2507
Langmuir 1994,10, 2507-2510
Thermodynamics of Microemulsion Formation. 2. Enthalpy of Solution of Water in Binary Mixtures of Aerosol OT and Heptane and Heat Capacity of the Resulting Systems S. Ray, S. R. Bisal, and S. P. Moulik* Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta 700 032, India Received May 4, 1993. In Final Form: March 10, 1994@ The heat of water dissolution in binary mixtures of Aerosol OT (AOT) and heptane (Hp) leading to increasing mole ratios of water and AOT in the final microemulsion solution has been calorimetrically determined. The overall heat of solution has been found to be endothermic. On the basis of several heat involving processes, a rational analysis of the water dissolution phenomenon has been made which has helped to evaluate the enthalpy of water penetration. In the presence of NaCl and NaC the overall heat of solution (&sol) has decreased. The specificheats of the resulting solutions have supported the formation of an initial open structure followed by clustering of microdroplets at higher mole ratios of water and AOT.
Introduction
Experimental Section
Although a thermodynamic study on the formation of ternary and quaternary systems of microemulsion is important, the area has not been amply explored.1*2 Energetic information is required for the understanding ofthe formation and stability of such systems. The method of calorimetry can be advantageous in this respect. The heat change associated with the dissolution of oil into water amphiphile or water into oil amphiphile can be conveniently determined in a calorimeter. Very recently, we have calorimetrically studied the multicomponent systems comprising heptanemriton X 100/water and heptane/Triton X lOO/butanol/water and have realized interesting results, a quantitative rationalization of which has been attempted. The thermodynamics of clustering of microdroplets in water1AOTheptanemicroemulsion has been contemporarily ~ t u d i e d .In ~ ,recent ~ years, the heats of dissolution of water in AOTIoil mixtures have been The endothermic heat has been found to strikingly dependent on water1AOT mole ratio. In this presentation, calorimetric investigation on the solubilization of water in AOTheptane medium both in absence and presence of the salts sodium chloride and sodium cholate (a surfactant) has been made. Sodium cholate can strikingly affect the percolation of conductance of a water in oil (W/O) microemulsion which sodium chloride ~ a n n o t Its . ~ influence on the thermodynamics of microemulsion formation on a comparative basis with NaCl has been, therefore, explored. Both the enthalpy of water dissolution and the heat capacity of the resulting solutions have been measured with a view to understand the thermodynamics of the microheterogeneous ternary inclusions.
+
+
* Abstract published in Advance ACS Abstracts, June 15, 1994.
(1) Kedes, A. S.; Chasten, S.; Lai, W.C. J. Colloid Interface Sei, 1980,73,94. (2) Lai, W.C.;Kertes, A. S.Colloids Surf. 1982,4 , 379. (3) Moulik, S.P.;Ray, S. Pure Appl. Chem. 1994,66,521. (4) Ray, S.;Bisal, S. R.; Moulik, S. P. J. Chem. Soc., Faraday Trans. 1993,89, 3277. (5) Goto,A.; Harada, S.; Fujita,T.;Micra,Y.;Yioshioka, H.; Kishimoh H. Langmuir 1993,9, 86. (6) Goto, A.;Yioshioka, H.; Kishimoto, H.; Fujita, T.Langmuir 1992, 8, 441. (7) DAprano, A.;Limo, A.; Turco Liveri, V. J.Phys. Chem. 1987,91, 4749.
Materials. Aerosol OT (AOT), heptane (Hp), NaCl, and
sodiumcholate (NaC)used in the study were the same as reported earlier.8 The water used was doublydistilledconductivitywater. Conductance Measurements. Conductancemeasurements were taken at a frequency of 1 kHz in a Jenway (England) conductometer in a temperature compensated cell ofcell constant 1.11cm-l. Allmeasurementsweretakeninathermostatedwater bath accurate within f0.02"C. A mixture ofknown amounts of AOT and Hp was taken in a container, which was placed in the water bath and water or additive in water was added in small installments from a microburet to the mixture with constant stirring, the conductancebeing measured at each addition after allowing sufficient time for the attainment of constant temperature (303 K). The course was continued until the mixture exhibited turbidity. For further detailsofexperimentationearlier reports may be ~onsulted.~ Calorimetric Measurements. Thermometric measurements were taken at 298 K in a Tronac Model 458 automatic titration calorimeter. The calibration was checkedby measuring the heat of neutralization of potassium hydrogen phthalate and caustic soda as described earlier.g The instrument was accurate to the extent of 0.5% on 2 calories. The thermistor constant was determined by comparingthe heat absorbed by known amounts of a pair of pure liquids (water and heptane) taken in the reaction vesseLg During experimentation, 20 mL of the titrant mixture was taken in the reaction vessel and water was taken in the buret. The whole system was immersed in the 60-L capacity water bath maintained at a temperature 25 0.0002 "C by a Tronac F'TC probe. After the attainment of thermal equilibrium the titer was delivered at the rate of0.316mUmin under constantstirring. The heat change in electrical equivalents was recorded on a Houston Instruments (Omniscribe D 5000) strip chart recorder with time and processed in the usual way.g In the actual run water was added in several installments. After the first installment,the volume of the mixture was reduced to 20 mL by withdrawing the excess amount and water was added for the second installment followed by measurement of the heat change. The same procedure was- adopted for subsequent expressing the associatedinte~alheat at any stage to be the cumulativeheat changesup to the stagewith appropriate correctionfor the withdrawal of solution for adjustment ofinitial volume of each thermometric run to 20 mL. To determine the heat capacity,a known quantity of heat was sent into the solution with the help of the calibrationheater and the change in temperature was recordedin milfivolts. meheat
*
(8) Bisal, S. R.;Bhattacharya,P.K.;Moulik, S. P. J.Su$. Sci. Technol. 1988,4, 121. (9) Moulik,S. P.;Das, M. L.;Das, A. R. Langmuir 1992,8, 2135.
0743-746319412410-2507$04.50/00 1994 American Chemical Society
Ray et al.
2508 Langmuir, Vol. 10,No. 8, 1994 AOT
/* - \ 2@
Heptane
Water
BLUlSh WHITE GEL
0
Figure 1. (A)Ternary phase diagram of W/AOT/Hp at 298 K. N l circles indicates the compositionsfor thermometric measurements. (B) Percolation of conductance of W/AOT/Hp microemulsion at 303 K 1,O.Olmol dm-3 NaCl; 2, without additive; 3,O.Ol mol dm-3 NaC; 4, 0.05 mol dm-3 NaC.
formed reverse micelles with very tiny water pools (1.3
A). With addition ofwater a number of primary processes
occurred. The effective processes were (1)penetration of water molecules into the microdroplets through the interfacial barrier (with associated endothermic heat Qpent),(2) solvation of Na+ ions and the SOa- group produced by AOT (with associated exothermic Qsolv), (3) reorganization of the surfactant molecules at the interface (associated with endo or exothermic heat, Qorg),and (4) clustering of the microdroplets (associated with endothermic heat, Qclus). The overall heat is therefore equated as
0
5
10 Time/cm
15
Figure 2. Thermograms of W/AOT/Hp system at 298 K 1,no additive; 2,0.05 mol dm-3 NaC; 3,O.Ol dm-3 NaC; 4,0.05 mol dm-3 NaC1; 5,O.Ol mol dm-3 NaC1. S and E represent time of start and end of titration, respectively. w varies in the range of 0-9. Scale is as follows: ordinate, 2.55 cm = 1mV, abcissa, 1 cm = 1 min.
capacity of the solution was found out by using a thermistor constant (Tk= 0.032894 "C/mV) along with the measured heat. The maximum error limit of the heat capacity measurements was +4%.
Results and Discussion Heat of Solution and Related Properties. The microemulsion formulations studied evidenced percolat i ~ n .Rapid ~ rise in conductance was observed after a threshold volume fraction of the dispersed droplets assumed to be consisting of the added water and the whole of AOT. The ternary phase diagram of the system is presented in Figure 1in which the compositions attained during the thermometric measurements are indicated with full circles. In the inset of the figure, the nature of percolation for several compositions is depicted. The overall heat of water dissolution (Qsol) in AOTheptane a t constant mole ratio (1:lO) was found to be endothermic, which parallels the observations of Goto et al.596and D'Aprano et al.' Several typical thermograms are presented in Figure 2. The presence of additives had a diminishing effect on Qsol. The contributions of several heat involvingprocessesmade the overall thermodynamics complex. A simplified rational analysis is presented with scope for further refinement. The amphiphile (AOT)used contained 0.7 mol/mol water;l0when added to heptane it ~
(10)Kotlarchyk, M.; Huang, J. S.; Chen, S. H. J.Phys. Chem. 1985, 89,4382.
The Qorgis a compensated heat of two opposing processes (redistribution and orientation of the amphiphiles at the interface) and was expected to be small. In the beginning of the water-dissolution process, the Qclus was minor, therefore Qsol
Qpent
+ Qsolv
Since Qs0l was found to be positive and small, Qpent > Qsolv and the difference is only moderate. AccordingtoJain et al.,ll the total water in the micropool in W/O microemulsion remains in three forms, trapped water (very low and constant), bound water, and free water. The bound water reaches maximum value of 12 at w = 18 whereas the free water increases after w = 18. In the present study the lowest value ofw was "9, the rest were '18. Except for the lowest w the level of bound water per mole of AOT was thus always 12. Following the proposition of Wong et al.12J450% of this bound water can be assigned to the Na+ ion, the rest goes to solvate the AOT anion by hydrogen bonding. Therefore, the enthalpy of solvation of RS03- (i.e. the AOT ion) was taken to be -12 k J mol-' of water (equivalent to that of hydrogen bonding), and -407 k J mol-' was taken as the enthalpy of absolute hydration of Na+ The Qsolv = [Qsolv(RS03-) Qsolv(Na+)l was then estimated from microemulsion composition. Low values of Qsol in comparison with Qsolv suggested that QWnt was close to Qsolv; in actual measure, the former was higher. These values are
+
(11)Jain, T. K.; Varshney, M.; Maitra, A. J.Phys. Chem. 1989,93, 7409. (12)Wong, M.;Thomas, J. K.; Nowak, J. J . Am. Chem. SOC.1976, 98,2391. (13) Marcus, Y. Ion Soluation; Wiley-Interscience: New York, 1985. (14)Wong, M.; Thomas, J. K.; Nowak, J. J . Am. Chem. SOC.1977, 99,4730.
Langmuir, Vol. 10,No.8,1994 2509
Thermodynamics of Microemulsion Formation
Table 1. Thermometric Results of Water Dissolutionain HeDtane-AOT Mixture at 298 K Additive = 0 18.6 29.4 43.9 55.9
9.3 19.8 31.3 46.8 59.7
9.3 (-2.4) 11.7 (8.1) 11.7 (19.6) 11.7 (35.1) 11.7 (47.9)
36.4 (390) 50.3 (253) 57.8 (184) 62.7 (134) 65.9 (110)
9.6 20.5 32.1 46.5 61.2
10.2 21.6 34.3 49.5 65.2
10.2 (-1.5) 11.7 (9.9) 11.7 (22.6) 11.7 (32.8) 11.7 (53.5)
Additive 0.01 mol dm-3 NaCl 31.9 (313) 4.79 44.1 (202) 5.11 50.4 (147) 5.11 54.4 (110) 5.11 57.2 (88) 5.11
8.6 18.7 29.0 31.8
9.2 19.2 30.9 33.9
9.2 (-2.5) 11.7 (8.2) 11.7 (19.2) 11.7 (22.2)
29.0 (317) 39.2 (197) 44.4 (144) 45.1 (133)
8.4 18.3 29.0 39.5
8.9 19.5 31.0 42.1
8.9 (-2.8) 11.7 (7.7) 11.7 (19.3) 11.7 (30.4)
31.6 (354) 44.3 (228) 51.2 (165) 55.5 (132)
9.1 20.8 33.7 47.2 50.6
9.7 22.1 35.9 50.3 54.0
9.7 (-2.0) 11.7 (10.4) 11.7 (24.2) 11.7 (38.6) 11.7 (42.3)
30.6 (316) 43.0 (194) 49.1 (137) 52.7 (105) 53.5 (99)
8.8
4.69 5.11 5.11 5.11 5.11
50.6 26.0 16.5 11.0 8.7
3.42 4.71 5.42 5.87 6.11
2.56 2.79 2.90 2.88 3.10
47.3 23.9 15.0 10.4 7.9
2.99 4.14 4.73 5.10 5.36
2.36 2.57 2.88 2.95 3.01
51.1 25.9 16.7 15.2
2.72 3.67 4.16 4.23
2.37 2.61 3.03
52.4 26.5 16.6 12.3
2.96 4.15 4.80 5.20
2.44 2.57 2.86 3.16
49.1 23.3 14.4 10.3 9.6
2.87 4.03 4.61 4.94 5.02
2.36 2.54 2.52 2.71 3.05
Additive 0.05 mol dmW3NaCl 4.67 5.11 5.11 5.11
Additive 0.05 mol dm-3 NaC 4.64 5.11 5.11 5.11
Additive 0.01 mol dm-3 NaC 4.73 5.11 5.11 5.11 5.11
a nwt.nwb.and nwfrewesent total moles of water added. moles of bound water, and moles of free water, respectively. mole of water. Expressed per mole of AOT. ,
I
presented in Table 1. The extents ofbound and free water in the studied microemulsion compositions are also presented there. At the lowest water-AOT mole ratio, n&.Am (or w ) , i.e. at minimum water addition, there was no free water left. The negative nwfstands for the deficit amount of water required for complete solvation. At the highest level of water addition (maximum w ) , nearly 80% of the water remained free. Except for w = 8.8, the other sets had water contents greater than the full share of 12 (6 for RS03- and 6 for Na+). The amount of AOT was same in all the sets, the Qsolvvalues were therefore the same. At the initial stage, water penetration and ionic solvation were the major thermochemical events, therefore Qsol was equal to Qpent Qsolv. In Figure 3, Qsol is plotted against the moles of free water (n,? wherefrom the Qsol a t nWf= 0 was found out. These values are written against the curves in the figure. The Qsol-Qso~v (according to eq 2) gave the measure of the endothermic heat of water penetration (Qpent)in the process. Expressed as per mole of added water, it corresponded to the enthalpy of penetration (AH",&,the average ofthe five studied cases was 50.1 kJ mol-l. The enthalpy of evaporation of water is 45 kJ mol-' and the heat of solution of water in heptane is 34.4kJ mol-'. Recognizing that the water penetration process through the amphiphile populated oil/water interface is more ordered, the estimated 50.1 kJ mol-' evaluated for A€Ppent is not unrealistic. At higher levels ofwater addition, QclUshould contribute to Qsol. It has recently been reported3that on the average 100kJ mol-l is the enthalpy of cluster formation in water/ AOTheptane microemulsions. Calculation on this basis finds Qeluato be very minor (only 0.5-2 J)for the studied ternary compositions. Qsolis, therefore, essentially given by eq 2. The data presented in Table 1 show that Qsol varies minorly with w . The other term Qpentis also a decreasing function of w . Expressing Qpentper mole of added water, the AHopnt values at all w have been obtained (column 6 of Table 1). They are found to decrease with increased addition of water. At fxed AOT, bigger droplets
70
r
65
-
60
-
Expressed per
55 -
50
-
+
F"' I
10
201 0
I
I
1
I
I
I
I
10
20
30
40
50
60
70
n$x
io2/
mol
Figure 3. Heat of solution vs free water profiles at 298 K 1, no additive; 2, 0.01 mol dm-3 NaC; 3, 0.01 mol cm-3 NaC1; 4, 0.05 mol dmn3NaC; 5,0.05 mol dm-3 NaCl. The zero free water Qaol values a r e shown against t h e curves with arrows.
were formed with larger addition of water, the oillwater interface was thinly populated with the amphiphile molecules, and water penetration was easier; the Mopent was thus lower. The AHopent vs w-l profile is presented in Figure 4. The excellent linear plot passes through the origin; i.e. LWOpent = 0. This is expected because in excess water the W/O microemulsion is nonexistent and the
Ray et al.
2510 Langmuir, Vol. 10, No. 8,1994
w
u-’ Figure 4. AHopent vs 0-l plot at 298 K 0, no additive; 0 , O . O l mol dm-3 NaC; QO.01 mol dm-3NaC1; A,0.05 mol dm-3NaC1; 8,0.05 mol dm-3 NaC.
Figure 5. Dependence of specific heat of W/AOT/Hp micro-
emulsion on w at AOT/Hp = 10 (moYmo1)in different environments at 298 K 1 , O . O l mol dm-3 NaCl; 2, no additive; 3 , O . O l mol dm-3 NaC; 4,no additive (calculated on the basis of ideal mixing); 5 , 0.05 mol dm-3 NaC; 6, 0.05 mol dm-3 NaCl.
question of water dispersion does not arise; the related the transient fusion of the droplets and exchange of mass energetic parameters ought to be zero. On the higher among them.4 The droplet clustering can thus affect the of side ( w - l > 0.11, the line levels off at 50.1, the epent specific heat by way of altered internal arrangement. All water through a compact amphiphilic interface between the curves in Figure 5 evidence a rapid increase in C, Hp/AOT. above w = 10 (except curve 3) which levels off a t w = 40 Goto et aL5s6have reported that the heats of water for no additive (curve 2) and in presence of 0.01 mol dm-3 dissolution in AOT/oil mixtures are endothermic in nature NaCl (curve 1). The calculated C, values corresponding and have a striking dependence on w . Three different to the curve 2 assuming ideal mixing presented in curve states of water in the micro-water pool have been 4 evidence overestimation at higher w compared with the ~ u g g e s t e d .A~maximum enthalpy of 5.3 kJ mol-l of AOT experimental results. With reference to the conduchas been reported using isooctane as the oil. The present tance-o profiles of the microemulsion system (inset study has also yielded endothermic heat that has sysFigure 11, the trends in specific conductance (a) and C, tematically increased and leveled off a t higher w close to with w are not parallel. The systems with additives (0.01 the phase separation stage. The results also corroborate mol dm-3 NaC1, 0.01 mol dm-3 NaC, and 0.05 mol dm-3 with the findings of D’Aprano et aL7 The trend in the NaC) have sharp rises beyond the range of percolation. partial molar enthalpy of water with w in the work of Although 0.05 mol dm-3 NaCl containing sample showed Goto et al.5 demonstrated three stages of the solvent in no evidence of percolation, the C, increased sharply with the domains of w 5 4, w = 4-11, and w > 11. In the w in the range 10-30. The trend in C, (an index ofinternal present study, w was varied in the range 8.8-61.2; the consistency), therefore, evidenced only a n apparent corvariation in enthalpy was thus smooth and continuous. relation with percolation. However, in deciphering the heat contributed by the The heat capacity results for NaCl and NaC were quite several internal processes, the solvation of AOT molecule different. At equal concentration (0.01 mol dm-3), C, (one of the important states of water in the micropool) declined up to w = 10 for NaC1, thereafter, it increased was considered. and leveled off beyond w = 40. In the case of NaC, there The influence of the salts NaCl and NaC on AWpent was was a gradual rise in C, up to w 30; then it increased perceptible; lowering effects were observed. At equal sharply. At 0.05 mol dm-3 of NaC1, the C,, values concentration, NaCl was more effective than NaC. This systematically rose with a tendency of leveling off a t higher was also true for the two M o s 0 l expressed per mole of w . The reason for the difference between the effects of water and per mole of AOT (given in columns 4 and 7, NaCl and NaC can be ascribed to their place of residence respectively, in Table 1). The salt NaCl essentially in the water pool; the first remained in the bulk whereas remained in the bulk region in the water pool whereas the second remained at the interface. The significant being surface active, NaC preferentially remained a t the increase in C, at higher w in the presence of NaC was a interface; there effects were therefore not identical. striking structural change of the compartmentalized Specific Heat of the Resulting Microemulsion. liquid. But the trends in C, did not correlate with the The specific heat, C,, ofthe microemulsion system was a n percolation behaviors exhibited in the presence of NaCl increasing function of w (Figure 5). At lower w , values for and NaC. The transport behavior (conductance)was not most of the systems were not significantly different. On primarily guided by the internal consistency as reflected the whole the results were parallel to the earlier observaon the C, of the system. t i o n on ~ ~the HzO/TX 100hutanol-Hp system in the lower The significant rise in C, was a reflection of reasonable range of w . The previous system was not of percolating internal structuredness: the microdropletsformed infinite type; therefore, an overall parallelism ofit with the present clusters (as manifested in percolation). The initial system was not expected. Thus the internal thermodydepression in C, in the presence of all the additives was namic conditions of the two systems were not identical. a striking feature which spoke in favor of formation of an For percolating microemulsion systems (as demonstrated initial open structure. in the inset of Figure 11, a t w values exceeding 15, clustering of the microdroplets occurs and as a result Acknowledgement. This work was supported by conductance increases many fold by either “hopping” of CSIR, Government of India, with fellowships to S. Ray the surfactant (AOT) ions from droplet to droplet or by and S. R. Bisal.
