I600
J. Phys. Chem..l981, 85, 1600-1605
Heat Capacities and Volumes of the Ternary System Benzene-Water-2-Propanol Jalme Lara, GOrald Perron, and Jacques E. Desnoyers” Deparfment of Chemlstry, Unlversl de Sherbrooke, Sherbrooke, Quebec, Canada JlK 2R 1 (Received: November 19, 1980; In Final Form: February 6, 198 1)
The densities and heat capacities per unit volume were measured over the whole miscibility region of the ternary system benzene-2-propanol-water. From these data, the apparent molal volumes and heat capacities of benzene and water were calculated. These functions for benzene show large changes in the water-rich region while the functions for water change rapidly in the benzene-rich region. In the intermediate range all changes are gradual. These results suggest that microtransitions,which are analogous to the formation of micelles and inverse micelles, occur in the two extremes of the phase diagram, and in that this system behaves similarly to a microemulsion. The absence of sudden changes in the thermodynamic functions in the intermediate region is consistent with a bicontinuous model for microemulsions.
Introduction Microemulsions have attracted much attention in recent years in view of their important applications in industry, medicine, and oil re~overy.l-~These systems, composed of large proportions of oil, water, surfactant, cosurfactant (usually an alcohol), and sometimes salt, are usually transparent and thermodynamically stable. It is generally believed that the surfactant is the important component in stabilizing these systems. However, in recent years, much emphasis has been placed on the role of the cosurfactant. Alcohols alone can solubilize an appreciable quantity of light oils in water. For example, Vorob’eva and Karapet’yant~,~ Fedoseeva et al.,5 and Kahlweit et ala6 made systematic studies of the phase diagrams of the ternary systems hydrocarbon-alcohol-water. Knickerbocker et al.’ investigated the effect of NaCl on these phase equilibria and Huyskens et alS8studied the effect of alcohols in solubilizing water in organic solvents. Barden et al.9J0 measured many physical properties of the system hexane-water-2-propanol and found that this system has some properties which are typical of microemulsion. Our interest in these systems stems from previous work in this laboratory on the thermodynamic studies of organic-water mixtures. Transitions were found in aqueous solutions of alcohols,11J2alkoxyethanols,13 and amines12 (1) L. M. Prince, Ed., “Microemulsions”, Academic Press, New York, 1977. (2) K. L. Mittal, Ed., “Micellization Solubilization and Microemulsion”, Plenum Press, New York, 1976. (3) K. L. Mittal, Ed., “Solution Chemistry of Surfactants”, Plenum Press, New York, 1979. (4) A. L. Vorob’eva and M. Kh. Karapet’yants, Russ. J. Phys. Chem., 40, 1619 (1966); 41, 602, 1061 (1967). (5) N. P. Fedoseeva, V. M. Kuchumova, L. A. Kochanova, and E. D. Shchukin, Russ. J. Colloid Sci., 39,1055 (1977). (6) C.-U. Herrmann, U. Wiirz, and M. Kahlweit, Ber. Bunsenges. Phys. Chem., 82, 560 (1978); see also ref 3, Vol. 2, p 879. (7) B. M. Knickerbocker, C. V. Pesheck, L. E. Schriven, and H. T. Davis, J. Phys. Chem., 83,1984 (1979). (8) P. L. Huyskens, M. C1. Haulait-Pirson, I. Hanssens, and J. Mullens, J. Phys. Chem., 84, 28 (1980). (9) G. D. Smith, C. E. Donelan, and R. E. Barden, J. Colloid Interface Sci., 60, 488 (1977). (10) B. A. Keiser, D. Varle R. E. Barden, and S. L. Holt, J. Phys. Chem., 83,1276 (1979). (11) C. de Visser, G. Perron, and J. E. Desnoyers, Can. J. Chem., 55,
which were qualitatively similar to micellization. For example, the partial molal heat capacity of the hydrophobic solute went through a maximum or hump and then decreased rapidly to the value of the pure liquid solute in the mole fraction range 0.03-0.15. It seems that these transitions occur mostly with hydrophobic solutes which also have strong acid-base type of interactions with water. It therefore occurred to us that there might be a relationship between the microheterogeneity of these binary systems and the role of the cosurfactant in microemulsions. A study of the heat capacity and volume of the ternary system 2-butoxyethanol (BE)-H20-sodium octanoate (NaOCt)14was therefore made. In one experiment, BE was kept at a constant low concentration while the concentration of NaOct was varied so as to cover the pre- and post-micellar regions. In a second experiment, the surfactant was kept constant at a concentration below the critical micelle concentration and the concentration of BE varied. The heat capacities and volumes of transfer of the two converse systems were qualitatively similar and the changes in the transfer functions in the transitions area were larger than with the corresponding binary systems. This suggested that the microphases of the surfactant (micelles) and of the cosurfactant (BE) were both stabilized in the presence of the other component. In this paper, we will investigate the same two properties for the ternary system alcohol-oil-H20 over the whole miscibility region. As a typical oil, benzene (B) was used since it is one of the few hydrocarbons which is sufficiently soluble in water (1.80g L-l) to allow direct measurements of partial molal heat capacities and volumes. To determine the best alcohol to use for this study we examined the phase diagrams of the ternary systems ROH-H20-benzene using a modified cloud point technique, developed in a parallel study on microemulsions containing ionic surfactant.15 The results are shown in Figure 1. The most effective alcohol is 2-propanol (P) followed closely by 2methyl-2-propanol. Similar conclusions were reached by Herrmann et al.6 Another advantage of 2-propanol is that the changes in the partial molal heat capacities and volumes of the binary system are spread out over a reasonably large concentration region, thus making it easier to see modifications of the changes with the ternary systems.
RK6 (1977). _
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(12) G. Roux, D. Roberts, G. Perron, and J. E. Desnoyers, J.Solution Chem., 9,629 (1980). (13) G. Roux, G. Perron, and J. E. Desnoyers, J. Solution Chem., 7, 639 (1978). 0022-3654/81/2085-1600$01.25/0
(14) G. Perron, R. DeLisi, I. Davidson, S. G6n6reux, and J. E. Desnoyers, J. Colloid. Interface Sci., 79,432 (1981). (15) G. Perron, F. Quirion, D. Hetu, J. Lara, and J. E. Desnoyers, J. Colloid. Interface Sci., submitted for publication.
0 1981 American Chemical Society
The Journal of Physical Chemisfry, Vol. 85,No. 11, 1981 1601
Heat Capacities of Ternary Systems ROH
B4k
a",*( w+
P )
i
P
A
200
0
E
7 Y
160
3
L
Y
v
"
"
v
v
v
"
WATER
'UI
v
BENZENE
160
Figure 1. Phase diagram of the ternary systems alcohol-benzenewater at 25 OC. The numeral refers to the number of carbon atoms in the alcohols.
02
OB
04
WtP
Consequently, 2-propanol was taken as the active component of our "detergentless microemulsion".
Experimental Section The two organic liquids, 2-propanol (Fisher Certified ACS) and benzene (Fisher spectral analyzed), were used as such after drying over molecular sieves. The water was distilled and deionized (Continental deionized water system) and degased. The temperature was kept a t 25 "C throughout the experiments. The heat capacity and density techniques for binary and ternary systems have been well described in our previous ~tudies.ll-'~ In the volumetric heat capacity measurements the flow rate was kept at 0.9397 cm3m i d , the basic power was 21.62 mW, and the temperature change during the measurement was 1.506 K. The present investigation differs to some extent from earlier ones in that the properties are determined over the whole miscibility range. The procedure is as follows. The densities d and heat capacities per unit volume u of the ternary system W + P + B are measured relative to the binary systens W + P or P + B (doand uo). The apparent molal properties of component 3 (W or B) are then calculated from
where the subscripts 1 and 2 refer to W and P or P and B, M are the molecular weights, and cp the specific heat capacities given by CP - CPO =
cpoU1
+ (a - co)/uoIdo/d
- 11
(3)
In addition to direct measurements, it is possible to calculate the apparent molal properties of W in the ternary system from the data on B in the same system. This is done by recalculating the densities and specific heat capacities of the solution a t fixed ratios of P + B. In these measurements, some precautions had to be taken to prevent changes in B concentration due to the high volatility of this component. For this purpose, the sample bottle of the system to be investigated was connected to another bottle containing vapors of a sample of the same composition, thus preventing any evaporation of the volatile components.
06
B
X Flgure 2. Apparent molal volumes and heat Capacities of benzene over the monophasic region of the system benzene-water-2-propanol at 25 OC.
All original data of this investigation are given in the miniprint material.
Results and Discussion Volumes and heat capacities of B were measured under different conditions so as to cover the whole miscibility region. Some examples of +V,B and $C,B data are shown in Figure 2. The two components W and P were kept at a fixed ratio and the mole fraction of B was varied up to saturation. The limiting values as Xp 0 are the standard doca(W+P) and $Ova(W+P). From these standard values and the corresponding values in pure water the thermodynamic function of transfer of B, at infinite dilution, from W to W + P can be calculated. Many data points were obtained in the water-rich region to define the trends in these functions. For mole fraction Xp > 0.1 the solubility of B is sufficiently high to allow reasonably good density and heat capacity measurements. The limiting factor in precision is essentially the determination of the B concentration during the experiments. In view of the volatility of €3 and the difficulty in handling the solutions, we can expect an uncertainty of about 1%on XB and, consequently, an uncertainty of about 2 J K-' mol-' on $c,~(W+ P) and of 0.2 cm3mol-' on $,B(W+P). For Xp 0.1 the solubility of B decreases significantly and so does the precision, Some idea of the accuracy of the data can be obtained from our extrapolated data to pure water, $"C,B(W) and $oV,B(W), with direct measurements from the literature. Heat capacities were measured by Jolicoeur et a1.16 (374 f 2 J K-' mol-') and by Gill et a1.I' (356 f 9 J K-I mol-'). Depending on the method of extrapolation, our 4°C B(W) in water would be between 350 and 365 J K-'mol-l, which is in excellent agreement considering the large uncertainty of these measurements in pure water. Masterton16 obtained a value of 83.2 cm3 mol-' for $ O V B in water while our extrapolation would indicate something of the same order.
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(16)C.Jolicoeur, P. Picker, and G. Perron, Can. J. Chem., 53, 3634 (1975);see also W. L. Masterton, J. Chem. Phys., 22, 1830 (1964). (17)S.J. Gill, N. F. Nichols, and I. Wadeo, J. Chem. Thermodyn., 8, 445 (1976).
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The Journal of Physical Chemistry, Vol. 85, No. 11, 1981
Lara et al.
12* 91.9
100.9 96.1 P9 6I . ., 3
5
= 0.x
2.37 6.14 15.6 11.11
The best way of representing these data is as a profile of $,B(W+P) and $,B(W+P) over the whole miscibility region of the ternary system. These are shown in Figure 3 and 4. The three heavier lines represent the standard values of B in the binary mixed solvent W + P, the apparent molal values of B in the binary system P + B, and the apparent molal values of B along the solubility curve. The lighter lines going from the P + W axis to pure B correspond to the actual ternary systems investigated. The other lines from pure W to the binary P + B axis are added
to better illustrate the changes in the profiles of &a(W+P) and $v,B(W+P). In the water-rich region, ~ c , ~ ( W + are P ) large while $,,(W+P) are small. In this region, both functions are changing with X, and X,, C$~~(W+P) reaching a maximum value near Xp 0.03 and $va(W+P) a minimum value. However, above Xp= 0.05, there are large changes in these functions, especially when X B 0 and other extrema are observed. The large dip in ~c,B(W+P) in the water-rich region rapidly disappears as the concentrations of B and P increase (X, < 0.8). Along the phase change line the
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Heat Capacities of Ternary Systems
The Journal of Physical Chemistty, Vol. 85, No. 11, 1981 1603
V
w
\I 02
06
04
OS
B
X Figure 3. Profile of the apparent molal volume of benzene in the monophasic region of the system benzene-water-2-propanol at 25 OC.
where18J9that similar trends are observed for enthalpies and compressibilities. Thermodynamic properties such as 4v and 4~ are very sensitive to the local average environment of a molecule in solution. Volumes depend on the packing density of the molecules and heat capacities on the energy fluctuations. In the water-rich region, Xw > 0.98, B is obviously in an aqueous environment and the dependences of &B( W+P) and d,B(W+P) on X p and XB are reflecting the various pair interactions B-P and B-B. While the thermodynamic pair interaction parameters have been well studied for alcohols, there are only a few data for benzene in water and these concern mostly free energies.20*21All we can say at the moment for the interactions B-P and B-B is that they appear to be of the same sign as the alcohol-alcohol ones in the case of volumes and heat capacities. At higher concentration of the nonaqueous components, Xw < 0.8, B is now primarily in a nonaqueous medium since +,,B(W+P) and $,,(W+P) are of the same magnitude as 4ca(P) and 4va(P). Since it was shown12that the binary system P + W exists as microphases at high concentrations, B is dissolving preferentially in the organic microphase. To understand the trends in the region where Xw N 0.9, the large dip in $cB(W+P) and large hump in $v,(W+P), it is necessary to recall the trends of the binary system W + P. For example, the 4~ of alcohols in water generally increase slowly with concentration, pass through a maximum in the water-rich region, and then decrease rapidly to their molar heat capacity.12 The changes become sharper as the hydrophobic character increases, and with but~xyethanol'~ approach a first-order transition. The trends in dVof alcohols in water are generally the mirror image of 4 ~ The . region where C#JCdecreases rapidly and 4v increases corresponds to the beginning of the microphase formation. With W P mixtures the extrema occur at about X, = 0.07. The dip in $c,B(W+P) and hump in $,B(W+P) are in the same direction as the changes accompanying microphase formation in the binary system W + P. The same is also true for enthalpies and compressibi1ities.'$lg These trends are also qualitatively similar to the transfer functions of surfactants from water to water plus alcohol solutions and to the transfer functions of alcohols to water plus surfactant solution^.'^ Mixed micelles are formed between alcohols and surfactants, and something similar is occurring between B and P. Not only is B dissolving preferentially in the organic microphases but it enhances the agregation process, in the same way as alcohols lower the critical micelle concentration of surfactants at low concentration. The small change in @Ova(P+W)and $"ca(P+W) as Xp 1 is probably real since it is also observed with other properties presently under consideration. Unfortunately, the changes are too small for us to venture an unambiguous interpretation at present time. The functions $,,(W+P) and @,B(W+P)are rather insensitive to changes occurring in the B-rich region. All interactions are spread over a large number of molecules. To investigate the structural changes in the nonaqueous end, we found it better to examine 4c,w(P+B) and 4v,w(P+B) in the binary and ternary systems. The functions 4c,w(P)and $v,w(P) of the binary system W P are shown in Figure 5. These results are somewhat typical of
+
400
0
300 Y -3
-a + -3
200
m
I&"
-
100
0
w
O2
OS
04
X
OS
B
Flgure 4. Profile of the apparent molal heat capacity of benzene in the monophasic region of the system benzene-water-2-propanol at 25 'C.
variation of +c,B(W+P)is more as expected. The values of $,,(W+P) are much less affected by concentration of B in the water-rich region. The properties of the binary system P-B are nearly ideal on this scale and c$C,~(W+P) and # J ~ ~ ( W +change P ) gradually from their standard value in pure P to their molar value in pure B. With the exception of the small changes in the P-rich end, the changes in ~c,B(W+P) and 4,B(W+P) are all quite regular in the central part of the phase diagram. It will be shown else-
+
(18)J. Lara, L. Avedikian, G. Perron, and J. E. Desnoyers,J.Solution Chem., in press. (19)J. Lara and J. E. Desnoyers, manuscript in preparation. (20) E. E. Tucker and S. D. Christian,J.Phys. Chem., 83,426 (1979). (21)W.T.Green and H. S. Frank, J. Solution Chem., 8,187 (1979).
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The Journal of Physical Chemlstty, Vol. 85,No. 11, 198 1
Lara et al.
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15
I
1
w
I
I
06
02
04
P+B
Flgure 5. Apparent molal volumes and heat capacities of water in the binary system water-2-propanol at 25 "C.
Io
01
P
A
I
02
0 4
X
I
I
06
0 8
O B
0 4
08
X
P
O 8
X
300
02
W
Figure 7. Some examples of the apparent molal volumes and heat capacities of water over the monophasic region of the system benzene-water-2-propanol at 25 "C.
15
I
8
Flgure 6. Standard apparent molal volumes and heat capacities of water in the binary system 2-propanol-benzene at 25 OC.
aqueous-organic mixtures; c $ ~ , ~ decreases (P) in a fairly regular fashion from its molar value to about 15 cm3mol-' while q!~c,~(P) remains very large even when Xp 1. Similar trends were observed for water in other alcoh ~ l s , ~ acetonitrile,22 'J~ and amides.23 On the other hand, the binary system P + B appears to be quite regular. As seen in Figures 3 and 4,~ c , B ( P and ) &$(P) change nearly linearly with X,. However, if a trace of water (0.024.1 mol kg-l) is placed in this binary system, large changes are observed. This can be seen in Figure 6 where the standard functions 4°V,w(P+B)and 4°c,w(P+B) are plotted against the mole fraction of the binary system. These functions change very rapidly in the benzene-rich region. By analogy with inverse micelles, it would seem that the presence of water causes the P molecules to re-
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(22) R. Zana, G. Perron, and J. E. Desnoyers, J. Solution Chem., 8, 729 (1979). (23) C. de Visser, W. J. M. Heuvelsland, and G. Somsen, J. Solution Chem., 7, 193 (1978).
V
W
0 8
06
0 4
X
02
B
Figure 8. Proflle of the apparent molal heat capacity of water in the monophasic region of the system benzene-water-2-propanol at 25
"C.
orient themselves such that the OH groups will form hydrogen bonds with water. Some examples of (bc,w(P+B)and 4V,w(P+B)in the ternary system are shown in Figure 7 for increasing mole fractions of W. The data for X B = 0.44 and 0.76 were measured directly while those at XB= 0.1 were calculated from the data of (bca(W+P) and 4va(W+P) in the ternary systems. As expected from Figure 6, the larger changes are observed in the B-rich region. The three-dimensional profiles of 4,w(P+B) and 4 v , ~ (P+B) over the whole miscibility region are shown in Figures 8 and 9. All these changes are fairly gradual except in the region where XB 1. The large changes occur when there is relatively few W and P molecules in an excess of B molecules. The structure of the system in this region probably has some resemblance to reverse
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The Journal of Physical Chemistty, Vol. 85, No. 11, 198 1 1605
Heat Capacities of Ternary Systems
22
well-known microemulsion toluene-water-l-butanol-sodium dodecyl sulfate show very similar trends for r,hoc,T and 4°V,Tof toluene (T) as the mole fraction of the pseudobinary system changes from pure H20 to the active mixture 1-butanol-sodiumdodecyl sulfate. This reinforces the claim of Barden et aL9J0that the ternary systems hydrocarbons-water-2-propanol have many of the features of microemulsions, even though we cannot observe some of the transitions they seem to observed with the system hexane-W-P. Our data would be more consistent with a bicontinuous model for the central part of the phase diagram, as suggested by Schriven,26since only gradual changes seem to occur in the thermodynamic properties. Some of the work in progress on conductivities might help to further define these systems.
1
Acknowledgment. We are grateful to the National Science and Engineering Research Council of Canada and to the Quebec Ministry of Education for financial support. We also thank Professor Viallard for making some of his results on microemulsions available to us prior to publication.
w
E
X
Flgure 9. Profile of the apparent molal volume of water in the monophasic region of the system benzene-water-2-propanol at 25 OC.
micelles, the H20 molecules helping to promote P-P association through H bonds. It should be mentioned finally that parallel studies in C l e r m ~ non t ~the ~ same thermodynamic properties of the
Miniprint Material Available: Full-sized photocopies of the Appendix giving all the original data are available (10 pages). Ordering information is available on any current masthead page.
(24) A. Roux, G. Roux-Desgranges, J. P. Grolier, and A. Viallard, J. Colloid Interface Sci., in press. (25) L. E. Schriven in ref 2, Vol. 2, p 877.
