Fluid Phase Equilibria, 25 (1986) 209-230 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

209

ROLE OF ALCOHOL IN MICROEMULSIONS. 111. VOLUMES AND HEAT CAPACITIES IN THE CONTINUOUS PHASE WATER-n- BUTANOL-TOLUENE OF REVERSE MICELLES *

GENEVIEVE ROUX-DESGRANGES ** and JEAN-PIERRE E. GROLIER **

Laboratoire de Thermodynamique et Cin&ique Chimique, UA C. N. R.S. 434, Unioersitk de Clermont - Ferrand 2, 63170 Aubikre (France)

MIGUEL ANGEL VILLAMARAN *** and CARLOS CASANOVA

Departamento de Termologia. Fact&ad de Ciencias, Universidad de VaNadolid, Valladolid

(Spain)

(Received March 2, 1985; accepted in final form August 2, 1985)

ABSTRACT

Roux-Desgranges, G., Grolier, J-P.E., Villamahan, M.A. and Casanova, C., 1986. Role of alcohol in microemulsions. III. Volumes and heat capacities in the continuous phase water-n-butanol-toluene of reverse micelles. Fluid Phase Equilibria, 25: 209-230.

Densities and volumetric heat capacities of the ternary system water (l)+ n-butanol (2)+ toluene (3) were determined in the homogeneous single phase region of the diagram. From these quantities the apparent molar volumes V+, and heat capacities C,, of species i are calculated. An analysis of the variation of these apparent molar quantities as a function of concentration is carried out taking either water, n-butanol or toluene as a molecular probe of the structural behavior of the medium. In particular, apparent molar properties of n-butanol,

V& and Cal, in the binary n:butanol + toluene and even more the apparent molar properties of water at infinite dilution, V:tand Cl,, in the ternary water+ n-butanol+ toluene show sharp changes in the toluene rich domain of the diagram. These changes are due to the self-association of n-butanol in the binary; the self-association being most likely enhanced by water in the ternary. This kind of behavior confirms the ‘detergentless microemulsion’ nature of such ternary systems where alcohol acts as both,a surfactant and a cosurfactant.

* Communicated in part at the 3rd International Conference on Thermodynamics of Solu- tions of Non Electrolytes - Universite de Clermont-Ferrand 2, Aubitre, France, 2-6 July 1984, the proceedings of which were published in Volume 20 of this journal. ** To whom correspondence should be addressed *** Present address: E.T.S. de Ingenieros Industriales, Universidad de Valladolid, Vallado- lid, Spain.

037%3812/86/$03.50 0 1986 Elsevier Science Publishers B.V.

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INTRODUCTION

In recent years interest in microemulsions has grown rapidly due to their numerous applications in many fields. Usually these systems are formed by mixing water (with or without salt), a surfactant, a cosurfactant (usually an alcohol) and hydrocarbons. For some time, the surfactant has been consid- ered as a key constituent in such systems, however, recent studies have emphasized the important role played by the alcohol (Roux-Desgranges et al., 1982). This behavior is merely the consequence of the typical self-associa- tion of an alcohol in aqueous (De Visser et al., 1977; Iwasaki and Fujiyama, 1977, 1979; Roux et al., 1978, 1980) as well as in nonaqueous solutions (Nagata, 1977; Costas and Patterson, 1985). This self-association takes place above a critical concentration as for a surfactant. What is remarkable is that, in the presence of a surfactant, weakly water-soluble or even insoluble alcohols can be solubilized in mixed micelles, i.e., surfactant + alcohol mixed micelles. Thus, with a given surfactant concentration, rather large quatities of alcohol can be solubilized in a process where the surfactant favors the formation and the stabilization of alcohol microaggregates (Roux-Des- granges et al., 1982; Majer et al., 1983).

In this respect the study of the ternaries water + alcohol + hydrocarbon is essential to understand the quaternary systems such as microemulsions. As concerns the latter systems it has been shown (Bellocq et al., 1979) that the structures present in these quaternary systems are highly composition depen- dent. Typically in the water rich region so-called direct micelles predominate in water, considered as the ‘continuous phase’, while reverse micelles are the main organized structures in the organic domain of the phase diagram. According to the picture proposed by Graciaa (1978) and generally adopted, the reverse micelles are water droplets surrounded by membranes made of surfactant, alcohol and hydrocarbon and dispersed in an organic (hydro- carbon + alcohol + water homogeneous phase) ‘continuous phase’. In regard to the composition and stability of reverse micelles the importance of the continuous phase has been stressed by Biais et al. (1981a, b). Their ‘pseudo- phase’ model used to interpret the properties of reverse micelles is based on the self-association of the alcohol in the continuous organic phase, and its distribution between the core and membrane of the droplets, and the continuous phase. This phase is then acting as a ‘feedstock’ of alcohol to maintain constant the micelle composition: therefore, the study of its ther- modynamic properties is essential. In these ternary systems the alcohol favors the solubilization of appreciable quantities of water in hydrocarbons, Systematic investigations of ternary phase diagrams (water + alcohol + hydrocarbon) have been made by Vorob’eva and Karapet’yants (1967), Herrmann et al. (1978) and Huyskens et al. (1980). On the basis of

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conductivity and centrifugation data as well as visual examination Smith et al. (1977, 1982) established that the systems water + 2-propanol + n-hexane and water + 2-propanol -t toluene behave like microemulsions at certain compositions although a surfactant is not present. Their expression ‘deter- gentless microemulsion’ was further confirmed and adopted by Lara et al. (1981a,b) who studied the thermodynamic properties of the ternary system water + 2-propanol + benzene. Presently, thermodynamic investigations of such ternary systems are in progress to understand better more complex systems like microemulsions (Biais et al., 1982; Backlund et al., 1984).

We previously studied (Roux et al., 1981, Roux-Desgranges et al., 1981) some thermodynamic properties of the quaternary system water + sodium dodecylsulfate + n-butanol + toluene considered as a model system of micro- emulsions. In the same way the ternary system water + n-butanol + toluene is treated as a model system for studying the organic continuous phase of these microemulsions. We report here, for this ternary system, thermody- namic properties such as volumes and heat capacities which are particularly sensitive to structural changes in liquid solutions.

EXPERIMENTAL

n-Butanol and toluene (stated purity > 99 mol% for both components) were puriss grade reagents from Fluka. Prior to actual measurements all liquids were carefully dried with a molecular sieve (Union Carbide type 4 A beads from Fluka) and used without further purification. At 298.15 K, our observed densities are p(g cmp3) = 0.80573 for n-butanol and p(g cme3) = 0.86219 for toluene and our observed heat capacities are Cp (J K-’ mol-‘) = 175.97 for n-butanol and Cp (J K-’ mol-‘) = 157.08 for toluene, values which are in good agreement with the most reliable literature values as can be seen from Table 1. Water used for solutions was deionized degassed doubly distilled water. All solutions were prepared by weighing. Densities, p,

of pure liquids and solutions were determined with a vibrating-tube densime- ter from Sodev (Model 02 D). Heat capacities per unit volume, u, were measured using a Picker flow microcalorimeter from Setaram. For both measurements, temperature was controlled to better than +0.003 K, as checked by a quartz thermometer (Hewlett-Packard, Model 2801 A). The maximum inaccuracy of the temperature readings is estimated to be less than kO.01 K. All experimental procedures were the same as used previously (Picker et al., 1971, 1974; Roux et al., 1981; Roux-Desgranges et al., 1981) all measurements being performed at 298.15 K.

The phase diagram (Fuoss, 1943) of the ternary system water (1) + n- butanol(2) + toluene (3), as shown in Fig. 1, is characterized by a homogen- eous domain in two parts, a rather large one in the organic region and a

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TABLE 1

Experimental densities, p, and molar heat capacities, Cp, at 298.15 K and atmospheric

pressure of toluene and n-butanol

Toluene

n-Butanol

P (g cmm3)

This work

0.86219

0.80573

Literature

0.86219 a 0.86224 b 0.86228 ’

0.80575 s 0.80586 h

Cp (J K-’ mol-‘)

This work Literature

157.08 157.08 d 157.00 e 157.20 ’

175.97 177.02 ’

a Hales and Townsend, 1972.

b Tanaka et al., 1975. ’ API Project.44, 1952. d Fortier and Benson, 1977.

’ Holzhauer and Ziegler, 1975. ’ Scott et al., 1962. s Hales and Ellender, 1976.

h Treszczanowicz and Benson, 1977.

’ Counsel1 et al., 1965.

and narrow one in the aqueous dilute region. Measurements of densities and of heat capacities were made for solutions, in the homogeneous organic phase, along .dilution lines from initial b.inaries, water + n-butanol and n-butanol + toluene, at given concentrations, by either toluene (line T) or water (line W), respectively. The values of the corresponding thermodynamic

BUTANOL

H20 TOLUENE

Fig. 1. Ternary phase diagram of the system water (l)+ n-butanol (2)+ toluene (3). Hatched area is the non-homogeneous region. The straight lines T represent the dilution lines by toluene. The straight lines W represent the dilution lines by water. Scale in mole fraction.

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namic properties for the initial binaries were measured against the related pure components taken as references.

The heat capacities per unit mass, cP, are calculated according to the following equation where subscript o refers to each initial binary taken as the reference ‘solvent’ for a given dilution line

0)

Apparent molar volumes and heat capacities for component i are calculated from experimental p and cP data using the usual relations

C,, = M,c, + 103kp - cp,,>

mi

(4

(3)

where Mi is the molar mass and m, the molality of component i in the initial binary reference ‘solvent’.

RESULTS AND DISCUSSION

When dealing with such ternary systems it is appropriate to look at the partial or apparent properties of one component taken as a molecular probe to investigate the molecular interactions in the medium. The variations of these properties when the concentration varies reveal the changes in struc- ture which take place in the medium around the molecular probe (Roux et al., 1981; Roux-Desgranges et al., 1981). To carry out the analysis from our experimental data we use the apparent properties of toluene and of water along the respective dilution lines T and W (see Fig. 1).

To conduct the discussion it is convenient to consider three regions of interest in the single phase of the diagram: the bulk solution and the two regions where either toluene or water is at infinite dilution, i.e., along the water + n-butanol and n-butanol + toluene sides of the diagram, respec- tively. Furthermore, it is interesting to take into account the properties of the binary itself when discussing the behavior of the third component at infinite dilution in the binary. The apparent molar properties at infinite dilution of the solute in the binary solvent reflect the interactions of a molecule of solute with its environment since the solute-solute interactions are negligi- ble. In fact the values of these properties and their variations with concentra- tion give an indirect insight on the local composition and therefore on the microstructures ‘seen’ by a molecule of solute, and their evolution. Using this kind of approach we have evidenced (Roux et al., 1981; Roux-Des-

214

granges et al., 1981) the remarkable structural changes which take place in the microemulsion water-sodium dodecylsulfate-n-butanol-toluene, in con- sidering toluene at infinite dilution in the ternary ‘solvent’ water + sodium dodecylsulfate + n-butanol.

The apparent molar volumes at infinite dilution V$ and the apparent molar heat capacities at infinite dilution Cli are obtained by extrapolation of values given, respectively, by eqns. (2) and (3) when the concentration of species i tends toward zero. To obtain a detailed picture of these properties .)ver the concentration range corresponding to the homogeneous region of each binary a great number of experimental data would be necessary. However, the variations of V,; and C,, being rather small along dilution lines (see Figs. 2 and 3), it is possible to consider the values of these properties at a finite but small molality of i (WI,) as the values at infinite dilution denoted by ‘.

The values of the apparent molar properties in the binary systems are listed in Table 2 for water in the binary water + n-butanol along with the mole fraction of water x1, and in Table 3 for n-butanol in the binary

0 0.1 0.2 0.3 x3

Fig. 2. Apparent molar volumes Vex and heat capacities C,+* of toluene in water (1) + n-butanol (2)+ toluene (3) at 298.15 K versus the toluene mole fraction xgr along dilution lines (T) by toluene, from initial binary mixtures water (l)+ n-butanol (2) in which x, = H, 0.44; 0, 0.35; A, 0.31; A, 0.25; ., 0.21; 0, 0.15.

215

Fig. 3. Apparent molar volumes V+, and heat capacities C& of water in water (l)+ n-butanol (2)+toluene (3) at 298.15 K versus the water mole fraction x1, along dilution lines (W) by water from initial binary mixtures n-butanol (2)+ toluene (3) in which x2 = 0, 0.36; A, 0.57;

0, 0.84.

n-butanol + toluene along with the mole fraction of n-butanol x2. The values of the apparent molar properties of toluene at infinite dilution, k$ and C,$, obtained as indicated previously, in the binary water + n-butanol are listed in Table 4. In the same way, those for water, V:, and Ci, in the binary n-butanol + toluene are gathered in Table 5.

Properties of concentrated solutions

Curves showing the variations of the apparent molar properties of toluene,

I& and &, versus the mole fraction of toluene are given in Fig. 2 for different compositions x1 of the binary water (1) + n-butanol (2), i.e., along different T-lines. The apparent molar properties vary smoothly; in all cases VG3 is almost constant, close to the value of toluene at infinite dilution in n-butanol(106.3 cm3 mol-‘) and C,, slightly decreases from a value close to that of toluene at infinite dilution in n-butanol(204.7 J K-’ mol-‘) toward the value of heat capacity of pure toluene (157.08 J K-’ mol-‘). This shows

216

TABLE 2

Water (l)+ n-butanol(2) binary system (homogeneous phase) at 298.15 K

Xl (“g Cm-3) V +1 c (cm3 mol-‘)

01 (J K-’ mol-‘)

0.0145 0.806508 16.32 2.3920 133.5 0.0387 0.807813 16.42 2.4203 128.7

0.0662 0.809351 16.46 2.4508 124.4

0.0949 0.810957 16.56 2.4816 120.7

0.1434 0.813781 16.70 2.5323 115.7 0.1488 0.814078 16.73 2.5381 115.3

0.2097 0.817820 16.90 2.6046 111.3 0.2521 0.820708 16.97 2.6511 108.7 0.3140 0.825288 17.07 2.7266 106.2 0.3495 0.828106 17.13 2.7652 103.8 0.4022 0.832743 17.20 2.8413 102.7 0.4475 0.837063 17.27 2.9067 101.1

Densities p and specific heat capacities at constant pressure cP; apparent molar volumes V+,

and heat capacities C,, of water (1) in the binary system; x,, mole fraction of water.

TABLE 3

n-Butanol (2)+ toluene (3) binary system at 298.15 K

x2

(“g cme3) b2

C (cm3 mol-‘) ff K-1 g-‘) (.G1 mol-‘)

0.0112 0.861483 93.76 1.7134 197.8 0.0239 0.860725 93.53 1.7348 241.8 0.0248 0.860668 93.55 1.7373 246.9 0.0292 0.860428 93.43 1.7460 256.2 0.0349 0.860102 93.35 1.7598 270.8 0.0422 0.859691 93.27 1.7737 275.8 0.0495 0.859292 93.18 1.7895 282.7 0.0650 0.858508 92.95 1.8156 281.6 0.0742 0.857978 92.94 1.8298 279.5 0.0808 0.857606 92.93 1.8384 276.4 0.0992 0.856628 92.83 1.8651 272.5 0.1498 0.853992 92.62 1.9221 256.2 0.1751 0.852708 92.53 1.9473 249.6 0.1972 0.851568 92.47 1.9667 244.1 0.2487 0.848841 92.40 2.0121 234.7 0.2965 0.846398 92.31 2.0518 228.0 0.3987 0.840920 92.21 2.1238 215.7 0.5467 0.833064 92.08 2.2159 203.3 0.6995 0.824611 91.98 2.2909 193.0 0.8446 0.815906 91.96 2.3403 184.2

Densities p and specific heat capacities at constant pressure cP; apparent molar volumes Vez and heat capacities C,, of n-butanol(2) in the binary system; x2, mole fraction of n-butanol.

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TABLE 4

Ternary system water (l)+ n-butanol (2)+ toluene (3) in the homogeneous part at 298.15 K

Xl m3 (mol kg-‘) ;g cm-)

VI3 CB ) (cm3 mol-‘) ;; K-’ g-‘)

rp3 (J K-’ mol-‘)

0.01996 0.2031 0.807612 106.58 2.3943 203.9

0.0401 0.2063 0.808693 106.67 2.4171 203.3 0.0599 0.2078 0.809916 106.75 2.4378 202.7 0.0799 0.2078 0.811004 106.81 2.4591 201.8 0.1001 0.2057 0.812218 106.87 2.4798 200.8 0.1500 0.2067 0.815029 106.99 2.5325 200.9 0.2746 0.2035 0.823086 107.19 2.6688 200.7 0.3752 0.2104 0.830922 107.36 2.7918 201 .o 0.4494 0.2075 0.837783 107.50 2.8899 199.8

Densities p and specific heat capacities c,, of the solutions, determined at molalities m3 of

toluene versus the water mole fraction x, in the binary water+ n-butanol, and the corre- sponding apparent molar volumes and heat capacities of toluene (3) (the latter values are

taken as the values at infinite dilution V13 and Ci3 (see text)).

TABLE 5

Ternary system water (l)+ n-butanol (2)+ toluene (3) in the homogeneous part at 298.15 K

x2 ml

(mol kg-‘) Fg cmm3) $I

(cm3 mol-‘) ;$ K-r g-‘) q1

(J K-’ mol-‘)

0 0.0163 0.0248 0.0213 0.0349 0.0198 0.0495 0.0275 0.0650 0.0346 0.0742 0.0469 0.0992 0.0561 0.1498 0.0740 0.1751 0.0704 0.2487 0.1154 0.2965 0.1051 0.3987 0.1456 0.5467 0.1501 0.6995 0.1371 0.8446 0.1671 1 0.1191

0.862184 21.22 _ _

0.860114 20.13 0.859315 19.83 0.858563 18.83 0.858052 18.85 0.856758 17.87 0.854180 17.61 0.852881 17.74 0.849125 17.80 0.846671 17.65 0.841297 17.75 0.833489 17.54 0.825051 17.12 0.816487 16.85 0.806207 16.23

1.7055 79.8 1.7424 270.8 1.7638 233.8 1.7937 185.0 1.8211 191.8 1.8367 180.2 1.8717 149.6 1.9296 136.1 1.9537 126.1 2.0206 110.1 2.0586 101.8 2.1310 87.8 2.2231 88.0 2.2988 99.0 2.3519 111.8 2.3851 135.3

Densities p and specific heat capacities cP of the solutions determined at molalities m, of water versus the n-butanol mole fraction x2 in the binary n-butanolt toluene, and the corresponding apparent molar volumes and heat capacities of water (1) (these latter values are taken as the values at infinite dilution V,“1 and Cz, (see text)).

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that in the bulk region of the homogeneous domain toluene is always in an ‘organic environment’ without important modifications of the local struc- ture.

The variations of the apparent molar properties of water, V+,r and C+r, versus the mole fraction of water are shown in Fig. 3 for different composi- tions x2 of the binary n-butanol (2) + toluene (3)-i.e., along different W-lines. These variations are more pronounced than those of k& and C,, and their dependence with the composition of the initial binary is also more pronounced. Values of V+r are smaller than the value for pure water (VP = 18.069 cm3 mol-I); as the water content increases (along dilution lines W) V,, decreases in the toluene rich domain but increases in the n-butanol rich domain. On the contrary, the C,, values are all larger than the value for pure water (C,” = 75.29 J K-l mol-‘): C,, decreases in the n-butanol rich domain and increases in the toluene rich domain. At this stage it is worth noting that, for a constant mole fraction of water (Fig. 3), the values of C,+r , as functions of the initial binary composition, are a minimum near the mole fraction x2 equal to 0.5 (we come back to this point later in the discussion). This type of variation of V,, and C,, would be the consequence of a local microstructure around water molecules which is dependent on the composition of the binary. Since apparent molar proper- ties of water reflect not only the evolution of the local microstructures surrounding water molecules but also all interactions between three compo- nents involving a large number of molecules, it is difficult to attribute all of these variations in V+r and C+r only to changes in the local microstructures around water molecules.

Properties of toluene at infinite dilution in binaries water + n-butanol

The composition dependence of apparent molar volumes and heat capaci- ties of both water and n-butanol in their binary mixture are represented, versus the mole fraction x1 of water, Fig. 4. The composition dependence of apparent molar volumes V13 and heat capacities C,$ of toluene at infinite dilution in different binary mixtures water (1) + n-butanol (2) are repre- sented in Fig. 5, versus the water mole fraction x1. Variations of Vi3 and Ci3 appear to be rather monotonous showing nevertheless a change in the slope -almost a break- at a value of x1 around 0.1. This change, although small, toluene being mainly in an organic environment, is significant and would reflect a modification of the structure of the medium in the presence of small quantities of water. This interesting behavior must be referred to the same observation made in the case of benzene solubilized in water + isopropanol mixtures at small water concentrations (Lara et al., 1981a,b).

219

“@ cm’ ma c

1

7

6.

Fig. 4. Apparent molar volumes V+ and heat capacities C, at 298.15 K of water (1) n-butanol(2) in the binary water (l)+ n-butanol(2) versus the water mole fraction x,: 0,

n , C+,; A, v,,; A, C+z.

and

c$1;

In the water + n-butanol binary mixtures n-butanol can be considered as the solvent in the n-butanol rich domain; its apparent molar properties I$,* and C,, smoothly change with the water concentration (Fig. 4) remaining close to the values for pure n-butanol (91.966 cm3 mol-’ and 175.97 J K-’ mol-‘, respectively). This behavior is typical of binary mixtures water + alcohol (De Visser et al., 1977; Roux et al., 1980) or water + alkoxyethanol (Roux et al., 1978). Consequently, water can be considered as a solute and the variations, which are rather important, of its apparent molar properties I& and C,, (Fig. 4) are typical of the behavior of water in polar organic or associated solvents (De Visser et al., 1977, 1978). The value at infinite dilution I$ (16.23 cm3 mol-‘), which is in good agreement with the value given by Sakurai and Nakagawa (1984), is smaller than the value for pure water and it increases with the water concentration. The value at infinite dilution, C$ (135.3 J K-’ mol-‘), is much larger than the value of the molar heat capacity of pure water (75.29 J K-’ mol-‘) and decreases rapidly as the water concentration is increased (Fig. 4). Compared to the values for pure

210 _

G3

JKfnor

I I I I 0 0.2 0.4

Xl

Fig. 5. Apparent molar volumes V$ and heat capacities C& of toluene at infinite dilution of toluene (3) in binary mixtures water (l)+ n-butanol (2) at 298.15 K versus the water mole

fraction x’~ in the binary.

water smaller values of I$, and larger values of C,, are not easy to explain. They could correspond to an ‘enhanced structuration’ as compared to the

structure of water in pure liquid water. Recently, De Granpre et al. (1982) combined infra-red studies and heat capacity measurements to understand the behavior of water at small concentrations in an alcohol. It appears that the IR absorption band shifts are small and that the HO-HOR and the HO-HO interactions have similar spectroscopic intensities. This confirms that water and alcohol molecules are closely bounded together by hydrogen bonds leading then to a rather strongly structured ensemble. When the water concentration remains very small (i.e., in the dilute water region) in the water + n-butanol, the apparent molar properties of water seem to reflect a change in structure which may be interpreted as follows: the self-associated structures of alcohol are modified by addition of small amounts of water which in turn create a new molecular organization through strong hydrogen- bonds with the assumption of water molecules trapped in ‘reverse micelle- type’ structures made of alcohol molecules. When toluene is added to this

221

medium its interaction with its local environment is affected as reflected (Fig. 5) by V$ and C,$; the variation of V$ versus x1 being more monotonous.

Properties of water at infinite dilution in bharies cbutanol + toluene

The composition dependence of apparent molar volumes and heat capaci- ties of both n-butanol and toluene in their binary mixtures is represented, versus the mole fraction x2 of n-butanol, in Figs. 6 and 7. The composition dependence of apparent molar volumes V$ and heat capacities C,$ of water at infinite dilution in different binary mixtures n-butanol + toluene is repre- sented in Figs. 8 and 9, respectively, versus the n-butanol mole fraction. An analysis of these different curves shows a particular and similar behavior of water and n-butanol in the sense that the variations of their respective apparent molar properties are parallel in the toluene rich part of the diagram. The apparent molar volumes of both components (n-butanol and water) decrease rapidly (Figs. 6 and 8) when the alcohol concentration increases. Simultaneously their apparent molar heat capacities exhibit a

9

4_

Fig. 6. Apparent molar volumes V& and heat capacities C,, at 298.15 K, of butanol in the binary mixture n-butanol (2)+toluene (3) versus the n-butanol mole fraction x2: A, V&; A, c 62.

222

Fig. 7. Apparent molar volumes V& and heat capacities C&, at 298.15 K, of toluene in the binary mixture n-butanol (2)+toluene (3) versus the n-butanol mole fraction x2: 0, Vej; 0,

C+3.

sharp maximum (Figs. 6 and 9) at low alcohol concentration. On the contrary, the values of apparent molar properties of toluene (Fig. 7) vary progressively when crossing the whole concentration range from the value for pure toluene to the value of toluene at infinite dilution in n-butanol.

The binary n-butanol + toluene It is clear that the knowledge of the structure and of the behavior of the

binary system n-butanol + toluene (Figs. 6 and 7) is essential to understand and interpret the behavior of water in the ternary system water + n-butanol + toluene.

The variations of V& and C,, for toluene do not show significant trends meaning that toluene is not involved in particular structures. On the other hand, the peculiar variations of I’,, and C,, observed in the toluene-rich domain are typical of structural changes involving n-butanol. V,, decreases rapidly from the value for n-butanol at infinite dilution in toluene ( - 94 cm3 molt) until at mole fraction x2 - 0.1 and then decreases smoothly to eventually remain almost constant. As for Ce2, it goes through a sharp maximum at x2 - 0.05 and decreases gradually toward the value of CP for pure n-butanol. This maximum corresponds to a large value in heat capacity terms- C+y - 282 J K-’ mol-‘-as compared to the respective values for pure n-butanol (175.97 J K-i mol-‘) and for n-butanol at infinite dilution in toluene (- 170 J K-’ mol-‘).

223

16

Fig. 8. Apparent molar volumes Vi, of water at infinite dilution, at 298.15 K, in binary

mixtures n-butanol (2)+ toluene (3) versus the n-butanol mole fraction x2 in the binary. 0 values obtained from direct measurements at small finite water molalities (m,) (see Table 4).

l Values extrapolated at molalities m, = 0 along dilution lines W.

These peculiar variations of the apparent properties of n-butanol diluted in toluene are typical for alcohols in hydrocarbons. Similar effects have been observed with other thermodynamic properties of alcohol-hydrocarbon sys- tems. See, for example, works by Van Ness et al. (1967) Stokes and Adamson (1976) Treszczanowicz et al. (1981) Kumaran et al. (1983a,b), Costas and Patterson (1985). As expected, this kind of behavior is even more spectacular with alkanes than with aromatics due to weaker intermolecular interactions in the former case. Qualitatively, the self-association of alcohols in more or less inert solvents is responsible for such a behavior. At infinite dilution of n-butanol in toluene the values of F’$ and C& correspond to the properties of alcohol monomers in toluene. When the alcohol concentration increases, the alcohol-alcohol interactions by means of hydrogen bonds increase and, as a consequence, the apparent volume of the associated species appears smaller than for the free monomers. As concerns the sharp maximum of C,, it can be attributed to the shift with temperature of the association equilibrium of the alcohol (Costas and Patterson, 1985). The

224

Fig. 9. Apparent molar heat capacities C$ of water at infinite dilution, at 298.15 K, in binary mixtures n-butanol (2)+ toluene (3) versus the n-butanol mole fraction x2 in the binary. 0

Values obtained from direct measurements at small finite water molalities (m,) (see Table 4). l Values extrapolated at molalities m, = 0 along dilution lines W.

magnitude of this maximum is mainly due to the temperature dependence of the enthalpy of the hydrogen bonds: the stronger the hydrogen bonds the higher the maximum of C,,. This kind of phenomenon can be considered as general and is also encountered, for example, with solutes which are associ- ated in solution through an equilibrium between monomers and associated species: In particular, sharp maxima for the apparent molar heat capacities have been evidenced with aqueous solutions of surfactants at the critical micelle concentration (cmc) (De Lisi et al., 1980) and further proposed as a mean to characterize solutions where micellization, or more generally a

-‘transition’, takes place (Roux-Desgranges et al., 1985). The magnitude of the changes which undergoes the heat capacity at the vicinity of these ‘ transitions’ depends upon different parameters: the association constant of the I ‘structure’ and the corresponding enthalpy change, as well as the properties of pure components.

Several theoretical models have been proposed to interpret the properties of alcohol + hydrocarbon mixtures. These approaches use either a simple

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self-association model between monomers and associated species (Roux et al., 1984; Costas and Patterson, 1985) or an augmented self-association model taking into account additional terms such as physical or structural interactions (Kretschmer and Wiebe, 1949; Renon and Prausnitz, 1967; Smith and Brown, 1973; Nagata, 1977). Usually these models give a fairly good, at least qualitative, representation of the thermodynamic properties; in particular, the more recent calculations by Roux et al. and Costas and Patterson reproduce, almost quantitatively, the sharp maximum of the C,,‘s in the dilute region.

The ternary system: water + n-hutanol + toluene

The most striking feature of our present results is the shape of the curves V$ and C,$ (Figs. 8 and 9) for water at infinite dilution in The binaries n-butanol + toluene. These curves, similar to those obtained for n-butanol (Fig. 6), show that, qualitatively, the observed phenomena are similar, being more marked in the case of water. A large decrease in V:, followed by a break is observed (Fig. 8) at mole fraction of butanol x2 - 0.1, then V$ remains almost constant between 0.1 and 0.5 and eventually goes downward to the value (- 16.3 cm3 rnol-‘) for the molar volume of water at infinite dilution in n-butanol. Cl, goes through a very sharp maximum (Fig. 9) practically a peak, at x2 = 0.03, decreases rapidly and then goes through a shallow minimum around x2 - 0.5 after which it increases again up to the ‘high value’ (- 136 J K-’ mol-‘) for water at infinite dilution in n-butanol.

Again we note here the large difference between C$max ( = 270 J K- ’ molt ‘) and the value of heat capacity for water at infinite dilution in n-butanol (= 136 J K-’ mol-‘) or in toluene (80 J K-’ mol-‘). It is also interesting to

note the properties of water at infinite dilution in toluene. Its partial molar volume ( Vi1 = 22.2 cm3 mol-‘) which is much larger than for pure water can be attributed to the lower internal pressure of toluene, as compared with that of water (Masterson and Seiler, 1968; Sakurai and Nakagawa, 1982): The correlative contribution to the intermolecular interaction water-toluene is weak in comparison with that of water-n-butanol which results in a net decrease of the apparent molar volume of water (= 16.3 cm mol- ‘). The apparent molar heat capacity of water at infinite dilution in toluene C$ (= 80 J K-’ mol-‘) is close to the value for pure water.

The minimum around equimolar concentration (x2 - x3 7 0.5) in n-

butanol and toluene corresponds to the minimum in C,, values mentioned before. Most likely the concomitant decrease of V$ and increase of Cl, after mole fraction x2 = 0.5 is a consequence of the progressive replacement of toluene by n-butanol; the corresponding changes in the apparent properties of water are as expected in such a case. The peculiar variations of ?$ and Cl, of water in the binary system n-butanol -I- toluene observed here are very

226

similar to those observed by Lara et al. (1981b) for water in the binary

system isopropanol + benzene. In the ternary system water + n-butanol + toluene, water at infinite dilution can be used as a probe to investigate the microstructure of the medium, since the variations of Vii and C$, in the same way as the variations of I’& and C,+* in the binary n-butanol + toluene, are representative of the association of alcohol and of the evolution of the microstructure over the whole mole fraction scale.

To explain qualitatively these variations it is interesting to compare them with results obtained for the ternary systems water + alcohol + surfactant (De Lisi et al., 1984; Roux et al., 1984; Roux-Desgranges et al., 1985). In these latter systems, at the vicinity of the critical micelle concentration (cmc) of the surfactant, the apparent molar properties of the alcohol at infinite dilution go through extrema which are more or less pronounced depending

on the chain length of the alcohol. This behavior has been explained taking into account different effects: the influence of alcohol on the lowering of the cmc of surfactant leading to a shift of the equilibrium monomer * micelle, and the partition of alcohol between the aqueous and micellar phases. Moreover, in the case of heat capacity, there is an additional contribution due to the effect of temperature on the equilibrium monomer +-+ micelle and as a consequence of the partitioning of alcohol. Taking into account these equilibrium shifts, with an association model for the surfactant and the partitioning of alcohol between aqueous and micellar phases, Roux et al. (1984) have been able to explain the shape of the experimental curves and even reproduce fairly well the apparent molar properties of alcohol in the dilute aqueous surfactant solutions.

Similarly, for the ternary system water + n-butanol + toluene studied here, the particular variations of V$ and Cii can be explained qualitatively. In the same way as surfactant molecules associate in water to form micelles,

n-butanol molecules associate in toluene to form ‘microaggregates’. Upon addition of water to the binary n-butanol + toluene water molecules perturb

the association equilibrium of n-butanol by interacting strongly with alcohol, leading then to mixed structures (water-alcohol), which appear at a lower ‘critical’ concentration of alcohol. This perturbation of the medium as seen by water is well reflected by V$ and even more by C$ because of the additional temperature effect on the equilibrium. The effect is maximized near the ‘critical’ concentration of association of the alcohol (Figs. 8 and 9). More precisely, the peak observed for Ci, (Fig. 9) has the same origin as the peak observed for C $z (Fig. 6) in the binary n-butanol + toluene but it occurs at a smaller mole fraction x2 of n-butanol in the ternary system due to the influence of water on the n-butanol association.

As a matter of fact, this ternary system behaves like a ‘detergentless microemulsion’ and in the toluene rich domain we most likely have ‘mixed

221

microaggregates’ of n-butanol and water dispersed in toluene exactly in the

same way we had reverse micelles in toluene in the microemulsion with surfactant studied before (Roux et al., 1981; Roux-Desgranges et al., 1981). In the latter case water is trapped in mixed surfactant alcohol micelles whereas in the former case water is trapped in alcohol microaggregates; the alcohol acting then at the same time as surfactant and cosurfactant.

CONCLUSION

Systematic measurements of thermodynamic properties such as volumes and heat capacities allow the investigation of large domains of concentration in the homogeneous single phases of multicomponent systems. They do not give a direct insight on the microorganization resulting from molecular interactions but they present the unique advantage of revealing, almost quantitatively in concentration terms, where the structural changes take place. Another decisive advantage over other thermodynamic data (such as enthalpies and free energies) is to check theoretical calculations which in turn can be used to predict the thermodynamic behavior of other multicom- ponent liquid systems.

Our results bring more evidence of the important role played by alcohols in ternary systems considered as detergentless microemulsions; they also confirm the key role of alcohol as a cosurfactant in microemulsions, where a surfactant is present, especially in reverse type microemulsions.

ACKNOWLEDGMENTS

Financial support received within the framework of the Spanish-French program for scientific and technical cooperation (‘Action IntCgrCe’ between UniversitC de Clermont-Ferrand 2 and Universidad de Valladolid) is grate- fully acknowledged by the authors.

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