Download ROLE OF ALCOHOL IN MICROEMULSIONS. 111. VOLUMES AND

Fluid Phase Equilibria, 25 (1986) 209-230
Elsevier Science Publishers B.V., Amsterdam
209
-
Printed in The Netherlands
ROLE OF ALCOHOL IN MICROEMULSIONS. 111. VOLUMES
HEAT CAPACITIES IN THE CONTINUOUS PHASE WATER-nBUTANOL-TOLUENE
OF REVERSE MICELLES *
GENEVIEVE
ROUX-DESGRANGES
** and JEAN-PIERRE
Laboratoire de Thermodynamique et Cin&ique Chimique,
Clermont - Ferrand 2, 63170 Aubikre (France)
MIGUEL
ANGEL
Departamento
(Spain)
(Received
VILLAMARAN
de Termologia.
*** and CARLOS
Fact&ad
March 2, 1985; accepted
de Ciencias,
E. GROLIER
AND
**
UA C. N. R.S. 434, Unioersitk de
CASANOVA
Universidad de VaNadolid,
Valladolid
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 Solutions 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, Valladolid, Spain.
037%3812/86/$03.50
0 1986 Elsevier Science Publishers
B.V.
210
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 considered 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-association 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-Desgranges 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 dependent. 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 (hydrocarbon + 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 ‘pseudophase’ 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 thermodynamic
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
211
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 ‘detergentless 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 microemulsions. 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, thermodynamic 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 densimeter 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) + nbutanol(2) + toluene (3), as shown in Fig. 1, is characterized by a homogeneous domain in two parts, a rather large one in the organic region and a
212
TABLE
1
Experimental
densities,
p, and molar
pressure of toluene and n-butanol
heat capacities,
Cp (J K-’
P (g cmm3)
This work
Literature
Toluene
0.86219
n-Butanol
0.80573
0.86219
0.86224
0.86228
0.80575
0.80586
a
b
’
d
’
’
s
h
’
Cp, at 298.15 K and
a
b
’
s
h
atmospheric
mol-‘)
This work
Literature
157.08
157.08
157.00
157.20
177.02
175.97
d
e
’
’
Hales and Townsend, 1972.
Tanaka et al., 1975.
API Project.44, 1952.
Fortier and Benson, 1977.
Holzhauer and Ziegler, 1975.
Scott et al., 1962.
Hales and Ellender, 1976.
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.
213
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
from experimental p and cP data using the usual relations
i are calculated
(4
C,, = M,c, +
103kp- cp,,>
(3)
mi
where Mi is the molar mass and m, the molality
initial binary reference ‘solvent’.
RESULTS
of component
i in the
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 structure 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, respectively. 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 negligible. In fact the values of these properties and their variations with concentration 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 considering 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.
+ 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.
n-butanol
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)
Xl
0.0145
0.0387
0.0662
0.0949
0.1434
0.1488
0.2097
0.2521
0.3140
0.3495
0.4022
0.4475
binary system (homogeneous
(“g
Cm-3)
V+1
(cm3 mol-‘)
0.806508
0.807813
0.809351
0.810957
0.813781
0.814078
0.817820
0.820708
0.825288
0.828106
0.832743
0.837063
16.32
16.42
16.46
16.56
16.70
16.73
16.90
16.97
17.07
17.13
17.20
17.27
phase) at 298.15 K
c01
(J K-’
2.3920
2.4203
2.4508
2.4816
2.5323
2.5381
2.6046
2.6511
2.7266
2.7652
2.8413
2.9067
mol-‘)
133.5
128.7
124.4
120.7
115.7
115.3
111.3
108.7
106.2
103.8
102.7
101.1
Densities p and specific heat capacities at constant pressure cP; apparent molar volumes
and heat capacities C,, of water (1) in the binary system; x,, mole fraction of water.
V+,
TABLE 3
n-Butanol
(2)+ toluene (3) binary system at 298.15 K
(“gcme3)
(cm3 mol-‘)
ff K-1 g-‘)
C
(.G1
0.861483
0.860725
0.860668
0.860428
0.860102
0.859691
0.859292
0.858508
0.857978
0.857606
0.856628
0.853992
0.852708
0.851568
0.848841
0.846398
0.840920
0.833064
0.824611
0.815906
93.76
93.53
93.55
93.43
93.35
93.27
93.18
92.95
92.94
92.93
92.83
92.62
92.53
92.47
92.40
92.31
92.21
92.08
91.98
91.96
1.7134
1.7348
1.7373
1.7460
1.7598
1.7737
1.7895
1.8156
1.8298
1.8384
1.8651
1.9221
1.9473
1.9667
2.0121
2.0518
2.1238
2.2159
2.2909
2.3403
197.8
241.8
246.9
256.2
270.8
275.8
282.7
281.6
279.5
276.4
272.5
256.2
249.6
244.1
234.7
228.0
215.7
203.3
193.0
184.2
x2
0.0112
0.0239
0.0248
0.0292
0.0349
0.0422
0.0495
0.0650
0.0742
0.0808
0.0992
0.1498
0.1751
0.1972
0.2487
0.2965
0.3987
0.5467
0.6995
0.8446
b2
Densities p and specific heat capacities
and heat capacities C,, of n-butanol(2)
mol-‘)
at constant pressure cP; apparent molar volumes Vez
in the binary system; x2, mole fraction of n-butanol.
217
TABLE 4
Ternary
system water (l)+ n-butanol
Xl
0.01996
0.0401
0.0599
0.0799
0.1001
0.1500
0.2746
0.3752
0.4494
(2)+ toluene (3) in the homogeneous
VI3
part at 298.15 K
CB
rp3
m3
(mol kg-‘)
;gcm-) )
(cm3 mol-‘)
;; K-’
0.2031
0.2063
0.2078
0.2078
0.2057
0.2067
0.2035
0.2104
0.2075
0.807612
0.808693
0.809916
0.811004
0.812218
0.815029
0.823086
0.830922
0.837783
106.58
106.67
106.75
106.81
106.87
106.99
107.19
107.36
107.50
2.3943
2.4171
2.4378
2.4591
2.4798
2.5325
2.6688
2.7918
2.8899
g-‘)
(J K-’
mol-‘)
203.9
203.3
202.7
201.8
200.8
200.9
200.7
201 .o
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 corresponding 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
x2
0
0.0248
0.0349
0.0495
0.0650
0.0742
0.0992
0.1498
0.1751
0.2487
0.2965
0.3987
0.5467
0.6995
0.8446
1
system water (l)+ n-butanol
ml
(2)+ toluene (3) in the homogeneous
$I
part at 298.15 K
q1
(mol kg-‘)
Fg cmm3)
(cm3 mol-‘)
;$ K-r
0.0163
0.0213
0.0198
0.0275
0.0346
0.0469
0.0561
0.0740
0.0704
0.1154
0.1051
0.1456
0.1501
0.1371
0.1671
0.1191
0.862184
_
21.22
_
0.860114
0.859315
0.858563
0.858052
0.856758
0.854180
0.852881
0.849125
0.846671
0.841297
0.833489
0.825051
0.816487
0.806207
20.13
19.83
18.83
18.85
17.87
17.61
17.74
17.80
17.65
17.75
17.54
17.12
16.85
16.23
1.7055
1.7424
1.7638
1.7937
1.8211
1.8367
1.8717
1.9296
1.9537
2.0206
2.0586
2.1310
2.2231
2.2988
2.3519
2.3851
g-‘)
(J K-’
mol-‘)
79.8
270.8
233.8
185.0
191.8
180.2
149.6
136.1
126.1
110.1
101.8
87.8
88.0
99.0
111.8
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,“1and Cz, (see text)).
218
that in the bulk region of the homogeneous domain toluene is always in an
‘organic environment’ without important modifications of the local structure.
The variations of the apparent molar properties of water, V+,rand C+r,
versus the mole fraction of water are shown in Fig. 3 for different compositions 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 properties of water reflect not only the evolution of the local microstructures
surrounding water molecules but also all interactions between three components 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 capacities 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 represented 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
“@
c
cm’ ma
7
1
6.
Fig. 4. Apparent molar volumes V+ and heat capacities C, at 298.15 K of water (1) and
n-butanol(2)
in the binary water (l)+ n-butanol(2)
versus the water mole fraction x,: 0, c$1;
n, C+,; A, v,,;
A, C+z.
In the water + n-butanol binary mixtures n-butanol can be considered as
the solvent in the n-butanol rich domain; its apparent molar properties I$,*
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
0
I
I
0.2
I
I
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$, and larger values of C,, are not easy to explain.
They could correspond to an ‘enhanced structuration’
as compared to the
structure of water in pure liquid water. Recently, De Granpre et al. (1982)
combined infra-red studies and heat capacity measurements
to understand
the behavior of water at small concentrations
in an alcohol. It appears that
the IR absorption band shifts are small and that the HO-HOR
and the
HO-HO
interactions
have similar spectroscopic
intensities. This confirms
that water and alcohol molecules are closely bounded together by hydrogen
bonds leading then to a rather strongly structured ensemble. When the water
concentration
remains very small (i.e., in the dilute water region) in the
water + n-butanol, the apparent molar properties of water seem to reflect a
change in structure which may be interpreted as follows: the self-associated
structures of alcohol are modified by addition of small amounts of water
which in turn create a new molecular organization through strong hydrogenbonds with the assumption of water molecules trapped in ‘reverse micelletype’ structures made of alcohol molecules. When toluene is added to this
221
medium its interaction with its local environment is affected as reflected
(Fig. 5) by V$ 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 capacities 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 represented 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
4_
9
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
systems. 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 associated 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
225
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 nbutanol 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 multicomponent 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 gratefully acknowledged by the authors.
REFERENCES
American Petroleum Institute Research Project 44. Chemical Thermodynamic
Properties
Center, Texas A and M University, College Station, Texas. (Table 23 a dated October 31,
1952.)
Backlund, S., Heriland, H. and Vikholm, I., 1984. Water-alcohol
interactions in the two-phase
system water-alcohol-alkane.
J. Solution Chem., 10: 749-755.
Bellocq, A.M., Biais, J., Clin, B., Lalanne, P. and Lemanceau, B., 1979. Study of dynamical
and structural properties
of microemulsions
by chemical physics methods. J. Colloid
Interface Sci., 70: 524-536.
Biais, J., Bothorel, P., Clin, B. and Lalanne, P., 1981a. Theoretical behavior of microemulsions: geometrical aspects and dilution properties. J. Colloid Interface Sci., 80: 136-145.
Biais, J., Bothorel, P., Clin, B. and Lalanne, P., 1981b. Theoretical behavior of microemulsions: geometrical
aspects, dilution properties
and role of alcohol. Comparison
with
experimental results. J. Disp. Sci. Technol. 2: 67.
228
Biais, J., Odberg, L. and Stenius, P., 1982. Thermodynamic
properties of microemulsions:
pseudo-phase
equilibrium-vapor
pressure measurements.
J. Colloid Interface Sci., 86:
350-358.
Costas, M. and Patterson, D., 1985. Self association of alcohols in inert solvents. Apparent
heat capacities and volumes of linear alcohols in hydrocarbons.
J. Chem. Sot. Faraday
Trans. 1, 81: 635-654.
CounselI, J.F., Hales, J.L. and Martin, J.F., 1965. Thermodynamic
properties
of organic
oxygen compounds:
butyl alcohol. Trans. Faraday Sot., 61: 1869-1875.
De Granpre, Y., Rosenholm, J.B., Lemelin. L.L. and Jolicoeur, C., 1982. In: K.L. Mittal and
E.J. Fendler (Editors), Near-infrared
and Heat Capacity Investigations
of the State of
Water in Surfactant and Alcohol Solutions. Solution Behavior of Surfactants, Vol. 1, pp.
431-453.
De Lisi, R., Perron, G. and Desnoyers, J.E., 1980. Volumetric and thermochemical
properties
of ionic surfactants:
sodium decanoate and octylamine hydrobromide
in water. Can. J.
Chem., 58: 959-969.
De Lisi, R., Genova, C., Testa, R. and Turco Liveri, V., 1984. Thermodynamic
properties of
alcohols in a micellar phase. Binding constants and partial molar volumes of pentanol in
sodium dodecylsulfate
micelles at 15, 25 and 35°C. J. Solution Chem., 13: 121-150.
De Visser, C., Perron, G. and Desnoyers,
J.E., 1977. The heat capacities, volumes and
expansibilities
of ter-butyl alcohol-water
mixtures from 6 to 65°C. Can. J. Chem., 55:
856-862
De Visser, C., Heuvelsland, W.J.M., Dunn, L.A. and Somsen, G., 1978. Some properties of
binary aqueous liquid mixtures. Apparent molar volumes and heat capacities at 298.15 K
over the whole fraction range. J. Chem. Sot. Faraday Trans. 1, 74: 1159-1169.
Fortier, J.L. and Benson, G.C., 1977. Excess heat capacities of binary mixtures of tetrachloromethane with some aromatic liquids at 298.15 K. J. Chem. Thermodyn., 9: 1181-1188.
Fuoss, R.M.. 1943. The system water-n-butanol-toluene
at 30”. J. Am. Chem. Sot., 65:
78-81.
Graciaa, A., 1978. Contribution
a I’etude des proprietes physicochimiques
des microemulsions
transparentes.
These d’Etat. Universite de Pau et des Pay de I’Adour.
Hales, J.L. and Ellender, J.H., 1976. Liquid densities from 293 to 490 K of nine aliphatic
alcohols. J. Chem. Thermodyn., 8: 1177-1184.
Hales, J.L. and Townsend, R., 1972. Liquid densities from 293 to 490 K of nine aromatic
hydrocarbons.
J. Chem. Thermodyn., 4: 763-772.
Herrmann, C.U., Wtirz, U. and Kahlweit, M., 1978. The influence of amphiphilic substances
on the mutual solubilities in binary liquid systems (microemulsions).
Ber. Bunsenges. Phys.
Chem,., 82: 560-567.
Holzhauer, J.K. and Ziegler, W.T., 1975. Temperature
dependence of excess thermodynamic
properties
of n-heptane-toluene,
methylcyclohexane-toluene,
and n-heptane-methylcyclohexane systems. J. Phys. Chem., 79: 590-604.
Huyskens, P.L., Haulait-Pirson,
M.CI., Hanssens, I. and Mullens, J., 1980. Water absorption
by alcohols in organic solvents. J. Phys. Chem., 84: 28-33.
Iwasaki, K. and Fujiyama, T., 1977. Light scattering study of clathrate hydrate formation in
binary mixtures of ter-butyl alcohol and water. J. Phys. Chem., 81: 1908-1912.
Iwasaki, K. and Fujiyama, T., 1979. Light scattering study of clathrate hydrate formation in
binary mixtures of ter-butyl alcohol and water. 2. Temperature effect. J. Phys. Chem., 83:
463-468.
Kretschmer, C.B. and Wiebe, R., 1949. Liquid-vapor
equilibrium of ethanol-methylcyclohexane solutions. J. Am. Chem. Sot., 71: 317663179.
229
Kumaran,
M.K. and Benson, G.C., 1983a. Limiting partial molar volumes of ethanol,
propan-l-01, butan-l-01, and hexan-l-01 in n-heptane at 298.15 K. J. Chem. Thermodyn..
15: 245-248.
Kumaran, M.K., Halpin, C. and Benson, G.C., 1983b. Limiting excess partial molar’enthalpies of hexan-1-ol, methyl pentan-l-01 and 2-ethylbutan-l-01
in n-hexane at 298.15 K. J.
Chem. Thermodyn., 15: 249-254.
Lara, J., Avedikian, L., Perron, G. and Desnoyers, J.E., 1981a. Microheterogeneity
in aqueous
organic mixtures: thermodynamic
transfer functions for benzene from water to 2-propanol
aqueous systems at 25°C. J. Solution Chem., 10: 301-313.
Lara, J., Perron, G. and Desnoyers, J.E., 1981b. Heat capacities and volumes of the ternary
system benzene-water-2-propanol.
J. Phys. Chem.. 85: 1600-1605.
Majer, V., Roux, A.H., Roux-Desgranges,
G. and Viallard, A.. 1983. Role de l’alcool dans les
microtmulsions.
II. Volumes et capacites calorifiques
molaires apparents
du systeme
eau + dodecylsulfate de sodium + isopropanol a 298.15 K. Can. J. Chem., 61: 139-146.
Masterson, W.L. and Seiler, H.K., 1968. Apparent and partial molar volumes of water in
organic solvents. J. Phys. Chem., 72: 4257-4262.
Nagata, I., 1977. Thermodynamics
of associated solutions: new expressions for the excess
Gibbs free energy and excess enthalpy of mixing of alcohol-solvent
systems. Fluid Phase
Equilibria, 1: 93-111.
Picker, P., Leduc, P-A., Philip, P.R. and Desnoyers, J.E., 1971. Heat capacity of solutions by
flow microcalorimetry.
J. Chem. Thermodyn., 3: 631-642.
Picker, P., Tremblay, E. and Jolicoeur. C., 1974. A high-precision
digital readout flow
densimeter for liquids. J. Solution Chem., 3: 377-384.
Renon, H. and Prausnitz. J.M., 1967. On the thermodynamics
of alcohol-hydrocarbon
solutions. Chem. Eng. Sci., 22: 299-307.
Roux, A.H., Hetu, D., Perron, G. and Desnoyers, J.E.. 1984. Chemical equilibrium model for
the thermodynamic
properties of mixed aqueous micellar systems: application to thermodynamic functions of transfer. J. Solution Chem., 13: l-25.
Roux, A.H., Roux-Desgranges,
G., Grolier, J-P.E. and Viallard. A., 1981. Apparent molar
volumes in microemulsions:
an insight into the structures of these systems. J. Colloid
Interface Sci., 84: 250-262.
Roux, G., Perron, G. and Desnoyers, J.E., 1978. Model systems for hydrophobic interactions:
volumes and heat capacities of n-alkoxyethanols
in water. J. Solution Chem.. 7: 639-654.
Roux, G., Roberts, D., Perron, G. and Desnoyers, J.E., 1980. Microheterogeneity
in aqueousorganic solutions:
heat capacities.
volumes and expansibilities
of some alcohols,
aminoalcohol and tertiary amines in water. J. Solution Chem.. 9: 629-647.
Roux-Desgranges,
G., Roux, A.H., Grolier, J-P.E. and Viallard, A., 1981. Heat capacities of
toluene in the microemulsion water-sodium
dodecylsulfaten-butanol-toluene
at 25°C. J.
Colloid Interface Sci., 84: 536-545.
Roux-Desgranges,
G., Roux, A.H., Grolier, J-P.E. and Viallard, A., 1982. Role of alcohol in
microemulsions
as determined from volume and heat capacity data for the water-sodium
dodecylsulfate-n-butanol
system at 25°C. J. Solution Chem.. 11: 357-375.
Roux-Desgranges,
G., Roux, A.H. and Viallard, A., 1985. Volumes et capacitts calorifiques
molaires de transfert des alcools en milieu micellaire eau + dodecylsulfate
de sodium. J.
Chim. Phys., 82: 441-447.
Sakurai, M. and Nakagawa, T., 1982. Densities of dilute solutions of water in benzene and in
methanol at 278.15, 288.15, 298.15, 308.15 and 318.15 K. Partial molar volumes V,, and
values of 8&,/8r
for water in benzene and in methanol. J. Chem. Thermodyn.
14:
269-274.
230
Sakurai, M. and Nakagawa, T., 1984. Densities of dilute solutions of water in n-alkanols at
278.15, 288.15, 298.15, 308.15 and 318.15 K. Partial molar volumes of water in n-alkanols.
J. Chem. Thermodyn., 16: 171-174.
Scott, D.W., Guthrie, G.B., Messerly, J.F., Todd, S.S., Berg, W.T., Hossenlopp, LA. and MC
Cullough, J.P., 1962. Toluene: thermodynamic
properties, molecular vibrations and internal rotation. J. Phys. Chem., 66: 911-914.
Smith, F. and Brown, I., 1973. Thermodynamic
properties of alcohol + alkane mixtures. I.
Theoretical results for the hydrogen bond-contribution
to the excess energies. Aust. J.
Chem., 26: 691-704.
Smith, G.D. and Barden, R.E., 1982. In: K.L. Mittal and E.J. Fendler (Editors), Chemistry of
Detergentless
Microemulsions.
Solution Behavior of Surfactants, Vol. 2, pp. 1225-1235.
Smith, G.D., Donelan, C.E. and Barden, R.E., 1977. Oil-continuous
microemulsions
composed of hexane water and 2-propanol. J. Colloid Interface Sci., 60: 488-496.
Stokes, R.H. and Adamson, M., 1976. Enthalpies of dilution of ethanol in cyclohexane in the
temperature range 279.85 K to 318.15 K. J. Chem. Thermodyn., 8: 683-689.
Tanaka, R., Kiyohara, O., d’Arcy, P.J. and Benson, G.C., 1975. A micrometer
syringe
dilatometer:
application
to the measurement
of excess volumes of some ethylbenzene
systems at 298.15 K. Can. J. Chem., 53: 2262-2267.
Treszczanowicz,
A.J. and Benson, G.C., 1977. Excess volumes for n-alkanols-t
n-alkanes. I.
Binary mixtures of methanol, ethanol, n-popanol and n-butanol+
n-heptane. J. Chem.
Thermodyn., 9: 1189-1198.
Treszczanowicz,
A.J., Kiyohara, 0. and Benson, G.C., 1981. Excess volumes for n-alkanols+
n-alkane. IV. Binary mixtures of decan-1-ol+ npentane, + n-hexane, + n-octane, + n-decane, + n-hexadecane.
J. Chem. Thermodyn., 13: 253-260.
Van Ness, H.C., Soczek, CA. and Kochar, N.K., 1967. Thermodynamics
excess properties for
ethanol-n-heptane.
J. Chem. Eng. Data, 12: 346-351.
in systems water-aliphatic
Vorob’eva, AI. and Karapet’yants.
M.K., 1967. Miscibility
alcohol- n-alkane. III. The system water-isopropylalcohol-n-C,,Hz,,+z.
Russ. J. Phys.
Chem., 41: 1061-1063.