.; Boned, C. Phys. Rev. A 1988, 38,4155. (7)Letts, K.; Mackay, R. A.Inorg. Chem. 1975,14,2993. (8)Das, M. L.; Bhattacharya, P. K; Moulik, S. P. Langmuir 1991, 7,636. (9)Bunton, C. A.;DeBuzzaccarini, F. J.Phys. Chem. 1981,85,3142. (10)Mishra, B. K.; Valualikar, B. S.; Kunjappu, J. T.; Manohar, C. J . Colloid Interface Sci. 1989,127,373. (ll)Moulik,S. P.; Das, M. L.; B h a t t a c h q a , P. K.; Das, A. R. Langmuir 1992,8,2135. (12)Kertes, A. S.;Chaston, S.; Lai, W. C. J. Colloid Interface Sei. 1980,73,94. (13)Olofsson, G.; Kizhing, J.; Stenius, P. J. Colloid Interface Sei. 1980,111, 213. (14)Lai, W.C.; Kertes, A. S. Colloids Surf. 1982,4,379. (15)Friberg, S.E.; Podzimek, M. Colloid Polym. Sci. 1984,262,252. (16)Rico, I.; Lattes, A. Nouv. J. Chim. 1984,8,429. (17)Fletcher, P. D. I.: Golal, M. F.: Robinson, B. H. J. Chem. SOC. Faraday Trans. 1 1984,80,3307. (18)Friberg, 5. E.; Liang, Y. C. Surfactant Sci. Ser. 1987,24,103. (19)Rico, I.; Lattes, A. Surfactant Sei. Ser. 1987,24,357. @
this type of microemulsions is so far scanty. This provides directions for further investigations on such systems. In this presentation, we have made a phase behavioral study of nonaqueous microemulsions using five solvents (ethylene glycol, propylene glycol, formamide, dimethylformamide, dimethylacetamide) and five oils (heptane, isooctane, toluene, octane, and xylene) together with the amphiphile aerosol OT (AOT,sodium 1,4-bis(2-ethylhexyl) sulfosuccinate). In addition to this, the conductance behavior as well as the calorimetric determination of the enthalpy of microemulsification of several specified mixtures have been undertaken. An elaborate study of the viscous behavior of the microemulsions has been also made which includes several binary systems of AOT and oil. The moisture content ofAOT has imparted approximately 1%water into the preparations. Hence the microemulsions herein studied are not truely waterless.
Experimental Section Materials. The nonaqueous solvents formamide (FA), dimethylformamide (DMF), dimethylacetamide (DMA), ethylene glycol (EG),and propylene glycol (PG) were pure products of either BDH or E. Merck. Their densities, refractive indexes, and specific heats were determined calorimetrically followingthe procedure described earlierB and agree closely with the literature data. They were used without further purification. The oils heptane (Hp), octane (Oc),isooctane (i-Oc),toluene (TU, and xylene (Xy) were A.R. BDH products and were used as received. The surfactant AOT was obtained from Sigma. The water content in the sample determined by Carl Fisher titration was 3% (w/w). With respect to other impurities it was 99%pure. The salts sodium cholate (NaC)and NaCl were pure products as reported earlier.25 All solvents and agents were stored under desiccating conditions during the work. (20)Lattes, A.;Rico, I.; De Savignac, A,;Samii, A. A.Z. Tetrahedron 1987,43,1725. (21)Holmberg, K.;Lassen, Bo.; Stark, M. B. J.A m . Oil. Chem. SOC. 1989,66,1796. (22)Lattes, A.;Rico, I. Surfactant Sci. Ser. 1987,24,377. (23)Lattes, A.;Rico, I. Colloids Surf. 1989,35,221. (24)Mukherjee, K.;Moulik, S. P.; Mukherjee, D. C. Langmuir 1993, 9,1727. (25)Ray, S.;Bisal, S. R.; Moulik, S. P. J.Chem. Soc.,Faraday Trans. 1 1993,89,3277.
0743-7463/94/2410-2511$04.50/00 1994 American Chemical Society
Ray and Moulik
2512 Langmuir, Vol. 10, No. 8, 1994 AOT
AOT
Figure 1. Ternary phase diagrams of nonaqueous microemulsions at 303 K. The area under the boundary flanked by i-Oc and nonaqueous solvent is biphasic and that above it is monophasic. The marked areas in the monophasic zone above the respective boundaries are viscous. Methods. The conductance measurements were taken with a Jenway (UK)temperature-compensated conductometer in a stoppered cell of cell constant 1.11 cm-'. The procedures for the conductance measurements a t different volume fractions of the nonaqueous dispersions and a t different temperatures for dispersions of constant compositions were the same as reported earlier.25,26 The vicosities of the preparations were measured in a calibrated Ubbelohde viscometer with flow times for the oils in the range of 75-95 s. Duplicate measurements were taken for each sample. The microemulsion compositions were considered viscous ifthe solution did not trail when the test tube holding the sample was horizontally placed. Density mesaurements were taken in a calibrated pyknometer. For details of viscosity and density measurements we refer readers to our earlier publications.26 The viscosity, density, and conductance measurements were taken in a constant temperature water bath of accuracy within f 0 . 0 5 "C. An isoperibol titration calorimeter (TRONAC-458)was used for the determination of the enthalpy of microemulsification and specific heat of the resulting solution, followingthe procedure described ea~-lier.ll,~' In the actual procedure, addition of the nonaqueous solvent was made in AOT solution in a n oil at a constant mole ratio of the two. The heat of solution of the nonaqueous solvent in the AOT/oil medium was recorded in terms of the voltage change. The data were processed in the prescribed way.25 During the measurements, the nonaqueous solvent was added in three to four protions, the cummulative heat was divided by the total moles of added solvent to obtain the enthalpy of solution (AHH,) of the process. For the determination of specific heat, a known quantity of heat was supplied to the solution with the help of a calibrated heater, and the change in voltage was noted for the estimation of C, in the manner described earlier.11*27 Results and Discussion Phase Behaviors. The ternary phase diagrams with i-Oc as the oil are presented in Figure 1. They are on the whole symmetrical having distinct viscous zones a t the top. A comparatively very large viscous zone has been observed for the FA/AOT/i-Oc system. The temperature in the range of 30-50 "Chas little effect on the single and biphasic zones. But it has a moderately decreasing effect (26) Mukhopadhyay, L.; Bhattacharya, P. K.; Moulik, S. P. Colloids Surf. 1990, 50, 295. (27) Ray, S.; Bisal, S. R.; Moulik, S. P. Langmuir 1994, 1 0 , 2507.
04 0 4
,
0 5 , 06 , 04 05, 06 0 4 , 05 , 0'6 'I 0 5 , 06 0 4 , 06 04 05
t'C
'
@d
Figure 2. The conductance vs f$d profiles for several microemulsion-formingsystems at 303 K are shown in part A (I) FA/AOT/i-Oc,(11)PG/AOT/i-Oc,(111)DMF/AOT/i-Oc,(TV)EG/ AOT/i-Oc, (V) EG (with 0.1 mol dm-3 NaC)/AOT/i-Oc,(VI) EG (with 0.1 mol dm-3 NaCl)/AOT/i-Oc. The conductance vs temperature profiles for several microemulsion systems are shown in part B: (I) FA/AOT/i-Oc at w = 1.44, (11) EG/AOT/ i-Oc at o = 1.71, (111)FA/AOT/i-Oc at o = 3.69, (IV)EG/AOT/ i-Oc at w = 2.96. on the viscous zone. Of the total triangular area, the monophasic areas of the studied systems, EG/AOT/i-Oc, PG/AOT/i-Oc, FA/AOT/i-Oc, DMF/AOT/i-Oc, and D W AOT/i-Oc, have been found to be 45, 44.6, 44.4, 49, and 50%)respectively. Of the studied systems therefore, i-Od AOT can fairly homogenize EG, PG, FA, DMF, and DMA. Compared with waterF8 the polar nonaqueous solvents have shown symmetrical and broad single phase microemulsion zones. Conductance and Viscosity in Relation to Percolation. The water-in-oil microemulsions produced with ionic amphiphiles may exhibit a manyfold increase in conductance after a threshold volume fraction of the dispersed phase ($a) which is known as percolation.26The 4 d = (volume of water volume of amphiphile)/total volume of microemulsion was estimated on the basis of ideal mixing. At appreciable [waterY[amphiphilel mole ratio (a), the microemulsion may also exhibit percolation after a threshold temperature In both circumstances, the dispersed microdroplets cluster and the rapid increase in conductance results either by the "hopping" of the amphiphile ions from droplet to droplet or by transient fusion of droplets and mass exchange. The system EG/AOT/i-Oc a t [i-OcY[AOTl mole ratio = 3:1has exhibited distinct percolation in conductance with 0.1 mol dm-3 NaC and NaCl a t 4; = 0.45 (Figure 2A). The 4; (the threshold 4 d at the start of percolation) without salt is 0.43 which shows that the salts NaC and NaCl hardly affect the 4.; FA/AOT/i-Oc, PG/AOT/i-Oc, and DMF/AOT/i-Oc have also shown nearly the same 4; = 0.44. In water-in-oil microemulsions, appreciable effects of NaC and NaCl on 4; have been observed;25while NaC decreases 4;) NaCl increases it. The observed unaffected 4: in different environments for nonaqueous microemulsions is, therefore, a contrary event. The trends in conductance versus temperature for several systems, FA/AOT/i-Oc ( w = 3.69), EG/AOT/i-Oc
+
(28) Eicke, H. F.; Rubik, R.; Hasse, R.; Zschokki, I. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: NewYork, 1984; p 1533. (29) Rico, I.; Lattes, A. J. Colloid Interface Sei. 1984, 102, 285.
AOT-Aided Nonaqueous Microemulsions
Langmuir, Vol. 10, No. 8, 1994 2513
h
6t
0.1
0
0.2
C i g m CC'
Figure 4. The treatment of Viscosity data accordingto Huggin's equation at 303 K on the binary AOT/oil systems. The names of the oils are given against the graphs. 16
I
0
' 0.1
,
I
0.2
0.3
I
I
0.4
14-
+d
Figure 3. The Viscosity coefficient vs profiles for three microemulsion forming systems at 303 K (I) FA/AOT/i-Oc at w = 1.44,(11)EG/AOT/i-Oc at w = 1.85;(111)DMF/AOT/i-Ocat w = 2.03. (w = 3.2),and FA/AOT/i-Oc (w = 1.44)are illustrated in Figure 2B. The increase in conductance was mild for all the systems; there is a tendency to attain a plateau. The compositions are not a t all temperature-induced percolating type. Temperature-induced percolations in microemulsions at low w values (< 15)are not usually observed. Preparations with higher w values are not of single phase type for the microemulsion systems herein studied which are restricted to low w values. The viscosities of FA/AOT/i-Oc,EG/AOT/i-Oc,and DMF/ AOT/i-Oc a t nonaqueous solvent/AOT mole ratios ( w ) of 1.44, 1.85, and 2.03, respectively, in the percolation range,30 are presented in Figure 3. For the first two systems, the threshold volume fractions of the dispersed phase (4;) appear a t 0.11and 0.17,respectively; the third system lacks a well-formed threshold state although viscosity has increased nearly five times a t 4 d = 0.38. The results suggest that clustering with increased viscosity can take place for certain systems at low w . In terms of conductance the phenomenon occurs a t higher 4;. It cannot be induced by temperature. Although the low 4; in the viscosity profile and high 4; in the conductance profile are noncorroborative, the results have a t least suggested the possibility of structure formation in nonaqueous microemulsion. General Viscosity Behaviors. The overall geometry of the nonaqueous microdispersions with low volume fractions of the dispersed phase has been reported to be ~ p h e r i c a l . The ~ ~ possible departure from the spherical shape has been also estimated in a few cases.17 The determination of the intrinsic viscosity, [VI, of several systems from the intercept of the graphical plotting of v8JCagainst C has been demonstrated in Figures 4 and 5. The specific viscosity, vSp,is equal to vr - 1,where vr is the relative viscosity i.e. the ratio of the viscosity of microemulsion with that of the corresponding oil, and C is the concentration of AOT or the dispersed phase (nonaqueous solvent plus AOT), expressed in grams per (30)Saidi, 2.; Mathew, C.; Peyrelasse, J.; Boned, C. Phys. Rev. A. 1990,42,872. (31)Rico. I.: Laths. A,: Das, K. P.: Lindman, B. J.Am. Chem. SOC. 1989,I l l , 7266.
0
0,05
0.10
0.15
c / g m CC-' Figure 5. The viscosity data treatment accordingto Huggin's equation for ternary microemulsion systems at 303 K (I)PG/ AOTm at w = 1.64,(11) PG/AOT/'Xy at o = 1.64,(111) PG/ AOT/i-Oc at w = 1.64,(Iv)EG/AOT/Hp at w = 2.12,0FA/ AOT/Hp at w = 1.44(Thiscurve is nonlinear all the way through. milliliter. In all the cases, the plots deviate from linearity at higher concentrations, except for Xy and Tl, where linear variations have been witnessed. The deviation a t higher concentrations is due to droplet association which, for Xy and T1, is considered minimum.32 Microemulsions with these oils are effectively n ~ n p e r c o l a t i n g .In ~ ~the case of a curvature, extrapolation of the lower linear part of the plot has been considered to evaluate [VI. The [VI values for the binary as well as the ternary systems are presented in Table 1. The values are fairly low which suggest spherical dispersions with minor solvation (if any). Such low [VI has been also reported for a glycerol/AOT/Hp microemulsion system.l7 The Huggin's constant k H was obtained from the relation
The results found from the linear plots of v,dC against C are presented in Table 1. Most of the k H values are in the range 2-4: three values are higher (between 6 and 7) for EG/AOT/Oc, EG/AOT/Hp, and EG/AOT/i-Oc. Fletcher et al.17 have reported k~ values 3.8 and 6.4a t 10 and 30 "C,respectively, for glycerol/AOT/Hp system. Spherical (32)Dutkiewicz, E.; Robinson, B. H. J. Electroanal. Chem. 1988, 251, 11. (33)Mukhopadhyay, L.;Bhattacharya, P. K.; Moulik, S. P. Indian J . Chem. 1993,324,485.
Ray and Moulik
2514 Langmuir, Vol. 10, No. 8,1994 Table 1. The Viscosity Parameters of Two-and Three-Component Mixturecia of Oil, AOT, and Nonaqueous Solvent at 303 K system [ql KH v Q Z M V AOT/T1
1.95 2.30 AOT/i-Oc 2.70 1.83 AOT/Xy/EG AOTPTVEG 2.02 AOTMp/EG 1.34 1.53 AOT/Oc/EG AOT/i-Oc/EG 1.93 AOTKyPg 2.02 AOT/*I'I/PG 1.75 AOTMpPG 2.26 AOT/i-Oc/PG 2.37 2.03 A0TfI"FA
AOTMp
2.11 1.90 2.05 3.41 2.99 7.29 7.37 6.34 2.51 2.74 3.69 3.93 2.56
2.33 2.81 3.23 3.10 2.96 4.26 4.91 4.90 2.68 2.41 3.45 3.70 2.80
1.86 1.02 1.08 2.02 1.97 4.27 4.09 4.11 1.48 1.84 2.99 3.23 1.74
1.15 1.18 1.20 0.89 0.97 0.77 0.68 0.66 1.09 1.08 1.08 1.07 1.10
62.1 68.0 86.7 132.2 116.3 342.5 424.0 427.5 81.1 76.9 158.4 185.4 90.8
1.11 1.15 1.15 1.48 1.40 2.49 2.52 2.49 1.22 1.21 1.87 2.00 1.31
a In ternary mixtures, measurements were taken at constant w which were 2.11, 1.64, and 1.44 for EG-, PG-, and FA-containing systems, respectively.
neutral dispersions are expected to offer a value of 2.0;for hard spheres kH ranges between 0.7 and 0.8. The dispersed microparticles are not hard spheres. Water-in-oil microemulsions aided by AOT in hydrocarbon oils have shown more or less similar viscosity behavi01-s.~~ The relative viscosity vr of the microemulsions has been found to vary nonlinearly with f$d. The simple relation of Einstein (vr = 1 -t 2.5 & I ) intended for dilute noninteracting spherical dispersions is not obeyed. Attempts have been made to check the validity of the relations of Vand, Moulik, and Eiler a t constant w with variable 4d. The relations are as follows:
where Y is the shape factor (which is 2.5 for spheres and greater than 2.5 for ellipsoids), Q is the interaction coefficient of the flowing medium past the neighboring suspended or dispersed particles, I and M are empirical constants, and Vis the voluminosity (ratio of the solvated to the nonsolvated volume) of the droplets. Several representative illustrations are given in Figures 6 and 7. The derived parameters v and Q of Vand, I and M of Moulik, and V of Eiler are presented in Table 1. The values of the shape factor, mostly fall in the range 2.43.0; three sets have v = 4.0, and they have also shown higher Huggin's constant, kH (Table 1). Perfect spheres should show v = 2.5. The microemulsions are, therefore, on the whole spherical with a small degree of ellipsoidal character. The values of the interaction parameter, Q of Vand's equation, are also moderate; the flow of the solvent molecules past a dispersed particle, therefore, moderately interact with those flowing past another. The Q values for the three cases of higher v and k~ are also large, indicating greater interaction. The values of the parameter I of eq 3 are normal (close to unity). The M values are very large; such high values have been also found for aqueous micro emulsion^.^^ Except the three cases of EG/ AOT/Oc, EG/AOT/i-Oc,and EG/AOT/Hp, the voluminosity paramter, V of Eiler's equation are not far off from l.0.38 (34) Ray, S.; B i d , S. R.; Moulik, S. P. J . Surf.Sci. Technol. 1992, 8,191. (35) Vand, V. J . Phys. Colloid Chem. 1948, 52, 277. (36) Moulik, S. P. J . Phys. Chem. 1966, 72, 4682. (37) Eiler, H. Kolloid-2. 1941, 97, 313; 1943, 102, 154. (38) Ekwall, P.;Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1970, 33, 215.
0
I
I
10
20
'19 Figure 6. The treatment of viscosity data according to Vand's equation for ternary microemulsion systems at 303 K (I)EG/ ACT/i-Oc at w = 21.2, (11) EG/AOT/Hp at w = 2.12, (111) PG/ AOT/i-Oc at w = 1.64, (rv)PG/AOT/Hp at w = 1.64, (V) EG/ AOTPTI at w = 2.12.
01 0
I
0.01
I
0.02
0 33
#* Figure 7. The validity of Moulik's equation for ternary microemulsion systems at 303 K. (I) PG/AOT/Tl at o = 1.64, (11) PG/AOT/i-Oc at w = 1.64, (111) EG/AOT/i-Oc at w = 2.12, (IV)EG/AOT/Hp at w = 2.12. This has suggested a low degree of solvation of the dispersed particles which is in conformitywith the vvalues which are on the whole close to 2.5 and the kH values ranging between 2 and 4. The analysis of the viscosity results indicate that nonaqueous microdispersions are spherical o r minorly ellipsoidal, soft, and poorly solvated. There is a mild interparticle interaction. Of all the systems studied, those of EG with Hp, Oc, and i-Oc are exceptions. Among the oils the viscosity behaviors of the nonaqueous dispersions in Xy and TI follow eq 1a t all the studied concentrations; in other oils, deviations have occurred a t higher concentrations. The inverse micellar solutions of AOT in Hp, i-Oc, T1, and Xy also obey the viscosity equation (eq 1). Their hydrodynamic properties are comparable with the inverse LZtype microemulsions with Xy and TI; aqueous
AOT-Aided Nonaqueous Microemulsions
Langmuir, Vol. 10, No. 8, 1994 2515 Table 2. The Heats of Solution of Nonaqueous Solvents in AOT/i-Oc Mediuma at SO3 K amount added observed in successive heat Cm solvent portions x lo3, mol (QsOi), J kJ mol-l J g-' K-l
Timelcm
EG
6.75 24.19 3.37 12.40 mt = 0.04671 w = 3.2
-5.58 +12.34 +1.10 +3.98 Q~= 11.84
0.254
2.63
PG
6.02 9.03 8.60 5.61 mt = 0.02926 w = 2.0
-4.63 $9.29 +14.00 +7.94 Q~= 26.60
0.909
2.54
FA
5.91 35.8 mt = 0.04173 w = 2.86
-12.85 -56.43 Qt = -69.28
DMF
4.90 19.6 13.4 mt = 0.0379 w = 2.60
-5.16 f19.98 f11.82 Qt = 26.64
0.703
2.58
DMA
1.70 18.7 7.65 mt = 0.02805 w = 1.96
f5.76 +68.70 f20.66 Q~= 95.12
3.394
2.51
'"
Figure 8. Thermogramsof dissolution of nonaqueous solvents in an AOT/i-Oc medium leading to Winsor lV microemulsion formation at 303 K. For PG and EG, the ordinate shows the temperature change in 0.4 mV cm-l and for FA, DMF,and DMA the ordinate shows the temperature changein 4 mV cm-l. Abcissa, 1cm = 1min S and E signify the beginning and end of the burette run.
microemulsions of AOT and Xy and T1 have also demonstrated similar hydrodynamic behaviors.38 It should be noted that FA/AOT/i-Oc system has not obeyed any of the viscosity equations. Neither the qs& vs C plot has shown linearity. Therefore, estimation of the viscosity parameters could not be done for this system. Energetics of Microemulsification of the Polar Nonaqueous Solvents. The heats of solution of the nonaqueous solvents in AOT/oil mixtures (thermograms presented in Figure 8) and the corresponding enthalpies of solution for the Winsor IV compositions are presented in Table 2. Sharp changes in the energetic events for EG, PG, and DMF are witnessed in the thermograms. The overall enthalpy has been found to be exothermic for the FA/AOT/i-Ocsystem; the rest are endothermic. The gross overall heat of solution (&,,I) a t low o can be the result of different kinds of heat,11,27viz., heat of penetration of the polar solvent in the interior of the reverse micelles, heat of solvation of AOT anion and Na+ cation, heat of organization ofthe amphiphile at the interface, etc. These heat terms are difficult to estimate, and such data for AOT and the polar nonaqueous solvents studied herein are, a t present, not available in the literature. A rational analysis is therefore pending. Very recently, analyses of the resultant enthalpy of solution of water in TX 100/ butanolheptane and AOTheptane systems have been made.11$27The solution process has been found to be always endothermic without a n intermediate element of exothermicity. The Qs0l values expressed per mole of nonaqueous solvent added have yielded the enthalpies of solution (AH,) presented in column four of Table 2. Except for FA, the enthalpies of solution are all positive and small. The endothermicities of the solution process follow the order DMA > PG > DMF > EG. We have noted with interest that the solution process is not always thermodynamically unidirectional. For EG, PG, and DMF, the solution process has been initially exothermic and, thereafter, endothermic;
-1.660
2.54
Composition: 0.0146 mol AOT + 0.0859 mol i-Oc the endothermicity has not always followed a systematic trend (Figure 8). The results indicate that the solution process leading to the formation of different extents of microdispersions in the isooctane continuum is thermodynamically complex because of the interplay of several inherent physicochemical processes mentioned above. In the thermodynamic comparison, FA shows uniqueness with regard to the enthalpy of solution which is negative. Its exceptional nature has also been manifested in the viscosity behaviors where the results do not fit with any of the four equations considered. Compared with other nonaqueous solvents, it has resulted in the largest viscous region in the phase diagram (Figure 1). Except for DMA, the enthalpies of solution of EG, PG, and DMF in AOT/ i-Oc are much lower than that of ~ a t e r ~in' b, u~ t~a n o m 100heptane medium as well a s in the AOTheptane medium. With respect of specific heats, all the five ternary systems studied are parallel except EG/AOT/i-Oc,where the specific heat is slightly higher. The realized specific heats of the microemulsion systems are also only moderately higher than the specific heat of i-Ocz4(2.05 J g-l K-1). Conclusions The present investigation leads t o the following conclusions: (1)The AOT aided microdispersions of the nonaqueous solvents (FA, EG, PG, DMF, and DMA) in oils (Hp, Oc, i-Oc, Xy, and T1) obey established viscosity equations with a good degree of correlations. (2) The resultant microdispersions are nonsolvated and more or less spherical. (3) The microemsulsions can undergo internal structure formation manifested in conductance and viscosity. (4) The overall dispersion processes of EG, PG, DMF, and DMA in the AOT/i-Oc medium are endothermic, whereas that of FA is exothermic.
Acknowledgment. The work was done during the tenure of a CSIR, Junior Fellowship to S. Ray.
